A comprehensive review of glycosylated bacterial natural products

Abstract

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

1. Introduction

While it is well-established that the glycosylation of naturally-occurring and/or synthetic small molecule-based drugs can dramatically influence the pharmacological properties of the parent scaffold,^1–17 there remains a lack of accuracy regarding the prevalence and/or extent of glycosidic diversity in the context of naturally-occuring glycosides. The current review attempts to address this gap of knowledge by providing a systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature. AntiBase 2012¹⁸ served as a key resource for glycoside identification and all glycosides were structurally validated via an analysis of the primary literature and corrected where necessary prior to integration into this compilation. Based upon an analysis of 15 940 bacterial natural products, over one fifth (3426 compounds, 21.5%, Fig. 1) are glycosides wherein glycosylated macrolides and macrolactams represent the largest allocation (738 compounds, 21.5% of all bacterial glycosides, Fig. 2). Further analysis of the range of saccharides represented across all 3426 bacterial glycosides revealed 344 distinct carbohydrates (Fig. 3 and Table 1). For this latter consideration, carbohydrates were only designated as ‘distinct’ based upon differences within the fundamental monsaccharide core (specifically, notable stereochemical and/or functional group variation, including anomeric configuration) whereas simple modifications of a given common sugar core (e.g., O/N/S-alkyl/acyl substitutions) were designated as identical to the parental core saccharide. The content of this comprehensive review has been organized based upon aglycon class/structure as indicated in Fig. 2 wherein each section provides additional relevant information pertaining to the glycosides represented within the selected metabolite classification. Illustrations throughout this review employ two standard conventions. First, regiospecificity of glycosylation is represented as a colored ball within the context of a representative aglycon, the latter of which are presented in many cases in generic form in an effort to emphasize glycosylation. Second, for simplicity, D-pyranoses are represented in the ⁴C₁ conformation while L-pyranoses are illustrated as ¹C₄ conformers. For furanoses, and/or in cases where the chair conformation may obstruct field of view, a standard planar pentose or hexose ring is used. In addition, all pseudosugars found as part of the microbial glycosylated natural products discussed within this review are summarized in Fig. 4.

Fig. 1.

Bacterial glycosylated (21.5%; 3426 compounds) and unglycosylated (78.5%; 12 514 compounds) natural products.

Fig. 2.

Chemical classes of glycosylated bacterial natural products (total 3426 compounds).

Fig. 3.

Summary of all sugars present in bacterial natural products (1–344). Only sugars that displayed differences within the fundamental monosaccharide core (specifically, notable stereochemical and/or functional group variation, including anomeric configuration) were considered as distinct. Modifications of a common sugar core (e.g., O/N/S-alkyl/acyl substitutions) were designated as identical to the parental core saccharide.

Table 1.

Names of sugars present in bacterial natural products. Numbers correspond to sugars in Fig. 3. For simplicity and convenience for the readers, common names were preferred than IUPAC nomenclature

Sugar no.	Sugar name
1	2′,6′-Diamino-2′,3′,4′,6′-tetradeoxy-α-D-glucose
2	2′,6′-Diamino-2′,3′,4′,6′-tetradeoxy-4′,5′-unsaturated-α-D-glucose
3	(6′R)-2′,6′-Diamino-2′,3′,4′,6′,7′-pentadeoxy-β-L-glucoheptose
4	(6′S)-2′,6′-Diamino-2′,3′,4′,6′,7′-pentadeoxy-β-L-glucoheptose
5	2′-Amino-2′-deoxy-α-D-glucose
6	2′-Amino-2′,7′-dideoxy-α-D-glucoheptose
7	6′-Amino-6′-deoxy-α-D-glucose
8	2′,6′-Diamino-2′,3′,6′-trideoxy-α-D-glucose
9	6′-Amino-3′,6′-dideoxy-α-D-glucose
10	2′,6′-Diamino-2′,6′-dideoxy-α-D-glucose
11	2′,6′-Diamino-2′,6′,7′-trideoxy-α-D-glucoheptose
12	3′-Amino-3′-deoxy-α-D-glucose
13	2′,6′-Diamino-2′,4′,6′-trideoxy-α-D-glucose
14	2′-Amino-2′,3′-dideoxy-α-D-glucose
15	(6′R)-2′,6′-Diamino-2′,3′,4′,6′,7′-pentadeoxy-4′,5′-unsaturated-α-D-glucoheptose
16	(6′S)-2′,6′-Diamino-2′,3′,4′,6′,7′-pentadeoxy-4′,5′-unsaturated-α-D-glucoheptose
17	Cyclic unsaturated analog of 3/4
18	α-D-Glucose
19	3′-Amino-3′-deoxy-4′-C-methyl-4′-epi-α-D-xylose
20	3′-Amino-3′-deoxy-4′-C-methyl-α-D-galactose
21	3′-Amino-3′-deoxy-4′-epi-α-D-xylose
22	3′-Amino-3′-deoxy-β-L-xylose
23	2′,3′-Diamino-2′,3′-dideoxy-β-L-xylose
24	2′-Amino-2′-deoxy-α-D-xylose
25	β-D-Ribofuranose
26	β-D-Xylofuranose
27	2′,6′-Diamino-2′,6′-dideoxy-β-L-idose
28	3′-C-Carbaldehyde-5′-deoxy-α-L-lyxofuranose
29	5′-Deoxy-3′-C-hydroxymethyl-α-L-lyxofuranose
30	3′-C-Carbaldehyde-α-L-lyxofuranose
31	3′-C-Hydroxymethyl-α-L-lyxofuranose
32	α-L-Glucosamine
33	α-D-Spectinose
34	Unusual cyclized analog of 3′-deoxyglucosamine
35	Unusual cyclized analog of glucosamine
36	4′-Deoxy-4′-amino-β-L-glucose
37	5′-Keto-5′-C-methyl-β-D-arabinofuranose
38	5′-Keto-5′-C-methyl-α-L-xylofuranose
39	4′-Amino-4′,6′-dideoxy-α-D-glucose
40	2′-Amino-1′,2′-dideoxy-α-D-glucose
41	2′,4′-Diamino-2′,3′,4′,6′-tetradeoxy-α-D-mannose
42	3′-Amino-3′-deoxy-β-D-mannose
43	6′-Hydroxy-β-L-mannose
44	α-L-Rhodinose
45	α-L-Ristosamine
46	α-L-Rhamnose
47	β-D-Oliose
48	β-D-Amicetose
49	α-L-Oliose
50	α-L-Rednose
51	α-L-Cinerulose
52	α-D-Taluronic acid
53	β-D-Olivose
54	α-L-Aculose
55	β-D-Kerriose
56	β-D-Rhodinose
57	α-D-Boivinose
58	α-D-Rhodinose
59	3′-Amino-2′,3′,6′-trideoxy-β-D-glucose
60	4′-epi-α-L-Tolyposamine
61	2′-Thio-α-D-glucose
62	6′-Deoxy-α-L-altrose
63	4′-Amino-4′-deoxy-β-D-amicetose
64	β-D-Mycarose
65	3′-epi-4′-epi-α-L-Vancosamine
66	3′-Amino-2′,3′,6′-trideoxy-β-L-glucose
67	3′-Amino-2′,3′-dideoxy-β-L-fucose
68	α-L-Digitoxose
69	α-D-Ristosamine
70	3′-Amino-2′,3′-dideoxy-α-L-fucose
71	α-D-Oliose
72	β-D-Glucuronic acid
73	3′-Amino-3′,6′-dideoxy-α-L-talose
74	3′-Amino-3′,6′-dideoxy-α-L-galactose
75	β-D-Fucofuranose
76	4′-Hydroxy-β-D-fucofuranose
77	β-D-Virenose
78	3′-Deoxy-3′-amino-β-D-fucose (β-D-ravidosamine)
79	4′-Keto-β-D-virenose
80	3′-C-Acetylpentofuranose
81	4′-Amino-4′-deoxy-α-D-amicetose (α-D-forasmine)
82	3′-Amino-N′,N′-dimethyl-N-oxido-2′,3′,6′-trideoxy-β-D-glucose
83	Unusual cyclized sugar I
84	Unusual cyclized sugar II
85	Potential sugar I
86	Potential sugar II
87	4′-Amino-4′,6′-dideoxy-β-D-allose
88	α-D-Fucosamine; α-D-elsaminose
89	4′-Amino-4′-deoxy-3′-C-methyl-β-D-ribose
90	α-L-Mycarose
91	4′-Amino-2′,4′-dideoxy-β-L-fucose
92	3′-Keto-4′-C-methyl-β-D-fucose
93	4′,6′-Dideoxy-4′-carbonyl-β-D-glucose
94	4′-Thio-2′,4′,6′-trideoxy-β-D-altrose
95	4′-Amino-2′,4′-dideoxy-α-L-xylose
96	4′-Deoxy-4′-thio-β-D-fucose
97	α-L-Amicetose
98	β-D-Cinerulose
99	4′-Deoxy-α-L-daunosamine
100	β-L-Olivose
101	3′-C-Methyl-3′-nitrose-2′,3′,6′-trideoxy-β-L-gulose
102	β-L-Decilonitrose
103	4′-C-Acetyl-2′,6′-dideoxy-α-L-gulose (α-L-trioxacarcinose B)
104	β-D-Glucose
105	α-L-Decilonitrose
106	2′,6′-Dideoxy-α-L-gulose
107	4′-epi-β-L-Decilonitrose
108	3′-C-Methyl-α-L-rhamnose
109	β-L-Avidinosamine
110	4′-Amino-4′,6′-dideoxy-3′-C-methylhexose
111	α-L-Lyxose
112	2′,6′-Dideoxy-4′-C-hydroxyethyl-α-L-gulose
113	4′-Oximo-2′,3′,4′-trideoxy-α-L-fucose
114	4′-Amino-2′,3′,4′,6′-tetradeoxy-α-L-glucose
115	2′-Deoxy-β-D-ribose
116	3′-Amino-2′,3′-dideoxy-4′-epi-α-L-xylose
117	3′,6′-Dideoxy-4′-keto-α-L-hexose
118	α-L-Vancosamine
119	β-L-Rednose
120	3′-Hydroxy-3′-C-nitro-2′,6′-dideoxy-α-L-talose
121	3′-Nitro-2′,3′,6′-trideoxy-α-L-gulose (3′-desmethyl-α-L-decilonitrose)
122	4′-epi-α-L-Mycarose
123	4′-C-Acetyl-2′,3′,6′-trideoxy-α-L-gulose
124	α-L-Actinosamine
125	6′-Deoxy-α-L-talose
126	4′-Keto-α-L-olivose
127	3′-epi-4′-epi-α-L-Decilonitrose (4′-desmethyl-α-L-evernitrose)
128	3′-Denitro-3′-hydroxylamine-3′-epi-4′-epi-α-L-decilonitrose
129	α-D-Digitoxose
130	4′-C-Acetyl-2′,3′,6′-trideoxy-β-L-gulose
131	β-D-Digitoxose
132	4′-C-Hydroxyethanone-2′,3′,6′-trideoxy-α-L-gulose
133	4′-Keto-β-D-mycarose
134	4′-Hydroxy-β-D-olivose
135	6′-Deoxy-β-D-gulose
136	β-D-Xylose
137	4′-Amino-4′,6′-dideoxy-α-L-glucose
138	4′-Amino-2′,4′-dideoxy-α-L-xylose
139	4′-Deoxy-α-D-taluronic acid
140	β-D-Quinovose
141	4′-Amino-2′,4′,6′-trideoxy-β-L-galactose
142	β-L-Mycarose
143	3′-Nitro-2′,3′,6′-trideoxy-α-L-glucose
144	β-L-Talose
145	3′-C-Carboxy-3′-C-methyl-2′,3′,5′-trideoxy-α-L-xylofuranose
146	3′-C-Carboxy-2′,5′-dideoxy-α-L-ribofuranose
147	3′-Amino-3′,6′-dideoxy-α-L-altrose
148	2′,3′,6′-Trideoxy-3′-oximo-α-L-altrose
149	3′-Amino-3′-C-carboxy-2′,3′,5′-trideoxy-α-L-ribofuranose
150	3′-Keto-2′,3′,6′-trideoxy-α-L-glucose
151	2′-Deoxy-3′-keto-α-L-fucose
152	3′-Amino-5′-hydroxy-2′,3′,6′-trideoxyhexose
153	β-D-Glucosamine
154	α-D-Glucuronic acid
155	α-D-Quinovose-6′-sulfonic acid
156	β-D-Mannose
157	α-L-Fucose
158	β-D-Arabinose
159	α-D-Glucofuranose
160	α-D-Mannose
161	β-D-Galactose
162	α-D-Allose
163	α-D-Altrose
164	6′-Deoxyhexosamine
165	β-L-Xylose
166	6′-Amino-1′-carboxy-6′-deoxy-β-D-mannose
167	6′-Amino-6′,8′-dideoxy-α-D-galactoctose
168	β-L-Iduronic acid
169	β-L-Rhodinose
170	6′-Deoxy-β-D-allose
171	6′-Deoxy-3′-keto-β-L-glucose
172	β-D-Galacturonic acid
173	2′-Amino-2′-deoxy-β-D-xylose
174	α-D-Galacturonamide
175	α-D-Glucuronamide
176	β-L-Digitoxose
177	3′-Amino-3′,4′-dideoxy-β-D-fucose
178	3′-N-Oxido-β-D-desosamine
179	4′,6′-Dideoxy-3′-keto-β-D-fucose
180	3′-C-Acetyl-4′,6′-dideoxy-β-D-allose
181	4′-Deoxy-β-D-fucose
182	3′-C-Methyl-2′,3′,6′-trideoxy-2′,3′-unsaturated-β-D-glucose
183	2′,6′-Dideoxy-α-D-altrose
184	α-L-Olivose
185	3′-Amino-3′,6′-dideoxy-β-D-glucose
186	β-L-Chromose
187	β-D-Boivinose
188	3′,6′-Dideoxy-4′-keto-2′-O-methyl-2′,3′-unsaturated-β-D-glucose
189	α-D-Gulosamine
190	4′,6′-Dideoxy-3′-C-hydroxyethyl-β-D-allose
191	α-D-Xylofuranose
192	β-D-Olivomicose
193	5′-C-Methyl-α-L-rhamnose (4′-O-desmethyl-α-L-noviose)
194	α-L-Quinovose
195	α-L-Chromose
196	2′,6′-Dideoxyhexose
197	6′-Deoxyhexose
198	6′-Deoxy-β-L-talose
199	L-Axenose
200	2′-Deoxy-β-D-glucose
201	1′-C-Methylamine-β-D-arabinose
202	4′-Amino-4′,6′-dideoxy-β-D-glucose
203	4′-Amino-2′,3′,4′,6′-tetradeoxyhexose
204	α-D-Arabinofuranose
205	2′,3′,6′-Trideoxyhexose
206	β-D-Mycosamine
207	α-L-Mycosamine
208	4′-Amino-4′-deoxy-β-D-fucose
209	6′-Deoxy-β-D-mannose
210	β-L-Glucose
211	α-L-Glucose
212	N-Desmethyl-β-D-vicenisamine
213	β-D-Pyrrolosamine
214	N-Desmethylcarbamate-β-D-tetronitrose
215	α-D-Quinovose
216	5′-C-Carbaldehyde-4′,5′-unsaturated-β-D-digitoxose
217	6′-Deoxyhexose
218	3′,4′-Diamino-3′-C-methyl-2′,3′,4′,6′-tetradeoxy-β-D-gulose
219	3′,4′-Diamino-2′,3′,4′,6′-tetradeoxy-β-D-galactose
220	4′-Deoxy-α-L-digitoxose
221	5′-Amino-5′-deoxy-β-D-ribofuranose
222	Tunicamine
223	5′-Amino-5′-C-carboxy-5′-deoxy-β-D-ribofuranose
224	2′,5′-Dideoxy-5′-amino-β-D-ribofuranose
225	4′,5′-Unsaturated-α-D-mannuronic acid
226	3′-Hydroxylamine-2′,3′,6′-trideoxy-α-L-allose
227	Unusual bicyclic sugar I
228	Unusual bicyclic sugar II
229	Unusual bicyclic sugar III
230	Unusual bicyclic sugar IV
231	α-D-Ribofuranose
232	5′-Amino-5′-deoxy-4′,5′-unsaturated-β-D-ribofuranose
233	5′-Deoxy-β-D-ribo-octofuranuronic acid
234	2′-Deoxy-2′-aminohexose
235	3′-Amino-3′,4′-dideoxy-β-D-glucuronic acid
236	Unusual dicyclic sugar
237	2′-Amino-2′-deoxy-β-D-guluronic acid
238	Unusual β-D-sugar
239	Unusual β-D-sugar analog of 238
240	4′-Amino-4′-deoxy-2′,3′-unsaturated-β-D-glucuronic acid
241	4′-Amino-6′-C-carboxy-4′-deoxy-6′-C-propylamine-2′,3′-unsaturated-β-D-glucose
242	Hydroxy analog of 241
243	4′-Amino-3′-4′-dideoxy-β-D-glucuronic acid
244	4′-C-Amino-3′-deoxypentose
245	4′-Oximo-3′,4′,5′-trideoxy-β-D-xylose
246	4′-Keto-3′,4′,5′-trideoxy-β-D-xylose
247	4′-Amino-4′-deoxy-β-D-hexuronamide
248	6′-Deoxy-4′-thio-4′-S-methyl-4′-C-(1,2-dihydroxyethyl)-β-D-galactose
249	2′-Deoxy-β-D-glucuronic acid
250	3′-Amino-3′-deoxy-β-D-glucose
251	4′-Amino-4′-deoxy-β-D-glucuronamide
252	6′,9′-Diamino-5′,6′,7′,8′,9′-pentadexoy-4′,5′-unsaturated-α-L-ribo-decofuranuronic acid
253	3′-Amino-3′-deoxy-β-D-ribofuranose
254	Unusual tricyclic sugar
255	4′-Amino-4′-deoxy-β-L-mannoheptose
256	Unusual C–C sugar
257	4′-Amino-4′-deoxy-β-L-glucoheptose
258	β-D-Glucofuranose
259	3′-Deoxy-β-D-arabinofuranose
260	β-L-Gulosamine
261	2′,3′-Dideoxy-4′-epi-β-D-xylose
262	2′,3′-Dideoxy-4′-epi-α-L-xylose
263	3′-Deoxy-4′-epi-β-D-xylose
264	3′-Deoxy-4′-epi-α-L-arabinose
265	4′-α-L-Arabinose
266	β-D-Elsarose
267	6′-Deoxy-α-D-mannose
268	6′-Deoxy-3′-C-methyl-β-D-mannose
269	β-D-Saccharosamine
270	β-D-Fucose
271	β-D-Eurekanate I
272	β-D-Eurekanate II
273	β-D-Eurekanate III
274	β-D-Eurekanate IV
275	D-Destomic acid
276	4′-epi-Destomic acid
277	α-L-Gulose
278	6′-Deoxy-α-L-gulose
279	4′-Amino-4′,6′-dideoxy-α-L-talose
280	4′-epi-α-L-Vancosamine
281	4′-Keto-α-L-vancosamine
282	4′-C-Amino-4′-epi-α-L-vancosamine
283	2′-Amino-2′-deoxy-β-D-glucuronic acid
284	3′-C-Methyl-3′-nitroso-2′,3′,6′-trideoxy-β-D-gulose
285	2′-Amino-2′-deoxy-α-D-glucuronic acid
286	α-D-galactose
287	2′-Keto-α-D-glucose
288	6′-Amino-6′-deoxy-D-glucoheptose
289	2′-Oximo-α-D-glucose
290	7′-C-Methyl-destomic acid
291	4′-Deoxy-β-D-glucose
292	4′-Amino-4′-deoxy-3′-C-methyl-α-L-fucose
293	α-D-Xylose
294	α-D-Arabinose
295	2′,4′-Diamino-2′,4′,6′-trideoxy-α-D-glucose
296	2′,3′-Diamino-2′,3′-dideoxy-α-L-pentose
297	β-L-Quinovose
298	6′-Deoxy-β-D-talose
299	β-L-Vancosamine
300	3′-Amino-2′,3′-dideoxy-α-D-fucose
301	β-D-Vancosamine
302	3′-epi-4′-epi-5′-Hydroxy-β-D-vancosamine
303	2′,6′-Dideoxy-β-D-altrose
304	α-D-Vancosamine
305	3′-epi-β-L-Vancosamine
306	Δ1,2-D-Anglosamine
307	6′-Deoxy-α-D-altrose
308	α-D-Amicetose
309	β-L-Amicetose
310	3′-O-desmethyl-β-D-rubranitrose
311	3′-Deoxy-α-D-fucose
312	3′-Amino-3′,6′-dideoxy-α-L-glucose
313	3′-Deoxy-β-D-fucose
314	4′-Deoxy-α-D-taluronamide
315	6′-Amino-6′-deoxy-4′-thio-heptofuranuronic acid
316	2′-Amino-2′-deoxy-α-L-idose
317	α-D-Altruronic acid
318	6′-Amino-6′-deoxy-β-D-glucose
319	α-D-Fucose; 3-O-desmthyl-α-D-digitalose
320	2′-Deoxy-2′-amino-β-D-gulose
321	3′,5′-Unsaturated galactouronic acid
322	β-L-Rhamnose
323	2′,6′-Dideoxy-3′-hydroxyethyl-α-L-gulose (analog of 324)
324	β-D-Galacturonamide
325	4′-Keto-β-D-olivose
326	β-D-Allosamine
327	α-L-Sibirosamine
328	α-L-Paulomycose
329	α-L-Keto analog of 328
330	6′-Amino-6′-deoxy-β-D-ribofuranhepturonic acid
331	β-D-Ribofuranhexuronamide
332	enol form of spectinose (33)
333	2′,4′-Dideoxy-4′,5′-unsaturated-α-D-glucuronamide
334	Hexose
335	5′-O-Methyl-4′,5′-unsaturated-β-D-arabinofuranohexose
336	5′-O-Methyl-β-D-arabinofuranohexose
337	6′,9′-Diamino-5′,6′,7′,8′,9′-pentadeoxy-β-D-ribofuranodecuronic acid
338	6′,9′-Diamino-5′,6′,7′,8′,9′-pentadeoxy-4′,5′-unsaturated-β-D-ribofuranodecuronamide
339	6′,9′-Diamino-5′,6′,7′,8′,9′-pentadeoxy-β-D-ribofuranodecuronamide
340	Cyclic analog of 339
341	Unusual nine carbon bicyclic sugar I (cyclic form of 344)
342	Unusual nine carbon bicyclic sugar II
343	Unusual nine carbon bicyclic sugar III
344	6′-C-Carboxy-5′,6′,7′-trideoxy-β-D-ribofuranoocturonic acid

Fig. 4.

Summary of pseudosugars P1–P28 present in bacterial natural products. This list represents only those pseudosugar found within the context of bacterial glycosides and does not reflect an exhaustive list of naturally-occurring pseudosugars.

2. Aminoglycosides and related secondary metabolites

Aminoglycosides are structurally and functionally diverse and are noted for a range of biological activities including inhibition of bacterial protein synthesis (e.g., anti-infectives such as gentamicin, kanamycin, streptomycin), glucosidase inhibition (e.g., the antidiabetic acarbose), trehalase inhibition (e.g., the crop protectant validamycin) and inhibition of bacterial/eukaryotic protein synthesis (e.g. the cytotoxin pactamycin).¹⁹ The core sugar-derived carbocycle is the key structural signature of aminocylitols and this core is further diversified via variant functionalization (such as various degrees of deoxygenation and amination) where glycosylation at one or more positions is critical to the wide range of structural and functional diversity of this important natural product family.

2.1. Pactamycins

Members of this class include pactamycins, pactalactams, pactamycates, and cranomycin.^19–21 They all contain pseudosugars and are characterized by the presence of an acetophenone unit which is N-linked at the C-3 position to an aminocyclopentinol pseudosugar. Twenty compounds have been found to be produced naturally, through feeding studies, or through mutasynthesis.^22–27 Pseudosugar features common to all members include C-3′, C-4′ and C-5′-disubstitution (‘branching’) and conserved stereochemistry at all positions where pseudosugar divergence stems from variation of the C-2′ and C-3′-amine and/or C-9′-hydroxy substituents (Fig. 5). This family inhibits protein synthesis by binding the 30S subunit and displays broad activities including antibacterial, antiviral, antimalarial, and general cytotoxicity. This broad range of activities is due to the inhibition of protein synthesis in most organisms through binding with the 30S ribosomal subunit. The C-3′-1,3-dimethyl urea moiety of pactamycins at C-3′ (Fig. 5) has been noted to be important for activity while the C-9′ 6-methyl salicylic ester is considered dispensible.²⁵ Variation of the pseudosugar C-3′-branching (P27a and P16a) favored antimalarial acitivity with lowered overall mammalian cell line cytotoxicity.²⁶

Fig. 5.

Pactamycin aglycons and associated pseudosugars.

2.2. Other members

While a diverse array of carbocyles serves as the core aglycon of aminoglycosides, diamino carbocycles are among the most common. Metabolites that contain a 1,4-diamino carbocycle core include both the 2-deoxy variants (P1–P4, Fig. 6) such as that found within the istamycins and related metabolites sporaricins and sannamycins,^28–31 as well as 2-hydroxy-containing cores found within metabolites like lysinomicin (P6)³² and the fortimicins (P6, P14, P15, and P18).^33–36 The corresponding 2-deoxy-1,3-diamino carbocyclic aglycon 2-deoxystreptamine (2-DOS, P7) was found in majority of clinically used aminocyclitols including gentamicins,³⁷ kanamycins,³⁸ ribostamycin³⁹ and tobramycins while the related 2-hydroxy aglycon streptamine (P8) serves as the core of several aminoglycosides including streptomycins⁴⁰ and ashimycins.⁴¹ In other aminoglycosides, including gentamicins and sagamycins,⁴² a 1-deamino-1-hydroxy-2-DOS (P9) was reported as the aglycon. Aminoglycosides hygromycin A and epi-hygromycin contain the methylene-bridged aminocyclitol neo-inosamine (P19)⁴³ while a handful of secondary metabolites including acarbose,⁴⁴ amylostatins⁴⁵ and validamycins⁴⁶ utilize an unsaturated C7 cyclitol aglycon (P13). The D-myo-inositol (P12) and its aminated derivative P24 were recently discovered in mycothiol and minosaminomycin, respectively.^47,48 The epimer of myo-inositol (P25) found as the aminocyclitol core of kasugamycin⁴⁹ and carbamoylated inositol (P10) anchors boholmycin.⁵⁰

Fig. 6.

Aminoglycoside pseudosugars and associated sugars.

While a diverse range of aglycon glycosylation patterns exist among aminoglycosides, glycosylation at C-4 and C-6 are among the most prevalent. Sugars employed the context of C-4 glycosylation of core aglycons include: the highly deoxygenated diaminosugar 1 and its C-5′ branched 3 and 4 (gentamicins)^39,51 as well as the corresponding C-4′/C-5′-unsaturated variants 2 (sisomycin),⁵² 15 and 16 (verdamicins);⁵³ 2′-amino-2′-deoxy-α-D-glucose (5), its C-5′-methyl branched analog 6 (gentamicins)³⁷ or its 2′-deoxy 14 (nebramycin);⁵⁴ 6′-amino-6′-deoxy-α-D-glucose (7, gentamicins and combimicins)^37,55 its corresponding 3′-deoxy 9 (combimicins)⁵⁶ or its C-5′-methyl branched 11 (gentamicins);³⁷ 2′,6′-diamino-2′,6′-dideoxy-α-D-glucose 10 (combimicins)⁵⁵ and its corresponding 3′-deoxy 8 (nebramine and tobramycin);⁵⁷ 3′-amino-3′-deoxy-α-D-glucose (12, nebramycins)⁵⁴ and 2′,6′-diamino-4′-deoxy 13 (seldomycins).⁵⁸ The novel 2′,4′-diamino-2′,3′,4′,6′-tetradeoxyhexose 41 comprised the C-4 attachment in minosaminomycin and C-1 in kasugamycin (Fig. 6). The bicyclic iminosugar 17 (gentamicins)³⁷ stands out as particularly unique among those found within the C4 glycoside series. Also noteworthy are the C-4′ octadiose moieties (34 and 35) observed among saccharomycins and apramycin.⁵⁹ The C-8′ of this unusual eight carbon carbohydrate was found to be further modified by 4′-amino-4′-deoxy-β-L-glucose (36) or α-D-glucose (18) in apramycin. C-4 glycosylation using select members of the sugars described above is also represented among other aminoglycosides including butirosins,⁶⁰ xylostatins and ribostamycins.⁶¹

In addition to a few sugars highlighted in the previous paragraph found appended at C-6 of aminoglycoside core aglycons (2, 4, 8, 10, 12),^54,62,63 additional sugar diversity has been found at C-6 including: α-D-glucose (18);⁶⁴ 3′-amino-3′-deoxy-α-D-xylose 21 (gentamicins and sisomicins)^37,65 and its C-4′-methyl branched 19 (prevalent in a range of aminoglycosides);^51,66,67 C-4′-methyl branched 3′-amino-3′-deoxy-α-D-galactose 20 (combimicins);⁵⁶ 3′-amino-3′-deoxy-β-L-arabinose (22, gentamicins and related compounds);^37,65 2′-amino-2′-deoxy-α-D-xylose 24 and its corresponding 2′,3′-diamino-2′,3′-dideoxy analog 23 (seldomycins);^58,68 as well as 3′-amino-3′-deoxy-β-D-mannose (42) and the diasaccharide I [comprised of the uniquely C-6′ oxidized mannose 43 and 2′-amino-2′-deoxy-α-D-xylose (24) in boholmycin].⁵⁰ Other C-6′-appended sugars include L-streptose (28), 5′-hydroxy-L-streptose (30) and the corresponding reduced forms 29 and 31 have been found among streptomycins^69–71 wherein further C-2′ glycosylation of the streptose moiety with 2′-amino-2′-deoxy-α-L-glucose (32) has been observed.

In addition to the previously noted 1 and 2, D-xylose (26, xylostatin and butirosin A)⁶¹ and D-ribose (25) are also found among the core C5 glycosides with the latter being most prevalent (as exemplified by butirosin B,⁶⁰ lividomycins,⁷² paromomycins⁷² and ribostamycin⁶¹). In addition, glycosides of the C-3 position of the appended ribose with 5 (neomycin),⁷³ 27 (neomycin B,⁷³ paromomycin and lividomycin A⁷⁴) and 10 (neomycin C)⁷⁵ have been characterized. A C-4′/C-5′-diether bridged glycosidic connection to 3′-keto-sugar (33) and its enol form (332) has been observed in the context of spectinomycin and spenolimycin, respectively.

Additional glycosides that fall outside the scope of those described in the preceding paragraphs include the aromatic hexofuranose glycosides (37 and 38) of hygromycins,⁴³ various N-glycosides (4′,6′-dideoxy-α-D-glucose 39 in acarbose⁴⁴ and the unique 1′,2′-dideoxy-2′-aminohexose 40 in salbostatin⁷⁶). It is important to note that minor modifications of functional groups such as N- and O-methylation, acetylation, carbamoylation, and formylation of sugar monomers, not specificied herein, are also prevalent.

3. Angucyclines

With more than 200 known bacterial-derived members,¹⁸ angucyclines comprise one of the largest groups of polycyclic aromatic polyketides. Angucyclines are classified based upon their signature tetracyclic benz[a]anthracene system into five major classes: tetrangomycin-type (e.g., landomycins); aquayamycin-type (e.g., saquayamycins and urdamycins); benzanthrin-type (e.g., benzanthrins); gilvocarcin-type and jadomycin-type (Fig. 7).^77–79 Despite their remarkable chemical diversity and divergent biological activties, angucyclines have failed to advance in clinical development due to off-target toxicities and/or poor drug-like properties.⁷⁹

Fig. 7.

The five major types in angucyclines and glycosylation positions.

3.1. Tetrangomycin-type (landomycins)

The landomycins (Fig. 8) are one of the largest angucycline subclasses and all members contain a C-8 oligosaccharide of varying lengths.^77,79 Landomycins have been further subdivided based upon the polycyclic aromatic aglycon into three sub-types: landomycinone-based (sub-class I; landomycins A–L, S, V, X and Z); tetrangulol-based (sub-class II; landomycins M, O–R, T, U, W, and Y) and tetrangomycin-based (sub-class III; atramycins, brasiliquinones and TAN-1085). Despite their notable diversity, only three core sugars (β-D-olivose, 53; β-D-amicetose, 48 and α-L-rhodinose, 44) comprise the glycosyl components of landomycins.

Fig. 8.

Landomycin aglycons and associated sugars.

Natural variants within sub-class I contain disaccharide to hexasaccharide glycosyl substitutions and include landomycins A–E, K, L and S–U and members of this group display potent in vitro cytotoxicity against cancer cell lines.^80–84 A common theme is the prevalence of the α-L-rhodinose-(1→3)-β-D-olivose and β-D-olivose-(1→4)-β-D-olivose repeat units within the oligosaccharide chains followed by chain termination with β-D-olivose (53). A notable distinction of landomycins K and L is the alternative termination of the saccharide chain with α-L-rhodinose (44). Additional members of sub-class I (landomycins F–J) have been generated via strain engineering where H and J stand out as among the only monosaccharide and trisaccharide-substituted members, respectively.^85–87

For subclass II, landomycins M–Z have been isolated from a combination of native and engineered bacterial hosts.^88–90 Similar to sub-class I, a common glycosyl theme is the prevalence of β-D-olivose-3-1-α-L-rhodinose and β-D-olivose-4-1-β-D-olivose repeat units followed by chain termination with β-D-olivose (53). In contrast, in certain members of sub-class II (such as landomycins C, X–Z), β-D-olivose has been replaced with its corresponding 3-deoxy analog β-D-amicetose (48). A comparison of the in vitro cancer cell line cytotoxicities displayed by members within sub-class I and II revealed the aglycon C6 and C11 hydroxyls as important for activity and the saccharide length to also modulate potency. Specifically for the latter, the most active landomycins were those lacking sugars (e.g., landomycinone, tetrangomycin, or tetrangulol) and those appended by penta- or hexasaccharide chains (e.g., landomycins A and B). Based upon this comparison, it was proposed that the oligosaccharide-substituted landomycins and their sugar-free congeners may function via distinct mechanisms.^89,90

Members of sub-class III also display notable in vitro cytotoxitity against representative cancer cell lines and include the atramycins,⁹¹ brasiliquinones^92–94 and TAN-1085.^95–97 While the regiospecificity of glycosylation (aglycon C-8-glycosylation) of atramycins and brasiliquinones is reminiscent of that described for sub-classes I and II, the C-6 glycosylation of TAN-1085 stands out as an exception among tetrangomycin-type angucyclines. The brasiliquinones are unique among tetrangomycin-type angucyclines as they are the only members isolated from a non-Streptomyces strain (Nocardia brasiliensis) and the only members with glycosides reported to contain α-L-ristosamine-based sugars (45). These latter metabolites were active against Gram-positive bacteria (including Mycobacterium) and multiple drug-resistant P388/ADR tumor cell lines.

3.2. Aquayamycin-type (saquayamycin and urdamycin analogs)

C-9 C-glycosylation is a common signature among both the saquayamycin-type and urdamycin-type angucyclines and the main structural divergence stems from C-3 (saquayamycins) or C-12b (urdamycins) O-glycosylation. In addition to saquayamycins,^98–104 other structurally related saquayamycin-type bacterial metabolites include moromycins,¹⁰⁵ vieneomycins,^104,106,107 PI-080/083/085/087,^108,109 grincamycins,^110,111 Sch 47554/47555,^112,113 and amicenomycins (Fig. 9).¹¹⁴ In addition to potent antibacterial activity and in vitro cytotoxicity against cancer cell lines, other activities noted among this broad group of metabolites include antifungal activity, and inhibition of platelet aggregation, UDP-GlcNAc enolpyruvyl transferase (EPTase) and inducible nitric oxide synthase (iNOS). Many saquayamycin-type members have also been demonstrated to be efficacious in standard murine xenografts and, at least in some cases, to be reasonably well-tolerated (e.g., the acute IP LD₅₀ of the potent antibacterial amicenomycin A = 100.0 mg kg⁻¹).¹¹⁴

Fig. 9.

Saquayamycin aglycons and associated sugars.

Only ten distinct monosaccharides (β-D-olivose, 53; α-L-rhodinose, 44; β-L-rhodinose, 169; α-L-aculose, 54; α-L-cinerulose, 51; α-L-rednose, 50; α-L-oliose, 49; α-L-amicetose, 97; β-D-amicetose, 48; β-D-oliose, 47; β-L-amicetose, 309), have been observed among the saccharide constituents of saquayamycin-type bacterial natural products. Of these, only four (β-D-olivose, 53; β-D-amicetose, 48; α-L-rhodinose, 44 and β-L-rhodinose, 169) have been found as C-glycosides where β-D-olivose (53), α-L-rhodinose, (44) and the β-D-amicetose (48) enantiomer α-L-amicetose (97) were also noted among O-glycosides. With respect to general trends, β-D-olivose (53), α-L-rhodinose, (44) and, to a lesser extent, α-L-amicetose (97) and α-L-oliose (49), predominate as the ‘internal’ sugars of appended oligosaccharides within this series wherein uniquely oxidized sugars (α-L-aculose, 54; α-L-cinerulose, 51; α-L-rednose, 50) predominate among the terminal ‘capping’ sugars. Glycosidic bonds within the corresponding oligosaccharides extend from either C-3 or C-4 in β-D-olivose (53), C-4 of β-D-oliose (49), and the single C-4 hydroxyl group within rhodinose, (44/169) and amicetose (48/97/309). Among the ‘capping’ sugars, the terminal α-L-aculoside (54) was reported to convert to the corresponding α-L-cineruloside (51) during silica gel chromatography raising the question of whether this latter sugar is artifactual rather than a biosynthetic product. The aminated α-β-unsaturated ketosugar α-L-rednose (50) in saquayamycins H and I stands out as particularly unique and this sugar was found to contribute to enhanced in vitro cytotoxicity against certain cancer cell lines.¹⁰³

Urdamycins differ from saquayamycins based upon their distinct regiospecificity of glycosylation (C-9 and C-12b of the angucycline core; Fig. 10). In addition to the numerous natural^115–119 and engineered urdamycins,^120–125 members of this subclass include the kerriamycins,¹²⁶ the N05WA963 series¹²⁷ and the urdamycin BA-12100 set of metabolites,^77,128 all of which derive from Streptomyces. Members of this sub-type are best known for their anti-proliferative properties against various cancers where C-9 C-trisaccharyl-substituted analogs are generally more potent. Six distinct monosaccharides (β-D-olivose, 53; α-L-rhodinose, 44; α-L-aculose, 54; α-L-cinerulose, 51; β-D-rhodinose, 56; and β-D-kerriose, 55) have been observed among the saccharides of urdamycin-type bacterial natural products with latter (β-D-kerriose, 55) unique to this sub-type of aquayamycin-type angucyclines. Of these, only three (β-D-olivose, 53; α-L-rhodinose, 44; and β-D-rhodinose, 56) have been found as C-9 C-glycosides where the predominant substitution at C-9 is comprised of α-L-rhodinosyl-(1→3)-β-D-olivose disaccharide (III, Fig. 10). In most members, this disaccharide is further capped by one of four monosaccharides (β-D-olivose, 53; α-L-aculose, 54; α-L-cinerulose, 51; or β-D-kerriose, 55). Urdamycins M, R, S and 12b,4′-di-urdaderhodinosyl-urdamycin S serve as the exception in this regard wherein each bear distinct C-9 C-rhodinosyl-based disaccharides.

Fig. 10.

Urdamycin aglycons and associated sugars.

3.3. Other related aquayamycin-type angucyclines

In addition to the above described angucyclines, a number of additional miscellaneous angucyclines have been reported and are organized herein based upon the regiospecificity of glycosylation. Angucyline C-1 or C-2 glycosylation is relatively rare with the benzanthrins from Nocardia,^129,130 pseudonocardones from Pseudonocardia (associated with the fungus-growing ant Apterostigma dentigerum)¹³¹ and P371A1/A2 from Streptomyces^132,133 as the only representative bacterial metabolites within this subset. Members display variant activities including antibacterial, antiplasmodial, and/or cancer cell line cytotoxicity as well as inhibition of gastric acid secretion. Sugars employed in C-1-O-glycosides within this series include α-D-boivinose (57), β-L-rhodosamine (67), β-L-angolosamine (66), and a β-hexuronic acid moiety (72, absolute stereochemistry not determined). In contrast, glycosylation at C-2 is in the form of C-glycosylation with β-D-angolosamine (59b) as the sole sugar represented. P371A1/A2 also contain a C-9 C-trisaccharyl moiety (V, Fig. 11) initiating with the typical angucycline C-9 C-β-D-olivose (53) modified at C-3′ with a dideoxy branched β-D-mycarose (64) which is capped at C-4′ with a notably unique 4′-ureido tetradeoxysugar 4′-amino-4′-deoxy-β-D-amicetose (63).

Fig. 11.

Other related angucycline aglycons and associated sugars.

Angucycline glycosylation at C-4a, C-5 or C-12b is also relatively rare. Bacterial metabolites representing C-4a-O-glycosides include the JBIR series of metabolites from Streptomyces,¹³⁴ and the rhodonocardins from Nocardia¹³⁵ where the sugars employed include α-D-glucose (18), α-L-rhodosamine (70) and α-L-oliose (49) and, in the case of the JBIR series, the C-4a-appended sugar is further substituted at C-4′ by α-D-oliose (71). This latter set of metabolites also contains the typical angucycline C-9 C-glycosidic β-D-olivose (53). The rhodonocardins, and the structurally-related BE-7585A from Amycolatopsis,¹³⁶ also carry the rare sugar 2′-thio-α-D-glucose (61) at C-5, connected as a typical O-glycoside in rhodonocardins or via an unsual thioether bond at C-2′ of the thiosugar (which is part of a ‘head-to-head’ α-D-glucose-containing disaccharide, 18) in BE-7585A. In addition, rhodonocardin A and BE-7585A contain the C-12b-O-glycosidic α-D-rhodinose (58). Mayamycin from a marine Streptomyces¹³⁷ (unique among angucyclines as the only C-5 C-glycosyl member) contains N-desmethyl-β-D-angolosamine (59a) as the C-5 sugar. Grecocyclines from Streptomyces,¹³⁸ the other C-5-O-glycoside member within this subset, contains a C-5-appended O-4-epi-α-L-tolyposamine (60a) and a C-9 C-disaccharyl α-L-rhodinosyl-(1→4)-α-L-rhodinose moiety (disaccharide I, Fig. 11). The remaining C-12b-O-glycosides within this subset, sakyomicins from Nocardia,¹³⁹ contain α-D-rhodinose (58) at the C-12b-position reminiscent of rhodonocardin A and BE-7585A. Members within this cumulative subset have been reported as cytotoxic against representative cancer cell lines, to display antibacterial activity (including for antibiotic resistant strains) and as protein tyrosine phosphatase 1B and thymidylate synthase inhibitors.

Saccharothrixmicines A and B from a marine Saccharothrix,¹⁴⁰ represent the only angucycline C-8 and C-7 O-glycosides, respectively. Both metabolites contain the same sugar, α-L-6-deoxyaltrose (62), and were reported to display antifungal activities in vitro.

As with angucyclines discussed within preceding sections, C-9 C-glycosylation is a predominate modification among members described herein and include naturally-occurring simocyclinones,^141–144 capoamycin,¹⁴⁵ dioxamycin,¹⁴⁶ MK844-mF10,¹⁴⁷ fradimycins,¹⁴⁸ frigocyclinone,¹⁴⁹ balmoralmycin,¹⁵⁰ and marmycins¹⁵¹ as well as the engineered landomycin–urdamycin hybrid metabolite 9-C-D-olivosyl-tetrangulol.¹⁵² With the exception of marmycin (produced by a marine actinomycete), all other metabolites within this grouping are Streptomyces metabolites. In addition to the the typical β-D-olivose (53), three additional sugars are represented as C-9-C-glycosides among members within this subgroup including β-D-amicetose (48), 3′/4′-epi-α-L-vancosamine (65) and the rare aminodeoxysugar α-L-ossamine (60b). In contrast to many previously discussed C-9-modified angucyclines (which contain C-9 C-oligosaccharyl modifications), the C-9 C-glycosylation patterns within this subgroup are limited to monosaccharides. Typically within this subgroup, β-D-olivose (53) and β-D-amicetose (48) is further acylated at C-3′/4′ with, in many cases, uniquely functionalized lipids which, in some cases (e.g., simocyclinones), is further conjugated to an aminocoumarin. A notable standout within this series is the unique C-8/C-9 ring fusion that occurs from an additional bond between the angucycline C-8 and C-3′-amine 3′/4′-epi-α-L-vancosamine (65) within the marmycins. In addition to reported cancer cell line cytotoxicity and antibacterial activities, other reported activities of members of this subgroup include differentiation-inducing activity on myeloid leukemia cells (M1) and inhibition of protein kinase C-α (PKC-α).

3.4. Jadomycin- and gilvocarcin-types (jadomycins, ravidomycins, chrysomycins and gilvocarcins)

The gilvocarcin-type aryl-C-glycosides⁷⁹ contain a number of naturally-occurring (gilvocarcins,^153–162 polycarcins, BE-12406A/B,^163–165 ravidomycins^166–171 chrysomycins¹⁷² and virenomycins^173,174) and engineered (gilvocarcins and polycarcins)^{172,175–177} Streptomyces metabolites wherein the signature C-glycosylation occurs at C-4 (Fig. 12). The monosaccharides found among naturally-occurring C-4-C-glycosides of this class include β-D-fucofuranose (75), α-L-rhamnose (46), β-D-ravidosamine (78), β-D-virenose (77), the 4′-keto analog of β-D-virenose (79) and a C-3′-branched analog of β-D-fucofuranose (80), wherein the presence of β-D-fucofuranose stands out as relatively rare among bacterial secondary metabolites. This set has been expanded to also include 4′-hydroxy-β-D-fucofuranose (76) and β-D-olivose (53) via metabolic engineering.^172,175,176 Some members of this subclass also contain a C-12-O-α-L-rhamnose (46) substituent. These metabolites are generally known as anticancer antibiotics where subtle differences within the appended sugar were found to improve the perceived therapeutic index (potency versus general toxicity).

Fig. 12.

Gilvocarcin and jadomycin aglycons and associated sugars.

The jadomycins from Streptomyces are angucyclines that contain a nitrogen at position 6 within ring B (Fig. 12).^79,178–181 Naturally-occurring jadomycins are C-8-O-glycosides of α-L-digitoxose (68) and an engineered variant that led to a replacement of this sugar with 6′-deoxy-α-L-altrose (62) has been reported.¹⁸⁰ The naturally-occurring B ring Streptomyces metabolite phenanthroviridin and is alternatively conjugated at C-1 to α-D-ristosamine (69).^182,183 Jadomycins display a range of bioactivities, including anticancer cytotoxicity, antimicrobial, anti-viral, and inhibition aurora-B kinase.

4. Anthracyclines, tetracyclines, quinones and tricyclines

4.1. Anthracyclines

According to 1963 Hans Brockman definition, anthracyclines are defined as red to orange natural/synthetic dyes with a skeleton of 7,8,9,10-tetrahydro-tetracene-5,12-quinone decorated with mono-, di-, tri-, tetra- and/or pentasaccharide side chains (Fig. 13, subclass I).¹⁸⁴ As exemplified by the early protypical members daunomycin (discovered in 1964)¹⁸⁵ and doxorubicin (adriamycin, discovered in 1969)^186,187 used to successfully treat cancer for nearly four decades, anthracyclines display a range of bioactivities of pharmaceutical utility.¹⁸⁸ To date,¹⁸ 407 bacterial-derived anthracycline glycosides have been reported from a range of bacteria including, but not limited to Streptomyces, Micromonospora, Actinomadura, Nomadura Actinosporangium, Chaetomium, Actinoplanes, Ampullariella and Nocardia. For the scope of this discussion, members are classified into five sub-classes I (365 compounds), II (3 compounds), III (6 compounds), IV (9 compounds), and V (24 compounds), based upon aglycon structural divergence (Fig. 13). Cumulatively, anthracycline glycosides integrate 51 structurally distinct monosaccharide units (Fig. 14) incorporated within the anthracycline-appended mono-, di-(15 variations, D1–D15; Fig. 15), tri- (35 variations, trisccharides I–XXXV; Fig. 16), tetra- (eight variations, T1–T8; Fig. 17), and pentasaccharides (two variations, P1 and P2; Fig. 18). While the predominant regiospecificity of anthracycline glycosylation occurs at C-7 and/or C-10, C-1-, C-3-, C-4-, and/or C-9-glycosylation (as well as C-4′-glycosylation in subclass V) has been observed (Fig. 13). Also, both O- and C-glycosides have been reported, the latter of which is less prevalent and mainly occurs at C-3 in sub-class IV (reminiscent of angucyclines and/or pluramycins).

Fig. 13.

The five anthracycline sub-classes I–V and glycosylation positions.

Fig. 14.

Sugars associated with anthracyclines.

Fig. 15.

Disaccharide variations D1–D15 observed in anthracyclines.

Fig. 16.

Trisaccharide variations (I–XXXV) observed in anthracyclines.

Fig. 17.

Tetrasaccharide variations (T1–T8) observed in anthracyclines.

Fig. 18.

Pentasaccharide variations P1–P2 observed in anthracyclines.

Sub-class I contains 365 anthracycline O-glycosides (and a few C-glycosides) bearing variant carbohydrate substitution ranging from mono- to pentasaccharides. C7- and/or C10-O-glycosylation predominates (358 compounds) within sub-class I anthracyclines including D-788,^189–193 oxaunomycins,¹⁹³ doxorubicins,^194,195 spartanamicins,¹⁹⁶ daunorubicins,^194,195 daunomycins,^185,197,198 aclacinomycins (aklavins),^199–205 rhodomycins,^{184,206–210} betaclamycins (and CG-21-C, CG1-C, CG21-B analogs),^211–213 auramycins,^214–219 MA-144-U and G analogs,^220–222 steffimycins,^223–226 baumycins,^227,228 pyrrocyclines,²²² rubeomycins,^229,230 pyrromycins,²³¹ oblemycins (cytorhodins),^232,233 sulfurmycins,^214–219 cinerubins,^{188,234–236} mutactimycins,^237–239 alldimycins,^233,240 nocardicyclins,²⁴¹ violamycins,²⁴² ditrisarubicins,²⁴³ aranciamycins,^244–247 A447A~D1,²⁴⁸ ciclamycins,²⁴⁹ isoquinocycline A,^188,250 kosinostatin (quinocycline B),^251,252 isoquinocycline B,^252,253 micromonomycin²⁵⁴ and other representative anthracyclines.^{100,188,201,202,211,229,255–263} Less common sub-class I O-glycoside regioselectivity was observed at C-1 (mutactimycin PR^237,238), C-4 [histomodulin,²⁶⁴ komodoquinone A^265,266 and 4-O-(β-D-glucopyranosyl)-ε-rhodomycinone²⁶⁷], and C-9 (cytorhodins X and Y).^188,268 In addition to the mono- to tetrasccharides observed, two pentasaccharide-containing members were reported [roseorubicin A (pentasaccharide P1 at C-10; Fig. 18)²²⁹ and β-rhodomycin-Roa2.deofuc.rod3 (pentasaccharide P2 at C-7, and 70b at C-10; Fig. 14 and 18)²⁶⁹] where roseorubicin SAR revealed improved potency to correlate with increase oligosaccharide length.²²⁹ Caeseorhodomycin²⁵⁶ is distinguished among this series as a C-1-C-glycoside (70c; α-L-rhodosamine).

A reduced ring-C is the distinguishing feature of sub-class II. Only 3 metabolites (antibiotics W, Y and Z) fall within this sub-class, all of which are C-7-O-glycosides bearing α-L-daunosamine (70a; Fig. 14).²⁷⁰ Sub-class III is unique by virtue of the C-10-keto ring D and contains only 6 metabolites, all of which are C-7-O-glycosides bearing α-L-sugars [2′-OMe-α-L-rhamnose (46a) or α-L-vancosamine (118)]. Sub-class III members include steffimycin A (46a at C-7),²²³ 5-iminoaranciamycin (46a at C-7),²⁴⁷ aranciamycin (46a at C-7),²⁴⁴ aranciamycin anhydride (46a, R³ = fragment E; at C-7),²⁴⁵ and nocardicyclins A (118 at C-7) and B (118 at C-7).²⁴¹ The 7 glycosidic sub-class IV members contain a fully aromatized tetracycline backbone with, typically, C-3-C-glycosylation, and include galtamycin(one)s,^100,271,272 balmoralmycin (β-L-olivose, 100; at C-3),²⁷² tetracenoquinocin (46a at C-7),²⁴⁷ quanolirones I (trisaccharide XXXII, at C-3; Fig. 16) and II (disaccharide D15, at C-3; Fig. 15),²⁷³ and the two D-ring-reduced metabolites N05WA963C¹²⁷ and grincamycin E¹¹¹ (both bearing the C-3-C-trisaccharide XXXIII). Finally, sub-class V members are differentiated by the presence of a fused ring A-sugar (connected 1-/1′-O- and 2/5′-C-) where the carbohydrate unit is comprised of α-L-mycaminose (74a) or 4′-epi-α-L-mycaminose (312). The 24 metabolites that fall within this sub-class include nogalamycin,^274–276 nogamycin,²²⁹ nogalarol,^276,277 7-O-methylnogalarol,²⁷⁶ 7-con-O-methylnogarol,²²⁹ arugorol,^277,278 arugomycin,^277–279 decilorubi-cin,^280–283 avidinorubicin,²⁸⁴ respinomycins,^285–287 cororubicin,²⁸⁸ and viriplanin and viriplanol analogs.^289,290 Additional C-4′ and/or C-7-O-glycosylation is observed with sugars 74a and 312 and/or various lengths of mono- to tetra-saccharide side chains.

From a global perspective (Fig. 14), anthracyclines present several unique carbohydrates. For example, C-3′- and C-4′-branching is prevalent as exemplified by 101–103, 105, 107–110, 112, 118, 120, 122, and 123. Additionally, nitroso (101), hydroxylimino (113) and nitro (102, 105, 107, 120, 121) sugars are quite common. Unsaturated (50, 54, 119) and ketosugars (50, 51, 54, 98, 117, 119) are also observed and the C-3′-N/C-4′-O-macrocylic substitution of 70d is uncommon. The C-3′-disubstitution pattern of 120 3′-methoxy-3′-nitro-2′,6′-dideoxy-α-L-talose in trisaccharide XXXV of 301-C²⁹¹ is of particular note as unprecedented carbohydrate functionality. In general, glycosylation is considered to be important to bioactivity in most cases studied. For example, Matsuzawa and Oki reported SAR on ~100 anthracyclines where they found the presence of an aminosugar (or amine-substituted aglycon) to be important and increased potency to also correlate with increased saccharide chain length.²²⁹

4.2. Hibarimicins

Hibarimicins are distinguished by their highly oxidized naphthyl-naphthoquinone aglycon which derives biosynthetically from dimerization of undecaketide-based precursors (Fig. 19).²⁹² To date, 11 herbimycin O-glycosides have been reported from Microbiospora where the regiospecificity of glycosylation is restricted to C-7/7′ and C-9/9′ (Fig. 19).^18,293–299 Glycosylation at these positions is composed of mono- and/or disaccharide side chains (Fig. 19) where five distinct sugar residues were observed including α-L-digitoxose (129), β-D-amicetose (48), 4′-C-acetyl-2′,3′,6′-trideoxy-α-L-gulose (123), 4′-C-acetyl-2′,3′,6′-trideoxy-β-L-gulose (130) and α-L-4-C-acetyl-2,6-dideoxy-xylo-hexopyranosyl (103). In all cases, the C-7/C-7′-sugar was α-L-digitoxose (129) and C-9-/9′-sugar was β-D-amicetose (48), the latter of which in certain analogs was further 4′-O-glycosylated (48, 103, 123 or 130 to present disaccharides I–IV, Fig. 19). HMP-P4 and HMB-Y6 are metabolites produced by Microbiospora engineered mutants.²⁹⁵ Hibarimcins are selective src tyrosine kinase inhibitors and do not inhibit protein kinase C.²⁹⁶ They display potent in vitro cytotoxicity against cancer cell lines and moderate Gram-positive antibacterial activity.²⁹⁶

Fig. 19.

Hibarimicin aglycons and associated sugars.

4.3. Tetracycline-type antibiotics

Tetracyclines are tetracyclic polyketide-based broad spectrum antibiotics with a rich history that includes the discovery and implementation of a number of clinically important tetracycline-based drugs, extensive litigation beginning in the 1950’s over tetracycline intellectual property and price fixing among major pharmaceutical companies of the time (Pfizer, American Cyanamid, Bristol-Myers) and a more recent discovery suggesting tetracycline ‘use’ dates back possibly to before 400–500 A.D. There are 26 glycosylated tetracyclines reported to date including the cervimycins,^300–302 HKI10311129³⁰³ dutomycin,^304,305 polyketomycin,^306–308 elloramycins,^176,309,310 tetracenomycins,^176,311,312 TAN-1518 A, B and X (also known as SF-2575),^313–315 and dactylocyclines,^316–319 the latter of which being the only member of non-Streptomyces origin (Dactylosporangium). Regiospecificity of tetracycline glycosylation is limited to C-4, C-8, C-11 and C-12a where all members with the exception of one subgroup (the C-8-C-glycosidic TAN-1518 analogs) are O-glycosides.

Cervimycins A–D are glycosylated at C-4 and C-12a with tetra-[consisting of one β-D-amicetose (48) and three α-L-rhodinoses (44), tetrasaccharide III] and disaccharide (β-D-amicetosyl-(1→4)-β-D-amicetose, disaccharide I) moieties, respectively (Fig. 20).^300–302 The terminal rhodinose (44) of tetrasaccharide III is further modified with a 4′-O-dimethylmalonyl/or monomethylmalonyl ester. The structurally similar HKI10311129³⁰³ displays an identical glycosylation pattern but lacks the terminal rhodinose ester modification while the related dutomycin and polyketomycin are C-4-O-glycosides bearing an a α-L-4′-epi-mycarosyl-(1′→4′)-β-D-amicetose (disaccharide II) but lacking C-12a glycosylation.^304–308 In these latter metabolites, the terminating mycarose is further modified via 4″-O-esterification and both have been noted as anticancer cytotoxins, antibiotics while dutomycin was also reported as a DNA methyltransferase inhibitor. Elloramycins are cytotoxic C-8-O-glycosides bearing per-methyl-α-L-rhamnose (46) where cytotoxicity, in this case, is attenuated via the presence of the sugar.^{176,309,310,320} The corresponding tetracenomycin C-8-O-glycosides from strain engineering include substitution with α-L-digitoxose (68), β-D-glucose (104), 4′-keto-α-L-olivose (126), α-L-olivose (184a) and α-L-oleandrose (184b).^176,311,312 Dactylocyclines A and B from Dactylosporangium sp. (ATCC 53693)^316–319 are C-11-O-glycosides substituted with α-L-evernitrose (127) and its hydroxyl-amino congener 128 (monosaccharides more commonly associated with the orthosomycin everninomicin, Section 14.1).³²¹ Intriguingly, removal of the dactylocycline sugar improves Gram-negative antibacterial activity.³¹⁸ Finally, the naphthacenecarboxamides TAN-1518 A, B and X (also known as SF-2575) are C-8-C-glycosides of β-D-olivose (C-53).^313–315 TAN-1518 A and B have been noted as topoismerase I inhibitors while TAN-1518 X displayed both Gram-positive antibacterial activity and in vitro and in vivo anticancer activity.³¹⁴ The structural elucidation of the TAN-X C-glycosyltransferase SsfS6 was recently reported as one among only a few C-glycosyltransferases to be structurally characterized to date.³²²

Fig. 20.

Tetracycline aglycons and associated sugars.

4.4. Aureolic acids and related tetracyclines

Mithramycin³¹⁴ is the prototypical member of the aureolic acid family of antitumor antibiotics that also includes chromomycins,³²³ olivomycins,^324–326 durhamycins,³²⁷ SR1768A,³²⁸ UCH9,^329,330 chromocyclomycin,^325,331 02-3D and 02-3G,³³² and variamycins^333,334 from Streptomyces and Actinoplanes as well as several other analogs (e.g., ketopremithramycins and ketomithramycins)³³⁵ generated by pathway engineering.^{323–326,331,333,336–340} The defining structural feature of aureolic acids is their tricyclic polyketide architecture and, to date, 45 glycosylated analogs have been reported from bacteria. The predominant regiospecificity of glycosylation among naturally-occurring aureolic acids is C-2-and/or C-7-O-glycosylation with two family members [aureolic acid C-glycoside I (C-47 at C-6) and aureolic acid C-glycoside II (C-53 at C-6); Fig. 21] displaying atypical C-6-C-glycosylation. For the scope of this discussion, members have been divided into subclasses I–III based upon aglycon distinctions where the tetracyclic subclass II are primarily considered as biosynthetic precursors and subclass III (pillaromycin A)³⁴¹ also displays similarity to II.

Fig. 21.

Aureolic acid analogs and associated sugars.

Family members are glycosylated with mono-, di-, tri-, or tetrasaccharide side chains (Fig. 21). Sugars found as monosaccharide substitutions include the uncommon C-6-C-glycosidic β-D-oliose (C-47) and β-D-olivose (C-53)-substituted aureolic acid C-glycosides I and II³⁴² and the unusual C4′-branched sugar 4′-C-hydroxyethanone-2′,3′,6′-trideoxy-α-L-gulose (132, pillaromycin A).³⁴¹ Six different disaccharide (D1–D6) and trisccharide (I–VI) variations are represented among family members, which are comprised of 10 different distinct monosaccharide units where β-D-oliose (47) and β-D-olivose (53), are the most predominant. Other saccharides employed include β-D-amicetose (48), β-D-mycarose (64), α-D-oliose (71, R = H; α-D-chromose, R = CH₃), β-D-digitoxose (131), 4′-C-hydroxyethanone-2′,3′,6′-trideoxy-α-L-gulose (132), 4′-keto-β-D-mycarose (133), 4′-hydroxy-β-D-olivose (134), and 3′-epi-αL-mycarose (195, α-L-chromose B). Tetrasaccharide substitution is uncommon with UCH-9 and durhamycin A as the only C2-tetrasaccharide-bearing examples [β-D-olivosyl-(1→3)-β-D-olivosyl-(1→3)-β-D-oliosyl-(1→3)-β-D-olivose, tetrasaccharide T1].

Aureolic acids display potent Gram-positive antibacterial and anticancer activities. Mithramycin (plicamycin) was originally approved for the treatment of cancer however, off-target toxicities, potentially deriving from the ability of mithramycin to target the ubiquitous transcription factor Sp1, limited clinical use. Recent studies revealed mithramycin to selectively target the EWS/FLI1 transcription factor fusion found within Ewing’s sarcoma, renewing interest in mithramycin and development of less toxic, more selective analogs.^343,344 Other activities noted among aureolic acids include inhibition of mdr1 gene expression and viral Tat transactivation inhibition. Intriguingly, pillaromycin A, a potent anticancer cytotoxin from this family, has been noted to display lower overall toxicity.^341,345

4.5. Pyranonaphthoquinones

Pyranonaphthoquinones contain a naphtho[2,3-c]pyran-5,10-dione core where some members present an additional γ-lactone ring, or corresponding open ring carboxylic acid, fused to the dihydropyran ring (Fig. 22). Pyranonaphthoquinone antibiotics isolated from various bacterial strains (including Streptomyces, Actinomycete, and alkaphilic Nocardiopsis) exhibited antibacterial, antifungal, antiviral, as well as cytotoxic activities.³⁴⁶ To date, 37 bacterial pyranonaphthoquinone glycosides have been reported, all of which contain single C-glycosidic sugars consisting of α-L-rhodinose (44), β-D-ribofuranose (25), β-D-angolosamine (59), α-D-forosamine (81), 3′-amino-N′,N′-dimethyl-N-oxido-2′,3′,6′-trideoxy-β-D-glucose (82), unusual cyclized sugars I (83) and II (84) or sugar mimetics I (85) and II (86) (Fig. 22). For the scope of this discussion, this family is further divided into subclasses I–III as further described below where the C6-C1′/C7-C5′ glycoside fusion found in Sch38519 from Thermomonospora is a notable outlier. This latter metabolite was reported to inhibit thrombin-induced aggregation of human platelets.^347,348

Fig. 22.

Pyranonaphthoquinone related aglycons and associated sugars.

Sub-class I is distinguished by a single C-8-C-glycosidic substitution and includes lactoquinomycins,^349–352 menoxymycins³⁵³ exfoliamycins,^354,355 mederrhodins,³⁵⁶ alnumycin (also known as K1115 B1 and K1115 A),^357,358 K 1115 B2,^355,356 and EI-2346.^359,360 Of the four sugars found among sub-class I members (59 or 25 or 82 or 83), N-oxide 82 (menoxymycin A) is unique.³⁵³ The C-ribosyl 25 (exfoliamycins) was also observed to rearrange to unusual dioxane 83 (alnumycin,³⁶¹ K 1115 B2,^355,356 and EI-2346^331,332).³⁶² Sub-class II members are distinguished by the 1,7-dioxaspiro-[5.5]undecane ring system (85 or 86), the C-1-spiroketal fused moiety of which, while likely polyketide-based, serves as a carbohydrate mimic.^363,364 Of the 12 griseusins mainly produced by Nocardiopsis,^{337–343,365–371} the C-3′-O-α-D-forosaminyl-(+)-griseusin A is the only O-glycosylated member. Finally, sub-class III contains a C-7-C-glycosidic linkage where the corresponding sugar 84 is also fused to the pyranonapthoquinone core via a C-8-C-4′ bond to provide a bicyclic structure (Fig. 22). Eleven sub-class III members have been reported including granatomycins, granaticins, dihydrogranatirhodins, 4-deoxy-4-S-(N-acetylcysteinyl)granaticinic acid, MM 44785 and MM 447876.^{256,346,372,373} Sugar 84 in three granaticin analogs [dihydrogranaticin B (MM 44325), MM 44785 and granaticin B (MM 44326)] is further C-3′-O-modified with α-L-rhodinose (44).

4.6. Benzoquinones related antibiotics

Napthoquinones represent a large class of bacterial natural products, the structural signature of which is a bicyclic aromatic-1,4-benzoquinone fused core structure 1,4-napthoquinone. Also included within this section are metabolites related to this core structure where the central benzoquinone has been reduced and/or further modified. The predominant regiospecificity of glycosylation across this series is glycosylation of the fused aromatic moiety either at the α- or β-carbon in relation to the ring junction or glycosylation of the quinone core (the precise numbering of which, in all cases, varies depending upon the specific aglycon, Fig. 23.

Fig. 23.

Benzoquinone-related aglycons and associated sugars.

A series of α-aromatic O-glucuronides including julichrome Q₆ glucuronide³⁷⁴ 671-C, genoketide A2, chrysophanol glucuronide and prechrysophanol glucuronide have been reported as metabolites of Streptomyces.³⁷⁵ All share a common C-4-β-D-glucuronic acid (72) substitution. Julichrome Q₆ glucuronide was reported as the first monomeric member of the julimycin-B analogs, and displayed moderate unselective cytotoxicity against human tumor cell lines.³⁷⁴ Genoketide A2 and prechrysophanol glucuronide were reported to inhibit the lymphoma cell proliferation in vitro.³⁷⁵

The β-aromatic glycosides include halawanones,³⁷⁶ fridamycins/himalomycins,^77,377 adxanthromycins^378–382 and grecoketides³⁸³ from various Streptomyces. Halawanones C and D, two tricyclic quinone-containing metabolites, are C-7-O-glycosides (β-D-olivose, 53).³⁷⁶ Fridamycins A (also known as vineomycinone B2), B, and D, and the structurally related himalomycins A and B,³⁷⁷ are angucycline-related antitumor antibiotics that likely arise from a cleavage of the corresponding C-12b/C-l bond to afford the substituted tricyclic angucyclinone lacking ring-A.^77,377 Similar to the saquayamycins (Section 3.2), they are C-3- or C-9-O-glycosides bearing mono- or disaccharides (IV–VI) comprised of β-D-olivose (53), 2′,6′-dideoxy-β-D-altrose (303), α-L-rhodinose (44), α-L-cinerulose (51) and α-L-amicetose (97).^77,377,384 Himalomycins A and B along with fridamycin D exhibited strong antibacterial activity against Gram-positive and Gram-negative bacteria.³⁷⁷ The two naphthoquinones grecoketides A and B are both disaccharyl-containing C-6-C-glycosides of the grecoketidone aglycon that differ in disaccharide composition.³⁸³ Specifically, the C-6-C-glycosyl moiety of grecoketide A is α-L-rhodinose (44) while that in grecoketide B is β-D-rhodinose (56). In both, the rhodinose is further C-4′-O-glycosylated with α-L-rhodinose (44).³⁸³ Finally, adxanthromycins A and B are unique dimeric peroxo-anthrone C-3-O-glycosides bearing α-D-galactose (286) and α-D-galactosyl-(1′→3′)-α-D-galactose (disaccharide I), respectively.^379–381 These compounds are reported as inhibitors of ICAM-1/LFA-1-mediated cell adhesion.^378–381

Glycosides of the core quinone include halawanones A and B,³⁷⁶ substituted naphthalene-1-ones,³⁸² lactonamycins^385,386 and lomaiviticins³⁸⁷ from Streptomyces and Micromonospora. Halawanones A and B are structurally related to other isochromane quinone antibiotics from Streptomyces (including exfoliamycins, granaticins and griseusins; see the lactoquinomycins, Section 4.5) and, as C-8-C-glycosides (2′-keto-β-D-oliose, 325) of the quinone core are unique among this class.³⁷⁶ A product of a mutant Streptomyces, 4β,8-dihydroxy-3α-O-(α-glucopyranosyl)hydroxymetyl-4α-methyl-1,2,3,4-tetrahydron-naphthalene-1-one is a simple side chain C-9-O-α-D-glucoside(44).³⁸² Lactonamycin, an antibiotic active against Gram-positive bacteria including MRSA and VRE, consists of a hexacyclic C-5a-O-glycoside bearing α-L-rhodinose (44).^385,386 In the related lactonamycin Z, rhodinose is replaced by α-L-digitoxose (68a).³⁸⁸ This latter compound displayed potent antiproliferative activity against gastric adenocarcinoma cell lines. Finally, lomaiviticins A and B are two unique diazobenzofluorene O-glycosides produced by Micromonospora.³⁸⁷ Both are C4/4′-O-glycosides of β-D-pyrrolosamine (213), a sugar previously identified in pyrrolosporin A.³⁸⁹ Lomaiviticin A is further C-3/3′-O-glycosylated with an α-L-oleandrose (184), presumably preventing the tetrahydrofuran ring fusion observed in lomaiviticin B. Diazo-containing natural products like lomaiviticins and the structurally-related kinamycins are rare.^390,391 Lomaiviticins function as DNA-damaging agents where lomaiviticin A cleaves double stranded DNA under reducing conditions.^387,392 Both were also reported as potent antibiotics against Gram-positive bacteria Staphylococcus aureus and Enterococcus faecium.³⁸⁷

Two metabolites display unique glycosylation patterns that fall outside the scope of those described in the preceding paragraphs namely, lemonomycin³⁹³ and heliquinomycin^394–396 from Streptomyces. Lemonomycin is C-18-O-glycoside (4′-amino-4′-deoxy-3′-C-methyl-6′-deoxy-α-L-fucose; 292) of a uniquely fused pyrrolidine-tetrahydroisoquinoline aglycon while heliquinomycin is a C-3-O-glycosyl rubromycin member bearing α-L-cymarose (68b).^394–396 A glycoside of the rubromycins/griseorhodins,³⁹⁵ this metabolite was found to selectively inhibit cellular DNA replication without affecting level of chromatin-bound MCM4 or activation of the DNA replication stress checkpoint system.^394–397

5. Benanomicins and pradimicins

Benanomicin and pradimicin analogs consist of a benzo[a]-naphthacene quinone conjugated to various modified aminoacids at C-15. Glycosylation within this subclass consists of mono or disaccharide side chains at C-5 and, in some cases, also C-11 of the benzoquinone nucleus (rings B and E; Fig. 24).^398–404 These compounds have excellent in vitro and in vivo activities against a wide range of fungal strains including Candida, Cryptococcus, Aspergillus and Trichophyton and also are effective inhibitors of HIV syncytium formation.^405,406 The pradamicins uniquely function as small molecule ‘lectins’ wherein antifungal activity derives from specific binding to terminal D-mannosides of the fungal cell wall while specific interactions with the N-glycosylation patterns of HIV-1 gp120 contribute to antiviral activity via inhibition of viral entry. The monomers β-D-glucose (104), β-D-xylose (136), 3′-amino-β-D-fucose (208), and β-D-fucose (270), or modifications thereof, encompass the sugars represented and SAR studies to date point the importance of both the C-15 amino acid substitution and C-5 glycosylation for antifungal and anti-HIV activities.³⁵⁷ C-5-glycosidic examples include pradimicins A–E,^{400,406–409} FA-1,⁴¹⁰ FA-2,⁴¹⁰ FB,⁴¹¹ FH, L, FL,⁴¹² S,^413–415 FS⁴¹¹ and other analogs.^{17,18,399,416–419} C-11 glycosylation occurs less frequently and is restricted to β-D-xylose (136) as illustrated by pradimicin T2,^420,421 11-O-L-xylosyl-pradimicin H⁴²² and 11-O-L-xylosyl-pradimicin FH,⁴²² all of which contain C-5 mono- or disaccharide substitutions.

Fig. 24.

Benanomicin and pradimicin aglycons and associated sugars.

6. Chartarin-analogs

Chartarin analogs are divided into two main classes distinguished by the B ring oxidation state and C-5 substitution of the fused pentacyclic ring aglycon (sub-classes I and II; Fig. 25). Sixteen naturally-occurring chartarin glycosides have been reported to date as bacterial metabolites (mainly from Streptomyces, Fig. 25) including the chartreusins (lambdamycins),^423–433 chrymutasins,^434–436 D329C,⁴³⁷ elsamicins,^438–440 and hayumicins.⁴⁴¹ These compounds display notable antimicrobial and antitumor activities where glyco-sylation plays a key role in improving potency, formulation and in vivo properties wherein aminosugar-containing variants are among the most advantagous.^{423,438–440,442–445} In all cases reported to date, O-glycosylation is restricted to C-10 mono-, di- or trisaccharide substitution. Four different appended modified sugars are found among existing charterin analogs [β-D-fucose (270), β-D-elsarose (266), α-D-fucosamine (also known as α-D-elsaminose, 88), and α-D-fucose (319)]. Of these four sugars, D-fucose (270) is the most predominant with a range of C-2′-, C-3′-, and/or C-4′-O-methyl/acetyl-substituted, as well as the C-3′-C-methyl-branched (β-D-elsarose, 266) or the corresponding α-D-fucosamine (also known as α-D-elsaminose, 88) analogs observed.

Fig. 25.

Chartarin aglycons and associated sugars.

7. Coumarins

The aminocoumarin group of antibiotics are characterized by their 3-amino-4,7-dihydroxycoumarin moiety, the microbial source for which is currently restricted to Streptomyces. This family of antibiotics [which includes novobiocin, isonovobiocin,⁴⁴⁶ 2562B, clorobiocin (2562A; RP 18 631),^447,448 coumermycin A1,^449,450 coumermycin A2,⁴⁵¹ vanillobiocin,⁴⁵² novenamine,^453,454 11-hydroxynovobiocin,⁴⁵⁵ isovanillobiocin,⁴⁵⁵ declovanillobiocin,⁴⁵⁵ biscarbamoylcoumermycin D,⁴⁵⁶ 3-chlorocoumarobiocin,⁴⁵⁷ and 8′-dechloro-3-chlorocoumarobiocin⁴⁵⁷] function as potent bacterial DNA gyrase inhibitors and were more recently discovered to inhibit Hsp90.^458–462 To date, 84 naturally-occurring aminocoumarins have been reported from bacteria, 65 of which are glycosylated representatives.^463–467 Importantly, the 3′-carbamoylation of the appended sugar in these metabolites is critical for DNA gyrase inhibitory activity but detrimental to Hsp90 inhibitory function.^460,468,469 The scope of glycosylated aminocoumarin derivatives has been notably expanded by Heide et al. via metabolic engineering, mutasynthesis and chemoenzymatic synthetic methods.^{456,467,470–473} It is also important to note that there are currently no examples of native bacterial coumarin/isocoumarin glycoside production.

The glycosylated aminocoumarins are classified into subclasses I–III based upon subtle substitution/conjugation distinctions within the core aminocoumarin (Fig. 26). From the perspective of glycosylation, subclasses I and II are markedly similar in both the regiospecificity of glycosylation (C-6 of the aminocoumarin core) and the predominate sugar employed (4′-O-methyl-5′-C-methyl-L-rhamnose, more commonly referred to as α-L-noviose, 193). In both subclasses I and II, the core sugar moiety is further modified via differential acylation of the 2′-OH and/or 3′-OH with a carbamoyl, 5-methyl-pyrrole-2-carboxyl or pyrrole-2-carboxyl moiety (Fig. 26, 193). In addition, 4′-OH methylation was observed among some members. A notable departure from the structural/biosynthetic conservation among subclass I members is TPU-0031-B (5′-demethyl-novobiocin) which reportedly bears the distinct sugar α-D-5′-demethyl-noviose (267) and, given the glycosidic conservation among the metabolites in this family, may raise questions regarding the structural assignment in this case.⁴⁷⁴

Fig. 26.

Coumarin aglycons and associated sugars.

Subclass III members are structurally distinguished by their conjugated system comprised of a novobiocin-type amino-coumarin connected to an ansamycin-like moiety via a 3,4-dihydroxydipioolinic acid central bridge, the latter of which serves as the site of glycosylation (specifically, C-4 of the 3,4-dihydroxydipioolinate). This subclass contains two antibiotics (rubradirin^475,476 and protorubradirin⁴⁷⁷ from Streptomyces) distinguished by their corresponding uniquely functionalized appended sugars [the C-3′-nitro sugar β-D-rubranitrose (310) and its C-3′-nitroso-analog (284), Fig. 26]. As in the case of the nitro sugar-containing orthosomycins (where aminosugars with differing levels of amine oxidation have been observed, Section 14), it is expected that NDP-rubranitrose biosynthesis proceeds via the C-3′-nitroso-analog. Interestingly, these two sugars have also been identified in anthracycline antibiotics (viriplanins A and D, Section 4).^289,290,477

8. Enediynes

Enediynes are a group of antibiotic anticancer compounds characterized by the presence of bicyclo[7.3.0]dodecadienediyne or bicyclo[7.3.1]tridecenediyne and this structural distinction serves as a basis for classification of family members within 9- or 10-membered subclasses, respectively.⁴⁷⁸ As an additional signature, 9-membered enediynes are commonly associated with a stabilizing apoprotein and 10-membered enediynes have been more recently been associdated with a novel self-sacrifice resistance mechansism.^479–481 Enediynes bind the minor groove of dsDNA in a sequence/context-specific fashion which is often mediated via their appended carbohydrates. Subsequent reductive/nucleophilic activation initiates a Bergman or Myers-Saito cyclization, the reactive diradical intermediate of which abstracts hydrogen from the DNA backbone to ultimately afford oxidative DNA strand cleavage.⁴⁸² Out of 46 enediynes and enediyne derived compounds isolated from bacteria, 38 glycosides have been characterized (Fig. 27). Members of this family are treated separately herein based upon enediyne ring size.

Fig. 27.

9-Membered enediyne aglycons and associated sugars. Ar, aromatic moiety.

8.1. 9-Membered enediynes

The glycosylated 9-membered enediynes (Fig. 27) include C-1027,^483–485 neocarzinostatin,^486–488 maduropeptin,⁴⁸⁹ kedar-cidin,^490–492 and two putative cycloaromatized analogs (NFκB inhibiting fijiolides⁴⁹³ and cyanosporasides⁴⁹⁴) from Streptomyces, Actinomadura, Streptoalloteichus, Salinospora, and Nocardiopsis. Nine-membered enediyne glycosylation regiospecificity is restricted to C-9 or C-10 of the enediyne core. With one exception (cyanosporoside), all 9-membered enediyne glycosides contain an amino sugar (C-1027 and fijiolides, 4′-deoxy-4′-dimethylamino-5′,5′-dimethyl-β-D-ribose 87; neocarzinostatin, N-methyl-α-D-fucosamine 88; maduropeptin, 4′-amino-4′-deoxy-3′-methyl-β-D-ribose 89; kedarcidin, α-L-kedarosamine 91) where kedarcidin is the only member containing more than one carbohydrate (an additional α-L-mycarose 90). Interestingly, the sugar appended to cyanosporoside is a rare oxo-β-D-fucose (92), reflecting a potential lack of a functional keto-sugar nucleotide aminotransferase common to members of this subclass. C-3′-, C-4′- or C-5′-branched sugars are also a predominate feature among 9-membered enediynes with N-methyl-α-D-fucosamine 88 (neocarzinostatin) and α-L-kedarosamine 91 (kedarcidin) as the only exceptions.

8.2. 10-Membered enediynes

Glycosylated 10-membered enediyne glycosides (Fig. 28) include the calicheamicins,^495–498 esperamicins,^499–505 shishijimicins,⁵⁰⁶ and namenamicin⁵⁰⁷ from Micromonospora, Actinomadura and potentially unidentified bacterial symbionts. With respect to the latter, shishijimicins and namenamicin were isolated from marine ascidians but based upon their structural resemblance to bacterial counterparts they are assumed to be of bacterial origin and therefore included herein. Among the glycosylated 10-membered enediynes, C-8 and C-12 of the enediyne core are the primary points of glycosylation (Fig. 28). Both the calicheamicins and esperamicins contain a conserved C-8 trisaccharide moeity I comprised of 4′,6′-dideoxy-4′-hydroxyamino-β-D-glucose (202), 4′-thio-2′,4′,6′-trideoxy-β-D-altrose (94) and 4′-amino-2′,4′-dideoxy-α-L-xylose (95) where, in some calicheamicins (calicheamicin ${\mathrm{\alpha }}_3^{\mathrm{I}}$, LL-E33288 B, and LL-E33288 ${\mathrm{\alpha }}_3^{\text{Br}}$), the aminopentose is lacking.^508–511 The corresponding terminal thiosugar is 4′-S-methylated (esperamicins) and 4′-S-acylated with a modified orsellinate (calicheamicins), the latter of which is typically O-glycosylated (3′-O-methyl-α-L-rhamnose, 46; exceptions being calicheamicin ${\mathrm{\alpha }}_2^{\mathrm{I}}$, LL-E33288 B and LL-E33288 ${\mathrm{\alpha }}_2^{\text{Br}}$). Esperamicin contains an additional C-12-O-glycoside (α-L-oliose, 49) which is further esterified at C-3′ (esperamicin A₁ series, 49a) or C-4′ (esperamicin A₂ series, 49b) with a modified anthranilate (Fig. 28). Shishijimicins A and C contain an alternative C-8 disaccharide III comprised of a branched thiosugar (6′-deoxy-4′-pyridoindolylcarbonyl-4′-S-methyl-4′-thio-β-D-galactose, 96) and 4′-amino-2′,4′-dideoxy-4′-O-methyl-α-L-xylose (95)⁵⁰⁶ while shishijimicin B contains disaccharide IV where the branched thio sugar 96 present in A and C derivatives is replaced with a 4′,6′-dideoxy-4′-pyridoindolylcarbonyl-β-D-glucose (93). Namenamicin⁵⁰⁷ contains the alternative C-8 trisaccharide V comprised of a non-branched thiosugar [6′-deoxy-4′-thio-4′-S-methyl-4′-C-(1,2-dihydroxyethyl)-β-D-galactose, 248], and 4′-amino-2′,4′-dideoxy-4′-O-methyl-α-L-xylose (95), the thiosugar branch of which is C-7′-O-glycosylated with the esperamicin 4′-S-methylthiosugar 94 (Fig. 28).⁵⁰⁶ It is also important to note that aminopentose 95 was also found appended to indolocarbazoles AT2433 (Section 10) and, as expected, common biosynthetic elements have been noted (Fig. 28).⁵¹² In the calicheamicins and esperamicins, the C-8-saccharide substitution is known to be important for DNA binding. While there remains some controversy regarding whether the metabolite or DNA alters configuration upon complex formation,^513–515 the unique conformation of the calicheamicin/esperamicin hydroxyaminosugar glycoside bond is believed to be a key contributor. Interesting, this hydroxylaminoglycosidic bond also was recently demonstrated to serve as a chemo-selective handle for neoglycosylation.^10,516 It should also be noted that the biochemical study of the glycosyltransferases involved in calicheamicin biosynthesis revealed these reactions to be much more reversible, a phenomenon that has since to be found as relatively universal among glycosyltransferase-catalyzed reactions.⁵⁰⁹

Fig. 28.

10-Membered enediyne aglycons and associated sugars. Ar, aromatic moiety.

9. Flavonoids and isoflavonoids

(Iso)flavonoids are a prevalent group of plant secondary metabolites that share a common three ring architecture with subtle ring regiospecificity distinctions as a defining feature. Specifically, the term ‘flavonoid’ generally encompasses scaffolds containing a 2-phenylchromen-4-one core while ‘isoflavonoid’ typically corresponds to compounds containing the corresponding 3-phenylchromen-4-one backbone. While the core scaffolds are not of bacterial origin, several novel analogs deriving from microbial bioconversion of soy-based media components have been reported in the context of bacterial natural products discovery.⁵¹⁷ Within this context, it is important to note recent strain engineering efforts to enable microbial (iso)flavonoid scaffold production^518,519 as well as engineered microbial strains recently developed to enable differential glycosylation of (iso)-flavonoids.^520–523 Many diverse activities have been attributed to the vast repertoire of naturally-occurring (iso)flavonoids including antioxidative properties and/or inhibition of lytic/digestive enzymes as well as anticancer, antifungal and/or estrogenic action.⁵²⁴ To date, 16 glycosylated (iso)flavonoid analogs have been reported from native bacterial media bioconversion by Streptomyces,^525–530 Kitasatospora,^531,532 and Sorangium^18,533 to afford the installation of five distinct sugars [α-L-rhamnose, 46; α-D-ristosamine, 69; 6′-deoxy-α-L-talose 125; α-L-chinovose (6′-deoxy-4′-epi-α-L-glucose), 157; and β-L-cymarose, 176], the most predominate of which being α-L-rhamnose (46) (Fig. 29).

Fig. 29.

Flavonoid and isoflavonoid aglycons and associated sugars.

The dominate glycoside regiospecificity within this class is C-4a- and/or C7-O-glycosylation as exemplified by the various daidzein and genistein rhamnosides from Streptomyces,^525,526,530 the corresponding genistein-derived talosins A and B from Kitasatospora kifunensis MJM341 [which contain the unusual sugar 6′-deoxy-α-L-talose (125)^531,532 better known as a monosaccharide integrated within cell wall oligosaccharides of certain Gram negative bacteria]^534–536 and the 4′,7-bis-(β-cymaropyranosyl)-genistein from Streptomyces.⁵²⁸ In addition, the Streptomyces-derived actinoflavoside carries the rare 3′-amino-2′,3′,6′-trideoxy-ribopyranoside (α-D-ristosamine, 69) at C-7.⁵²⁷ Intriguingly, while genistein rhamnosides lack antifungal activity, the corresponding talosides (talosins A and B)⁵³² were reported as potent antifungals, implicating the inversion of a single stereo-center (C-3′) within the appended sugar as the distinguishing feature that invokes antifungal activity.

C-3a and C-6 represent less common points of O-glycosylation with N-99-596 A (7,4a-dihydroxyisoflavone-3a-O-α-L-rhamno-pyranoside) and N-99-596 B (5,7,4a-trihydroxyisoflavone-3a-O-αL-rhamnopyranoside)⁵²⁹ from Streptomyces as the only examples of the former. For the latter, the α-L-chinovose (6′-deoxy-4′-epi-αL-glucose, 157) containing luteolin-6-chinovoside from Sorangium represents the only C-6-glycoside within this series.^18,533

10. Indoles and indolocarbazoles

For the scope of this discussion, indole-derived metabolites have been divided into two simple classes – those that contain a single indole as part of the aglycon core (indoles, kahakamides, neosidomycin, SF-2140, oxopropalines, pyridindolols, pyrroin-domycins and tryptamines), and aglycons comprised of fused bis-indole systems (indolocarbazoles, tjipanazoles, and akashins). Cumulatively, 79 glycosylated members are represented among these two classes where an intriguing bifurcation exists. Namely, with a few exceptions (kahakamides, neosidomycin, and SF-2140), indolocarbazole N-glycosylation is restricted to the fused bis-indole-containing members.

10.1. Fused indoles

Indolocarbazoles, the predominate metabolites within the fused indole series, are actinomycete-dervived alkaloids which inhibit topoisomerase I and kinases relevant to anticancer, antitubercular, antimalarial and antiviral lead development.^537–540 While many indolocarbazole analogs have been discovered or synthesized,^537,538 it is noteworthy that those that have advanced furthest clinically are either a known natural product (CT327, a pegylated formulation of the natural product K252a currently in phase IIb for treating psoriasis)⁵⁴¹ or subtle variations thereof including N-acylated staurosporine [midostaurin/PKC412, which recently completed a successful phase II trial for acute myeloid leukemia (AML) and is currently in phase II evaluation for metastatic melanoma],⁵⁴² the N-alkylated rebeccamycin (becatecarin)^543,544 and a reduced K252a (lestauritinib/CEP-701 currently in clinical phase II/III evaluation for treating FLT3-ITD AML).⁵⁴⁵

Indolocarbazoles are characterized by their indolo[2,3-a]pyrrolo[3,4-c]carbazole core and generally divided into two separate sub-classes exemplified by rebeccamycin⁵⁴⁶ and staurosporine⁵⁴⁷ with 64 glycosides cumulatively reported to date (Fig. 30). The distinguishing features of the rebeccamycin-like indolocarbazoles (including both AT2433 and rebeccamycin analogs) include a 1H-pyrrole-2,5-dione, indole N-glycosylation and often indole halogenation. In contrast, staurosporine-like indolocarbazoles (including, indocarbazostatins, K-252, MLR-52, RK-286, RK-1409, staurosporine, UCN and ZHD-0501 variants) contain a reduced 1H-pyrrol-2(5H)-one, a signature fused N,N′-12,13-glycoside, and typically lack indole halogenation (Fig. 30). In general, mechanism of action coincides with these structural distinctions where staurosporine-like indolocarbazoles typically function as kinase inhibitors⁵⁴⁸ while rebeccamycin-like indolocarbazoles inhibit topoisomerase I⁵⁴⁹ and, in either case, glycosylation generally contributes to improved bioactivity.⁵⁴⁹ Staurosporine-like indolocarbazoles have been reported from Micromonospora, Nocardiopsis, and Streptomyces while rebeccamycin-like analogs have been reported from Saccharothrix (later reclassified as Lechevalieria) and Actinomadura. In addition to the native natural products, indolocarbazole analogs have been generated via strain engineering,^540,548,549 feeding native producing strains with 6-fluoro-tryptophan,⁵⁵⁰ bioconversion of aglycon mimetics by E. coli strains expressing genes encoding tailoring enzymes (N-glycosyltransferases and N-methyltransferases),⁵⁵¹ and exploiting the permissive nature of corresponding methyltransferases for differential alkylation.^552,553 In addition, the heterologous production of rebeccamycin in E. coli has been achieved.⁵⁵⁴

Fig. 30.

Fused indole-related aglycons and associated sugars where different colors distinguish multiple points of attachment.

The staurosporine-like members can be further divided into N,N′-12,13-fused pyranosides and N,N′-12,13 furanosides. Of the former, a biosynthetic progression can be observed among isolates (suggesting notable sugar nucleotide tolerance within the corresponding N,N′-12,13 glycosyl-forming enzymes) beginning with the simple 6′-deoxy sugar α-L-rhamnose (46b) found in MLR-52 (Fig. 30).⁵⁵⁵ Continuing, sugar C-2′-deoxygenation and C-3′-epimerization presents the methylated α-L-digitoxose 68b of RK-286C^556,557 while corresponding sugar C-3′oxidation contributes to a staurosporine analog bearing the C-3′-keto sugar, 3′-keto-2′,3′,6′-trideoxy-α-L-glucose (150).⁵⁵⁸ Subsequent C-3′ sugar transamination and C-3′ sugar amino modification provides a range of analogs including the flagship 3′-N-methyl-4′-O-methyl-α-L-ristosamine (45b, staurosporine,^{547,559–561} TAN-999,⁵⁶² UCNs,^563,564 and RK-1409⁵⁶⁵) as well as N,N-dimethyl,⁵⁶⁶ N-desmethyl,⁵⁶⁷ and N-amide derivatives⁵⁶⁸ in staurosporines; N-formyl and the corresponding C-3′/C-4′-cyclic carboxamide 45c of ZHD-0501;⁵⁶⁹ and N-oxidation to afford the hydroxyl-amino analog (226, associated with two N-hydroxystaurosporine derivatives and 4′-demethyl-N-formyl-N-hydroxystaurosporine) and hydroxylimino analog (148) of the TAN-1030A series (Fig. 30).⁵⁵⁸ The same strain afforded metabolites containing the 3′-keto corresponding sugars 150 and 151.⁵⁵⁸ Three divergent N,N′-12,13-pyranosides have also been observed including: 4′-O-methyl-α-L-olivose (106, RK-1409B);⁵⁷⁰ 3′-nitro-2′,3′,6′-trideoxy-α-L-glucose (143, 4′-demethylamino-4′-nitro-staurosporine);⁵⁶⁸ and 3′-amino-3′,6′-dideoxy-4′-O-methyl-α-L-altrose (147, 5′-hydroxystaurosporine and 4′-N-methyl-5′-hydroxystaurosporine from Micromonospora).⁵⁷¹ The corresponding N,N′-12,13-fused furanoside-containing members of this group include indocarbazostatins A–D,^572–574 all of which contain a novel branched α-L pentose 145 [differing via a C-3′-side chain ethyl (A/B) or methyl (C/D) ester], and K-252a-b.^566,575,576 K-252a and K-252b contain a novel branched α-L pentose 146 [differing via the presence of a C-3′-side chain free acid (b) or methyl ester (a)] while, in N-methyl-3′-amino-3′-deoxy K-252a, the 146 C-3′-hydroxyl has been replaced with a N-methyl-amino group to afford 149 (Fig. 30). From a biosynthetic perspective, general formation of the indole-N-sugar-C-5′ fusion, sugar branching (145, 146 and 149), sugar N-oxidation (143, 148, 335) and formation of the C-3′/C-4′-cyclic carboxamide 45c are expected to offer potential new chemistries.

In contrast to the N,N′-12,13-fused pyranosides and furanosides discussed in the preceding paragraph, rebeccamycins (from Lechevalieria),^{546,577–579} AT2433 analogs (from Actinomadura),^580,581 holyrines (from a marine actinomycete strain N96C-47),⁵⁸² RK-286D (from Streptomyces),⁵⁵⁷ tjipanazoles (from Tolypothrix),⁵⁸³ akashins (from Streptomyces),^584,585 and K-252d (from a marine Streptomyces and Nocardiopsis)^566,575,576 represent related fused tryptophan ring-based systems that contain a single N-glycosidic attachment. A range of monosaccharides have been observed among the appended sugars within this subclass including: β-D-glucose (104) and its 4′-O-methyl-analog (rebeccamycins); α-L-digitoxose (68a) (RK-286D); α-L-ristosamine (45a) and the 2′,3′,6′-trideoxyaminohexose (152) (holyrines A and B, respectively); α-L-rhamnose 46a (K-252d); 6′-deoxy-β-D-gulose (135), α-L-rhamnose (46a), β-D-glucose (104), or β-D-xylose (136) (tjipanazoles); 4′-amino-4′,6′-dideoxy-α-L-glucose (137a), 4′-acetamido-4′,6′-dideoxy-α-L-glucose (137a) and the corresponding C-3′/C-4′-oxazole sugar 137b (akashins A, B and C, respectively), the latter of which is perhaps the most uniquely functionalized sugar among this series. The four AT2433 variants stand out as the only disaccharide-containing metabolites within the fused indole series where the deoxyaminopentose 4′-amino-2′,4′-dideoxy-α-L-xylose (138) or its 4′-N-methyl derivative is attached at 6′-position of indole 4′-O-methyl-β-D-N-glucoside (104) (reminiscent of rebeccamycin core structure) (Fig. 30). AT2433 and rebeccamycin share many common biosynthetic features and a biosynthetic relationship between AT2433 and the enediyne calicheamicin (Section 8.2, by virtue of their shared aminopentose) has also been noted.⁵¹²

10.2. Simple indoles

Unlike members discussed in the previous subsection, indole N-glycosylation is uncommon among the simple indoles metabolites. Specifically, kahakamides (a marine Nocardiopsis),⁵⁸⁶ neosidomycin (Streptomyces),⁵⁸⁷ and SF-2140 (Actinomadura)⁵⁸⁸ are the only members that contain indole N-glycosides, all of which contain either 4-deoxy-α-D-taluronamide (314; kahakamide A) or 4′-deoxy-α-D-taluronate (139; kahakamide B, neosidomycin, and SF-2140) (Fig. 31). Of these, SF-2140 displayed notable antiviral activity and a higher survival rate than amantadine in viral-challenged mice. Among other simple indole/tryptophan analogs, two C-5-glycosylated N-acetyl tryptamine derivatives [β-D-quinovose (140) or α-L-rhamnose (46)], were discovered in the context of metabolic feeding studies using the staurosporine producer.⁵⁸⁹ Sugar 46 was also present, as an unusual anomeric glycosyl ester, in an indolyl-3-carbonyl derivative isolated from Streptomyces.⁵⁹⁰

Fig. 31.

Simple indole-related aglycons and associated sugars.

The pyrroindomycins from Streptomyces represent the only trisaccharyl-containing members of the simple indole group. Unlike the indolyl-glycosyl esters described above, the trisaccharide of pyrroindomycins is connected via an ester bond to the C-3′ terminus of a 4′-amino-2′,4′,6′-trideoxy-β-L-galactosyl-(1′→4′)-β-L-mycarosyl-(1′→4′)-β-D-rhodinosyl trisaccharide appended to a polyketide tetramic acid-containing macro-ring system (trisaccharide I, Fig. 31). These unique metabolites were noted for their antibacterial activity against the drug resistant pathogens including MRSA and VRSA.

The β-carbolines represent the last group of simple indole-containing metabolites and are signified by a fused pyridine-indole skeleton (Fig. 31). Members include the oxopropalines^591,592 and pyridindolols^593–596 from Streptomyces and have been noted for herbicidal and anti-fungal activities and as benzodiazepine-receptor ligands.^597,598 To date, only six glycosylated β-carbolines have been reported from bacteria. Variant side-chain regiospecificity of O-glycosylation was observed among this group with C-10, C-11, and C-12 mono-glycosylation [β-D-glucose (104)] observed in pyridindolols and C-11 and C-12 mono-glycosylation [α-L-rhamnose (46)] found among oxopropalines (Fig. 31). In the latter case, rhamnosylation was found to be detrimental to the observed anticancer activity of the parental aglycon.⁵⁹²

11. Lipids, polyenes and carotenoids

Microbial glycosylated lipids are ubiquitous and include both primary metabolites common to cell wall/membrane architecture as well as a range of bioactive secondary metabolites. As aglycon structure and glycosyl regioselectivity is highly diverse, this section simply highlights the diverse sugar structures observed among bacterial glycolipids.

Not surprisingly, O-β-D-glucosides (104) are among the most prevalent glycosyl substitutions represented as exemplified by fattiviracins,⁵⁹⁹ moenomycins,⁶⁰⁰ arthrobacillins,⁶⁰¹ glucolipsins,⁶⁰² and carotenoids.⁶⁰³ The α-anomer 18 was also observed in a small set of glycosylated lipids^604,605 along with the corresponding common aminosugar variant (2′-amino-2′-deoxy-β-D-glucose, 153), α-D-anomeric phosphate [2-amino-2-deoxy-α-D-glucose-1-phosphate (5) of lipid A]⁶⁰⁶ α-D-glucuronic acid (154), and C-5′ sulfonates (155).^607,608 The common 2′-epimer β-D-mannose (156) is also represented among many bacterial carotenoids⁶⁰⁹ and Flavobacterium glycerolipids⁶¹⁰ while the corresponding C-2′, C-6′-disulfate (160),⁶¹¹ as well as α-L-fucose (157),⁶¹² β-D-arabinose (158),⁶¹³ α-L-rhamnose (46),⁶¹⁴ β-D-xylose (136),⁶¹⁵ β-D-glucofuranose (159),⁶⁰⁴ β-D-galactose (161),⁶⁰⁷ α-D-allose (162),⁶¹⁶ α-D-altrose (163),⁶¹⁷ Gram-negative phosphosphingolipid heptose sugar 166,⁶¹⁸ and the unique 8-carbon thio sugar 167 (desalicetin 2′-butyrate)⁶¹⁹ are additional sugars represented among bacterial glycolipids, carotenoids and related metabolites. Di-saccharides I and II (Fig. 32), consisting of β-L-hexanoic acid (168, β-L-iduronic acid) and β-D-glucosamine (153), or glucose-1-phosphate (18) and β-D-glucosamine (153), are also components of some glycolipids.^620,621

Fig. 32.

Glycolipids, polyenes and carotenoids and associated sugars.

Monosaccharides represented among secondary metabolites include the undefined 2′-amino-2′,6′-dideoxy hexose 164 (vancoresmycin),⁶²² β-L-xylose (165, malyngamide J),⁶²³ β-L-rhodinose (169, streptolydigin)⁶²⁴ β-D-digitoxose (131, α-lipomycin)⁶²⁵ and α-D-altrose (163, moritoside).⁶¹⁷ While glucosamine (153) is commonly found as a lipid O-glycoside, it exists as an N-glycoside among racemomycins.⁶²⁶ Alternatively, some secondary metabolites within this family contain O-oligosaccharyl substitutions as exemplified by moenomycin [Fig. 32; hexasaccharide chain V comprised of β-D-galacturonamide (324), 2′-N-acetyl-β-D-xylose (173), β-D-glucose (104), N-acetyl-β-D-2′-glucosamine (153) and phosphosugars 174 and 175].⁶⁰⁰

Elfamycins and phenelfamycins represent a structurally unique group of metabolites from Streptomyces.^627–629 The polyene moieties found within these molecules are glycosylated with mono-, di-, or trisaccharides (Fig. 32) where α-L-oliose (49a/49b) serves as the predominate monomer and typically as the point of contact to the aglycon. Exceptions to this include disaccharide III of efrotomycin and N-demethylefrotomycin, comprised of the 6′-deoxy-β-D-allose (170) and α-L-rhamnose (46),^630–632 and the inclusion of β-L-digitoxose (176) in elfamycin analogs (disaccharide VII and trisaccharide IV). Aurantinin B from Bacillus, while also structurally related to elfamycins, is also unique by virtue of its rare 3′-keto-β-L-sugar (171).⁶³³

12. Macrolides and macrolactams

Macrolides and macrolactams are highly functionalized macro-cyclic polyketide-derived metabolites, the ring structures of which are formed via an intramolecular ester (macrolides) or amide (macrolactams) bond. This section summarizes the vast array of glycosylated macrolides (categorized by ring size and architecture in Sections 12.1–12.7), macrodiolides (Section 12.8), super macrolactones (Section 12.9), polyene macrolides Section 12.10), macrolactams (Section 12.11), spirotetronates (Section 12.12) and spinosyns (Section 12.13).

12.1. 12-Membered macrolides

Methymycin and pseudoerythromycin derivatives constitute the known 12-membered bacterial glycosylated macrolides from Streptomyces, where O-glycosylation is restricted to C-3 and/or C-5 of the macrolide core (Fig. 33). The most predominate sugar found appended to this class is β-D-desosamine (177a), at either C-3 or C-5,^634,635 but other sugars observed include the corresponding desosamine analogs N-desmethyl 177b or N-oxide 178. In addition, C-3-appended α-L-cladinose (90b) is present pseudo-erythromycins⁶³⁴ while β-D-olivosyl (53) substituted methymycin has been generated via pathway engineering⁶³⁶ and a range of non-native desosamine replacements (in the context of 12-, 14- and 16-membered macrolides) have been accomplished via chemoenzymatic methods.⁶³⁷ Classical 12-, 14- and 16-membered macrolides function as antibacterials in a mechanistically similar manner as described in Section 12.3.

Fig. 33.

12-, 14- and 16-membered macrolactones and associated sugars.

12.2. 14-Member macrolides

As exemplified by the naturally-occurring erythromycin or semi-synthetic clarithromycin, 14-membered macrolides are a mainstay in the clinical treatment of Gram-positive/negative infections.^638,639 There are 96 naturally-occurring 14-membered glycosylated macrolides reported to date from bacterial sources including Streptomyces, Nocardia, Saccharopolyspora and Amycolatopsis. C-3- and C-5-O-glycosylation is the most predominate among 14-membered macrolides, but C-6-, C-7-, C-9- and, in one case (CP 64537, containing either 140 or 170c),⁶⁴⁰ C15-O-glycosylation has also been observed. Reminiscent of classical 12- and 16-membered macrolides, 14-membered macrolides (including erythromycins,^{634,641–643} megalomycin,^644,645 picromycin,⁶⁴⁶ oleandomycins,⁶⁴⁷ sporeamicins;^648–651 Sections 12.1 and 12.3) typically contain the common C-5 β-D-desosamine (177a). The corresponding N-desmethyl-β-D-desosamine (177b)⁶³⁴ is also found at this position as exemplified by the cineromycins [which also contain a C7 α-D-glucose (18)]⁶⁵² C-3-O-glycosylation is also prevalent, albeit less important for bioactivity, and includes 3-O-methyl-α-L-mycarose (also known as α-L-cladinose, 90b),^{634,648,653,654} α-L-mycarose (90a; as observed in CP64593,⁶⁴⁰ sporeamicin-C⁶⁵⁰ or the 2-norerythromycins deriving from pathway engineering),⁶⁵⁵ or the C-3′/C-4′-modified form (90c/90d, Fig. 33) in megalomycin analogs.⁶⁵⁶

Additional C-3-appended sugars observed include 4′-O-acetyl-αL-arcanose (122b) [and corresponding engineered 2′,3′-anhydro-arcanose (182) and 3′-acetylated 180^657,658] of the lankamycins⁶⁵⁹ and the kujimycins [which contain a C-3 α-L-arcanose (122a) or 4′-O-acetyl-α-L-arcanose (122b) and a C-5 β-D-lankavose (181b)]⁶⁶⁰ and α-L-oleandrose (184); found in oleandomycin^647,661 and the 3-O-oleandrosyl-5-O-desosaminyl-(8S)-8-hydroxyerythronolide B⁶⁶² from bioconversion. Additional sugars attached to C-5 include 3-keto-4,6-dideoxy-β-D-hexopyranose 179 (CP-63693⁶⁴⁰), β-D-chalcose (181b; lankamycins⁶⁵⁹ and kujimycins⁶⁵⁴) and β-D-mycaminose (185; narbolide,⁶⁶³ via biotransformation). C-6 substitution is restricted to α-L-megosamine (45; megalomycin⁶⁵⁶ and SF2748⁶⁶⁴) while that of C-9 is limited to variants of α-L-rhamnose (14–18; lyngbyaloside and lyngbuilloside^665,666). Arguably the most unique glycosylation among 14-membered macrolides is the α-L-cladinose (90e) orthoester of erythromycin E originally produced via bio-conversion⁶⁶⁷ and later discovered as a metabolite of a pathogenic Nocardia.⁶⁶⁸ Such orthoester glycosides are more typically associated with orthoester natural products (see Section 14). Classical 12-, 14- and 16-membered macrolides function as antibacterials in a mechanistically similar manner as described in Section 12.3.

12.3. 16-Membered macrolides

Bacterial 16-membered macrolide glycosides represent the largest subgroup with a total of 284 glycosylated members reported thus far primarly from actinomycetes. This group is further divided into two subgroups based upon macrolactone ring architecture of as depicted in Fig. 33, where the predominant subgroup 16-A contains classical antibacterial macrolides reminiscent of their smaller counterparts described in the preceding sections. As with other classical macrolides, the prominent observed glycosyl regioselectivity within subgroup 16-A is C-3 and/or C-5 where some members also display C-9 and/or 14-C O-glycosylation. A total of 21 sugar monomers are represented among 16-membered macrolides including β-D-desosamine (177a, typically C-5), β-D-mycaminose (185, typically C-5) and α-L-mycarose (90a, typically C-5/C-3) commonly found in 12- and/or 14-membered counterparts as illustrated by rosamicins,⁶⁶⁹ mycinamicins,^670–678 juvenimicins.⁶⁷⁹ Similar to the 12-membered comparators, C-3 3′-N-oxo-β-D-desosamine (also called desosamine N-oxide, 178) is observed among rosaramicins from Micromonospora⁶⁸⁰ while the only pentose (α-D-arabinofuranose, 191) is found at this position in epothilones.⁶⁸¹ C-5-O-glycosides of 4′,6′-dideoxy-β-D-glucose (181a), or subtle variations thereof (such as β-D-chalcose, 181b, or 2-propionyl 181c) are also observed as exemplified by chalcomycins,⁶⁸² GERI-155⁶⁸³ and neutramycin A.⁶⁸⁴ Angolamycins, in contrast, carry the 2′-deoxy-β-D-mycaminose (β-D-angolosamine, 59)⁶⁸⁵ while β-D-aldgarose (190b) and its acyclic precursor (190a) are found at the same position of aldgamycins^686–691 and swalpamycin.^692,693 C-4′-O-glycosylation of the C-5 β-D-mycaminose (185) is also a fairly common occurrence within this subgroup (disaccharides I–IV, Fig. 33)^694–696 where four L-sugars [α-L-amicetose (97), α-L-cinerulose (51), α-L-rhodinose (44) and α-L-mycarose (90a)], or subtle modifications thereof, make up the terminal sugar.^696–706 The angolamycins are distinguished within this context by their C-5-initiating sugar as described above (α-L-mycarose, 90; disaccharide V). The highly deoxygenated sugar β-D-forosamine (63) is restricted to C-9-O-glycosylation as found in the spiramycins (also known as shengjimycins)^707–711 and chimeramycins.⁶⁹⁹ Finally, C-14-O-glycosides have also been reported where the sugars utilized include: β-D-mycinose (170c in tylosin,⁷⁰³ neutramycins,⁶⁸⁴ mycinamycins⁶⁷⁸ and GERI-155⁶⁸³); 6′-deoxy-β-D-allose, 6′-deoxy-2′-O-methyl-β-D-allose, or 4′-O-propionyl-β-D-mycinose (170a, 170b or 170d, respectively, in mycinamycin⁶⁷⁷ and tylosin analogs⁷¹²); β-D-boivinose and β-D-digitoxose variants (187a/b and 131a/b, respectively in engineered tylosins⁷⁰³) as well as 3′,6′-dideoxy-4′-keto-2′-O-methyl-2′,3′-unsaturated-β-D-glucose (188) also via pathway engineering.⁷¹³

A small set of 16-membered glycosylated macrolides are distinguished by the subgroup 16-B branched macrocyclic lactone architecture, where O-glycosylation occurs at C-21 (Fig. 33). Examples include bafilomycin-A1-rhamnoside (α-L-rhamnose, 46a)⁷¹⁴ leucanicidin (2′-methoxy-α-L-rhamnose, 46e)⁷¹⁵ and formamicin (β-D-olivose, 53).^716,717

The final sub-group of the 16-membered macrolides are the avermectins (sub-class 16-C, Fig. 33).^718–720 They are 16-membered macrolides produced by Streptomyces, Amycolatopsis and Nocardia. Avermectins typically contain a C-13 disaccharyl substitution (disaccharide VI). More recently, a variety of sugar analogs of this sub-class have been made via chemoenzymatic methods.⁷²¹

Classical 12-, 14- and 16-membered macrolides inhibit bacterial protein synthesis by blocking the 50S ribosomal peptide exit tunnel.^722,723 Core contributions to the specific macrolide-ribosome interaction derive from the C-3 sugar (e.g., desosamine, 177a) 2′-hydroxyl and 3′-dimethylamino groups, the latter of which has been deemed most important, based upon both macrolide-ribosome complex structure elucidation⁷²⁴ and bioactivity assessment of analogs.^725–727 Glucosylation of 2′-hydroxyl group of this aminosugar is an established mechanism of macrolide resistance,^728–730 where the corresponding O-glucosyltransferases have been recognized as highly permissive.^731,732 C-5-disaccharyl-substituted macrolides further perturb the relative positioning of the 3′-end of P-site bound tRNA and 23 S rRNA in the ribosome where the terminal sugar of the disaccharide moiety extends into the peptidyl transferase center.^{684,702,733–735} Some classical macrolides (e.g., megalomicins) have been found to inhibit protein trafficking in the golgi and, while the megalomicin C-6 α-L-megosamine (45) has been put forth as a contributor to this activity,^736,737 this contention has not been experimentally validated. A number of 14-membered macrolides have also been noted for their immunomodulatory activities,^738–740 the mechanism for which remains poorly understood. Exceptions to the classical mode of action, the epothiolones are potent tubulin-targeted anticancer agents that advanced to clinical evaluation,^741,742 subgroup 16-B members such as bafilomycins are potent inhibitors of vacuolar type H⁺-ATPase,^743,744 and the avermectins are potent antihelmenthics.⁷⁴⁵

12.4. 18-Membered macrolides

There are only 23 reported bacterial 18-membered glycosylated macrolides the majority of which are tiacumycins and lipiarmycins^746,747 from Micromonospora, Dactylosporangium, Actinoplanes and Catellatospora. All analogs share a common macrolactone ring O-glycosylated at C-11 with variants of the branched 5′-C-methyl-β-L-rhamnose (193a–f) or at C-20 with variants of β-L-rhamnose (322a) (Fig. 34, 18-A). TAN-1323 A-C (18-B)⁷⁴⁸ and the recently isolated biselyngbyaside (18-C)⁷⁴⁹ are distinguished from the tiacumicins by their C-17 side chains and corresponding glycosylation patterns. TAN-1323 A-C are C-23-O-glycosides of β-D-olivose (53a), 4-carbamyol-β-D-olivose (53b) and β-L-rhamnose (322a), respectively, while biselyngbyaside is a C-3-O-glucoside (2′-O-methyl-β-D-glucose, 104b).

Fig. 34.

18-, 20-, 22-, 24-Membered macrolactones and macrodiolides and associated sugars.

12.5. 20-Membered macrolides

Venturicidins, irumamycins, ammodicin and apoptolidins are the only known glycosylated 20-membered macrolides from Nocardiopsis, Saccharotrix, and Streptomyces (Fig. 34). Glycosylation regiospecificity is variant across this series with C-9-O-glycosides represented by ammocidin (6-deoxy-α-L-glucose, 194a) and apoptolidin (4′-O-methyl-6-deoxy-α-L-glucose, 194b). C-13-O-glycosides include the venturicidins (β-D-olivose, 53a) and irumamycin (3′-carbamoyl-β-D-olivose, 53c).^750–752 C-24-O-glycosides are disaccharide substitutions as exemplified by ammocidin [disaccharide I comprised of β-D-olivomicose (192) and β-D-digitoxose (131)].⁷⁵³ In contrast, apoptolidin contains a C-27-O-disaccharide [disaccharide II comprised of 3′-O-methyl-β-D-olivose (53d) and α-L-chromose (195)].⁷⁵⁴ Apoptolidin specifically induces apoptosis in E1A-transformed glial cells with little or no detectable normal cell toxicity and is believed to function via inhibition of mitochondrial F1F0-ATPase where the disaccharide appears to be dispensable.^754–756 Venturicidins and irumamycins are potent antifungals believed to inhibit ATP-driven proton transport and hydrolytic processes.^757,758 Interestingly, X-14952B which differs from irumamycin in the C-19 alkyl side chain possesses antibacterial activities.⁷⁵⁹

12.6. 22-Membered macrolactones

Pulvomycin from Streptouerticillium represents the only bacterial 22-membered macrolide (Fig. 34).⁷⁶⁰ This metabolite is a C-33-O-glycoside of 2′,4′-di-O-methyl-β-D-fucose (270). Pulvomycin inhibits protein synthesis by preventing the formation of the elongation factor Tu (EF-Tu)/GTP/aa-tRNA ternary complex, the pulvomycin sugar contribution to which remains unknown.⁷⁶¹

12.7. 24-Membered macrolactones

Macrolactins,⁷⁶² archazolids⁷⁶³ and maduralides⁷⁶⁴ represent 24-membered glycosylated macrolactones from Bacillus, Cystobacter, Archangium and Maduramycetes (Fig. 34). Macrolactins and archazolids are C-7 and/or C-15-O-β-D-glucosides (104a). Glycosylation in these molecules has been demonstrated to suppress their cytotoxic effects. Maduralide, a weak Gram-positive antibacterial, is a C-13-O-glycoside of 6-deoxy-3′-O-methyl-β-L-talose (198).

12.8. Macrodiolides

A small set of glycosylated macrocyclic dilactones have been reported from Streptomyces and Microbiospora. Elaiophylin, SNA-4606-1 and efomycins are C-13/13a-di-O-α-L-oliosides (49a),⁷⁶⁵ where the sugars are 3′-O-methylated in (49b) efomycin A.⁷⁶⁶ Bispolides from Microbiospora are comprised of larger dilactone rings containing 2′,6′-dideoxyhexose 196 at C15a and the same sugar or 6′-deoxyhexose 197 at C-15.⁷⁶⁷ Axenomycins from Streptomyces are C-43-O-disaccharyl-substituted metabolites [disaccharide III comprised of β-D-amicetose (48) and L-axenose (199, anomeric stereocenter undefined)].⁷⁶⁸ Liposidolide A encompasses a C-15-O-β-D-mannoside (156) and C-41-O-β-D-olivoside (53e), the latter of which is modified as depicted (Fig. 34).⁷⁶⁹ Efomycins are non-toxic inhibitors of selectin-mediated leukocyte adhesion and have been considered as candidates for further development to treat autoimmune disorders.⁷⁷⁰ Elaiophylin, efomycin G and SNA-4606 inhibit testosterone 5α-reductase and display growth-promoting, antiviral and antibacterial activities. Glycosylation does not impact upon efomycin activity and the glycosyl contribution to the latter metabolites has not been studied.

12.9. Super macrolactones (ring size 32–48)

There are 25 glycosylated super macrolactones (defined as macrolides with ≥32-membered ring systems) from bacterial sources including Streptosporangium, Streptomyces and Nocardia (Fig. 35). Brasilinolides are C-37-O-α-D-oliosyl (71a) bearing 32-membered macrolides where brasilinolide C contains unmodified α-D-oliose (71a),⁷⁷¹ brasilinolide A carries 3′-O-acyl-α-D-oliose (71b),⁷⁷² and bracilinolide B has the permethylated counterpart.⁷⁷³ Liposidolide A has two appended sugars, a C-15-O-α-D-mannose (160) and a C-41-O-β-D-olivose (53) derivative modified at C-3′.⁷⁶⁹ The 34-membered macrocyclic sporaviridins from Streptosporangium are glycosylated to a greater extent with a C-13-O-pentasaccharide (III and IV), a C-21-O-β-D-glucose (104) and a C-47-O-α-L-vancosamine (118).⁷⁷⁴ The core of the sporaviridin pentasaccharide (III and IV) is a tri-substituted β-D-quinovose (140) which contains a C-2′-O-β-D-glucose (104) or 4′-amino-4,6-dideoxy-β-D-glucose (202), a C-3′-O-3′-amino-3,6-dideoxy-β-D-glucose (59) and a C-4′-O-disccharide comprised of β-D-quinovose and 3-amino-3,6-dideoxy-β-D-glucose. The 32-membered macrolides notonesomycin⁷⁷⁵ and A-77951⁷⁷⁶ both possess C-37–O-glycosides. The former contains both 3-O-methyl-β-D-olivose (53) and β-L-rhodinose (169), connected via a modified para-aminobenzoic acid (fragment II) while the latter contains an undefined modified disaccharide (205, disaccharide I). The recently discovered 51-membered stambomycins represent the largest glycosylated bacterial macrolide and share a common C-5-O-N,N-dimethyl-3′,6′-dideoxy-β-D-glucoside (185b).⁷⁷⁷ Primycins (C-18-O-α-D-arabinofuranose, 204) and mathemycins (C-18-O-β-D-mannose, 156) both carry an O-glycoside three carbons from the macro-cyclic ring forming oxygen. In addition, mathemycin contains a C-37-O-3′-amino-2,6-dideoxy-β-D-glucose (185a).⁷⁷⁸ Finally, the 42-membered desertomycins and 48-membered monazomycins both share a C-22-O-α-D-mannose (160).^779–783

Fig. 35.

Super macrolactone (ring size 32–48) aglycons and associated sugars.

In addition to antifungal activities, brasilinolide A and A-77951 were noted for their immunosuppressive effects.^772,776 Stambomycins, moderatively active against Gram-positive and Gram-negative bacteria, also were reportedly cytotoxic against a number of human cancer cell lines.⁷⁷⁷ It should also be noted that acetylation of the sporaviridin aminosugar abolished antimicrobial activity.⁷⁷⁴

12.10. Polyene macrolide

A total of 49 glycosylated polyene macrolides have been reported from Streptomyces, Actinoplanes, and Sorangium. Members within this group share a common macrocyclic lactone of various sizes (26 to 38-membered rings) with 4–7 conjugated double bonds where the classical antifungals amphotericin and nystatin serve as prototypical examples. With the exception of chivosazoles^784–786 aglycons can be further distinguished by the presence or lack of an endocyclic pyran ring (core structures A and B, respectively; Fig. 36).

Fig. 36.

Polyene and macrolactam aglycons and associated sugars.

The aminosugar β-D-mycosamine (206) is the predominate O-glycoside among polyene macrolides as exemplified by amphotericins,⁷⁸⁷ candicidins,⁷⁸⁸ candihexins,⁷⁸⁹ vacidin-A,⁷⁹⁰ SCH 16656 complex (67–121 analogs)⁷⁹¹ and polyfungins.⁷⁹² With the exception of sorangiosides (glycosylated with β-D-glucose, 104),⁷⁹³ regioselectivity of glycosylation as β to the pyran ring is strictly conserved among prototypical polyene members (Fig. 36, position a, β-D-mycosamine 206). Some polyfungins carry an additional C-35-O-α-L-digitoxose (68) and the mycosamine C-4′ is further modified with 6′-deoxy-β-D-mannose (209; disaccharide I) in 67-121C (Fig. 36). The 44-membered polyene antibiotic lienomycin is distinguished by its C-25-O-α-L-rhamnose (46),⁷⁹⁴ while DJ-400-B and B2 are the only members with a C-5 β-D-mycosamine (206) (Fig. 36, position a).⁷⁹⁵ The appended mycosamine is important to the antifungal activity of these metabolites where subtle changes dramatically influence potency.⁷⁹⁶ While systemic toxicity presents a challenge to the clinical use of amphotericin, recent work with analogs bearing 2-deoxy mycosamine suggest a notable improvement of selectivity for ergosterol-containing fungal membranes.⁷⁹⁷ As with other bacterial systems, reactions catalyzed by polyene glycosyltransferases have been found to be reversible and used to generate differentially-glycosylated polyene analogs.⁷⁹⁸

Distinct from the prototypical antifungal polyenes, chivosazoles are cytotoxins that contain an integrated aglycon oxazole ring and are C-11 glycosides (β-D-quinovose, 140).^784,785

12.11. Macrolactams and related glycosylated metabolites

Similar to the macrolactones discussed in the previous section, macrolactams, from Streptomyces, Actinosynnema, Amicolatopsis, Actinomadura, Maduromycetes, Microtetraspora, Nonomuraea and Pseudonocardia are large macrocyclic structures but are distinguished by a ring-closing amide bond. Compared to macrolactones, glycosylated macrolactams are fewer in number (53 total) and display variant glycosyl regiospecificity as summarized in Fig. 36.¹⁸ Many members contain an integrated aromatic ring as part of the overall macrolactam aglycon, the exceptions being fluvirubicins, SCH 39185, SCH 38511, SCH 38516, SCH 42729, SCH 42282, cremimycins, vicenistatin, and incednine. Tolypomycins from Streptomyces stand out by virtue of their conjugation to the C-4′-amine of tolyposamine (203a) or via a corresponding C-4′-imine in tolypomycin-Y (203b).¹⁸ Ansamitocinosides from Actinosynnema are unique as N-β-D-glucosides (104),⁷⁹⁹ the sugar of which can be further modified as in ansacarbamitocins from Amicolatopsis.⁸⁰⁰ In contrast, the phenolic hydroxyl is the glycosyl acceptor in the related C-22-Oβ-D-glucosyl-mycotrienin II.⁸⁰¹

Fluvirucins,^802,803 SCH 39185, SCH 42729, and SCH 42282 are 14-membered macrolactams bearing either a C-3-O-3′,6′-dideoxy-3′-amino-α-L-talose (73) or C-9-O-glycoside comprised of the same sugar or α-L-mycosamine (207) (Fig. 36).⁸⁰⁴ The C-2′-OH of α-L-mycosamine (207) is further glucosylated with α-L-glucose (211, disaccharide II, SCH-42729) which can also be C-4′-O-glucosylated with β-L-glucose (210, trisaccharide III, SCH-42282).⁸⁰⁵ Modified macrolactams, ansaetherone,⁸⁰⁶ and tetrapetalones (described in details in Section 23.5.3.),⁸⁰⁶ Q-1047-A⁸⁰⁷ and its hydroquinone analog Q-1047-R-A all are believed to contain C-9-O-β-D-rhodinose (56), although in some cases, the nature of the sugar has not been completely defined. The vicenistatins also contain a single C-7-O-glycoside comprised of β-D-vicenisamine (212), 3′-epi-β-D-vicenisamine (213), or β-D-mycarose (64)⁸⁰⁸ while cremimycin contains a C-10-O-3′-O-methyl-β-D-digitoxose (131).⁸⁰⁹ Finally, incedine from Streptomyces bears a C-11-O-disaccharyl moiety constructed from the two amino sugars 4′-N-methyl-β-D-forosamine (63) and 2′-deoxy-2′-methyl-amino-β-D-xylose (173).⁸¹⁰

Glycosylated macrolactams are known to possess a variety of biological activities including antifungal, antibacterial, antihelmintic, antiviral, antitumor, radical scavenger and phospholipase C inhibition.^{800,801,803,806,809–812} While the specific impact of the sugar upon bioactivity has not been delineated in most cases, recent studies in which the vicenistatin D-vicenisamine was replaced with the D-mycarose led to a complete loss of activity, implicating the aminosugar as important to metabolite cytotoxicity.⁸⁰⁸ It should be noted that the glycosyltransferase involved in vicenistatin biosynthesis displays broad substrate tolerance and also was among the first to be studied in the context of glycosyltransferase reversibility.^813,814

12.12. Spirotetronates

Spirotetronates are pentacyclic polyketides that contain a signature fused 6-membered/γ-butyrolactone spiro-bicyclic ring substructure and are produced by Amycolatopsis, Streptomyces, Actinomadura and Micromonospora. Several spirotetronate O-glycosides have been reported (Fig. 37) where dominant glycosyl regioselectivity observed is α- to the bicyclo[4.4.0]decene ring fusion (as exemplified by C-7 of chlorothricins or C-9 of tetrocarcins). Additional O-glycosylation is observed at C-17/19 of the largest macrocyclic ring in some members (as exemplified by tetrocarcins and versipelostatins, respectively) and, in one case, C-6 (PA-46101 B).

Fig. 37.

Spirotetronates and spinosyn aglycons and associated sugars.

As noted above, C-7/C-9 O-glycosylation is a predominate feature. Examples of C-7 glycosylation include disaccharide I comprised of two β-D-olivose (53) units (chlorothricins,⁸¹⁵ MC-031 and MC-033⁸¹⁶), disaccharide II comprised of a β-D-olivose (53) and β-D-quinovose (140) (hydroxychlorothricins,⁸¹⁵ K818,⁸¹⁷ MC-032, and MC-034),^816,817 or disaccharide III continaing 6′-deoxy-β-D-allose (170) and 5′-O-methyl-β-D-digitoxose (131) (PA-46101A and B).⁸¹⁸ The terminal olivose sugar (53) of these metabolites is often further modified as observed in chlorothricin and related analogs (Fig. 37, appendages i–iii) and, as previously noted, PA-46101 B also contains an additional C-6-appended 3′-C-methyl-α-L-rhamnose (108). C-9 glycosylation also predominates with monosaccharyl substitution observed on occasion (e.g., the acylated trideoxy amino sugar 213 in BE-45722,⁸¹⁹ BMY-42448,⁸¹⁹ decatromicins,⁸²⁰ pyrrolosporin A³⁸⁹) and oligosaccharyl substitutions units typically containing α- and/or β-L-digitoxose (68 and 176, respectively) and α-L-amicetose (97) as more prevalent (e.g., oligosaccharides IV–VII, tetrocarcins⁸²¹ and AC6H);⁸²² disaccharide VI, arisostatins;⁸²³ trisaccharide VIII, lobophorins;^824,825 tetrasaccharide IX, kijanimicin.^826,827 The α,β-unsaturated aldehyde-containing deoxysugar (216) of tetrocarcin K stands out as a unique glycoside among this series (Fig. 37, VII).⁸²⁸

C-17 and C-19 glycosylation is less common. Examples of C-17 glycosylation include the unique nitro sugar β-D-kijanose (also known as β-D-tetronitrose, 214; tetrocarcins,^821,822 lobophorins B,^824,825 kijanimicin,^826,827 arisostatin A⁸²³), the corresponding amino analog 218 (AC6H, lobophorin A,^824,825 arisostatin B⁸²³), and variations of 3′,4′-diamino-2′,3′,4′,6′-tetradeoxy-β-D-galactose (219, MM 46115).⁸²⁹ C-19 glycosylation is restricted to the larger versipelostatin aglycon and employs a series of oligosaccharides (X–XVI, Fig. 37) comprised of β–D-digitoxose (131), 3′-O-methyl-α-D-olivose (183) and/or α-L-oleandrose (184) monomers.^830–832

Spirotetronates possess antitumor, antiviral and antimicrobial activities where metabolite glycosylation, in most cases, is critical to activity.⁸²⁹ For example, a reduction in tetrocarcin glycosylation correlates to diminished antibacterial activity,⁸³³ while the sugar variation among lobophorin analogs contributes to antibacterial activity modulation.⁸²⁵ Sugar variation of MC-031 congeners also contributes to potency modulation in the context of cholesterol biosynthesis inhibition.⁸¹⁶ Versipelostatins were reported to down-regulate the GRP78 molecular chaperone which has potential application in cancer, Alzheimer’s and Parkinson’s.⁸³²

12.13. Spinosyns

Spinosyns are a structurally unique group of reduced polyketide macrolactams from Saccharopolyspora. A total of 23 spinosyn O-glycosides have been reported thus far.^18,834 Spinosyns contain two O-glycosides critical to their insecticidal activities, a C-9 α-L-rhamnose (46) and a C-17 β-D-forosamine (63) (Fig. 37). Spinosyn variation is based, in part, upon differential methylation of the corresponding glycosides and SAR studies revealed both sugars to contribute to bioactivity.⁸³⁵

13. Nucleosides and nucleoside-derived compounds

Compounds in this class are divided into pyrimidines (uracil- or cytosine-derived) and purines (adenine- or guanine- derived) metabolites. While metabolites with a modified ribose core are included herein, classical ribose-based nucleosides are considered outside the scope of this discussion. A total of 280 glycosylated bacterial metabolites are encompassed within this broad class where pyrimidine nucleoside antibiotics comprise the predominate group.⁸³⁶

13.1. Pyrimidine-derived nucleosides

13.1.1 Uracil-derived nucleosides

The first major subclass of uracil-derived nucleosides includes streptovirudins, tunicamycins, and corynetoxin isolated from Streptomyces, Bacillus, and Coryne-bacterium-infected Lolium.^837–840 This group includes 23 metabolites and structural signatures include a uracil or dihydrouracil (Fig. 38), an amide-linked fatty acid, a dialdose sugar attached to the N-1 of the uracil moeity, and N-acetylglucosamine (5). The corresponding dialdose sugar, a unique 11-carbon amino sugar (tunicamine, 222) comprised of a carbon-carbon bond fusion between C-6 of galactosamine and the uridine ribose C-5, is connected head-to-head to N-acetyl-α-D-glucosamine (5) (Fig. 39).⁸⁴¹ While potent antibacterial cell wall inhibitors, these agents lack clinical utility due to their inhibition of N-linked protein glycosylation in mammalian cells.⁸⁴²

Fig. 38.

Uracil-derived aglycons.

Fig. 39.

Sugars associated with uracil-derived aglycons. Pink fragment is absent in some derivatives.

The next subclass, the uridylpeptide antibiotics, includes mureidomycins,^843,844 napsamycins,⁸⁴⁵ pacidamycins,^846,847 and sansanmycins from Streptomyces and Pseudomonas.^848–850 The 25 members of this group share a common skeleton that consists of a uracil or dihydrouracil attached to a 4′,5′-unsaturated-3′,5′-dideoxyribose (332) at the uracil N-1. This modified ribose is connected to a 2,3-diaminobutyric acid (DABA) via a 4′,5′-enamide linkage to which are appended various amino acids (Fig. 39). The uridylpeptides selectively inhibit bacterial translocase (MraY) and, while both the modified ribose and uracil are critical to activity, analogs in which the enamide has been reduced retain activity.⁸⁵¹

Twenty-two glycosylated muraymycins and related metabolites from Streptomyces comprise the next subclass.⁸⁵² The core structure is composed of a peptide, uracil, uronic acid (330), and a modified ribose which terminates with a hexahydro-2-imino-4-pyrimidylglycyl (epicapreomycidine)–urea–valine moiety (Fig. 39, X = i). Members of this family differ via either the terminal amino sugar [5′-amino-5′-deoxy-β-D-ribose (221), 2′-methoxy-221, or 5′-amino-2′,5′-dideoxy-β-D-ribose (224)] and/or the fatty acid (Fig. 39). Three related epicapreomycidine-containing analogs [AA-896-A6, C5 and D4 (Fig. 39, X = i)] contain sugar 221 attached to C-5′ of the ribose moiety of uridine.⁸⁵³ Muraymycins also effectively inhibit bacterial translocase (MraY). Interestingly, while removal of the aminoribose primary amine abolishes activity,⁸⁵⁴ derivatives lacking aminoribose remain active.⁸⁵⁵

Eighteen glycosylated capuramycins (A-500359) and related metabolites produced by Streptomyces and Amycolatopsis comprise the next subclass. In addition to the structural aminocaprolactam signature (Fig. 39, X = ii), the capuramycin skeleton consists of a uracil nucleoside with a unique 4′,5′-unsaturated α-D-mannuronic acid (225) connected to the C-5′ of a modified ribose (331 forming disaccharide III, Fig. 39). Exceptions include A-500359 D, which contains the 2′-deoxy analog of 225 (333), and A500359 J which bears the C-4′-C-5′-hydrated form of 225 (α-D-taluronic acid, 52).⁸⁵⁶ Inhibition of lysine biosynthesis, via feeding the capuramycin production strain 2-aminoethyl-L-cysteine, led to additional analogs where the signature aminocaprolactam moiety was lacking (A-500359 E, F, F-amide, H), or replaced with acetylcystamine (M-1) (Fig. 39, X = iii) or a thiazepanone moeity (M-2) (Fig. 39, X = iv).^857–859 A-503083 A, B, E, and F also contain disaccharide III composed of 225 and 331.⁸⁶⁰ A-102395, which has a benzene group in place of the caprolactam moiety, is the only analog isolated from a non-Streptomyces species.⁸⁶¹ Additional modifications of capuramycin-type metabolites include ribose 2′-O-acylation and 3′-O-methylation, the former of which has been noted to improve cell permeability.⁸⁶² Members of this class also function as translocase (MraY) inhibitors and display notable anti-tubercular activity but suffer from poor bioavailability. Based upon existing SAR, a core structure containing the modified ribose, an unsaturated hexuronic acid and an aminocaprolactam offers the best activity.

The liposidomycins and caprazamycins from Streptomyces comprise the next subclass and the 38 glycosylated members within this group share a common structural skeleton that is composed of uracil, a modified ribose (330), a corresponding ribose C-5′-appended aminoribose (5′-amino-5′-deoxy-β-D-ribose, 221) and an acylated diazepanone (Fig. 39, X = v). Liposidomycins are subclassified into four types based on the presence or absence of the aminoribose (221) 2′-sulfate and/or the diazepanone side chain C-3 modifications (Fig. 39, X = v).^863–865 Type I contains both groups while type IV lacks both. Type II and III lack the diazepanone glutaryl modification and sulfation, respectively. Caprazamycins resemble type III, and to lesser extent type I, liposidomycins but contain an additional 2′,3′,4′-tri-O-methyl-α-L-rhamnose (46) attached to C-5 of diazepanone-appended methyl-glutaryl group (Fig. 39, appendage v). In A-90289 A and B, C-2′ sulfation of the modified ribose 330 rather than aminoribose 221 is observed.^866,867 Liposidomycins, caprazamycins, and A-90289 also inhibit translocase (MraY) where the type I analogs were found to be most active.^836,868 Synthetic derivatives modified at the ribose C-5′ display improved potency.⁸³⁶

Ezomycins are a group of pyrimidine-based N- and C- glycosidic antifungal compounds isolated from Streptomyces.^869–877 Glycosylation within this subclass occurs at either the N-1 (ezomycins A1 and A2), C-5 (ezomycins B1, B2, C1, C2), or C-8 (ezomycins D1/D2) (Fig. 38) where the predominate glycosylation signature is comprised of a trans-fused furopyranoside 3′,7′-anhydrooctose (236) which is further glycosylated at C-6′ with 3′-amino-3′,4′-dideoxy-β-D-glucuronic acid (235, also called ezoaminuroic acid).⁸⁷⁵ This octosyl nucleoside-ezoaminuroic acid (235) ezomycin core is sometimes referred to as ezomycin disaccharide (VII). Exceptions include ezomycins D₁ and D₂ which carry a 3′-amino-3′,4′-dideoxy-β-D-glucuronic acid-(1→4)-3′-deoxy-3′-ureal-β-D-guluronic acid disaccharide (VIII). Some ezomycins are further substituted via a hexuronic acid C-6′-L-cystathionine amide, a modification important to antifungal activity.

Nikkomycins (also referred to as neopolyoxins)^878–883 and polyoxins^884,885 are peptidyl nucleosides isolated from Streptomyces that contain a pyrimidine aglycon typically comprised of uracil, thymine or, in some cases, 4-formyl-4-imidazolin-2-one. For simplicity, all glycosylated nikkomycins and polyoxins regardless of the base are included herein. Nucleoside N-1-glycosylation with 5′-amino-5′-carboxy-5′-deoxy-β-D-ribose (223) is a predominate feature (Fig. 39) where this hexuronic acid is typically further modified in most nikkomycins by a rare amino acid hydroxypyridylhomo-threonine via a C-6′-amide linkage. Exceptions include nikkomycins S_x/S_z and So_x/So_z, which carry the bicyclic sugars 227 and 228, respectively,⁸⁸² and the uracil C-5-C-glycosides nikkomycins pseudo J/Z. Octosyl acids are also structurally related to nikkomycins and polyoxins but lack the aminohexuronic acid.⁸⁸⁶ Derivatives A and B contain modified pentose 227 while C contains the keto derivative 229. While nikkomycins and polyoxins are potent inhibitors of chitin biosynthesis in fungi and display notable antifungal activity,⁸⁸⁷ their unfavorable physicochemical properties has hampered their clinical development.

Other uracil-based metabolites include the C-5-C-nucleosides malayamycin A and de-O-methylmalayamycin A from Streptomyces, which are inhibitors of fungal sporulation and contain the bicyclic sugar 230 (Fig. 39).^888–890 A bacterial translocase inhibitor, A-94964A contains two unidentified sugars, 2′-deoxy-2′-aminohexosyl-1-phosphate (234) and a hexose (334), attached to a riboocturonic acid (233) appended to N-1 of uracil.^891,892

13.1.2 Cytosine-derived nucleosides

The first main group in this subclass includes amicetins, cytosaminomycins, and SF-2457. This group is comprised of eleven metabolites from Streptomyces, Arthrobacter, and Nocardia,^893–899 most of which contain a N-1-β-disaccharide (I or II) and p-aminobenzamide-modification of the cytosine 4-NH₂. The dissaccharide is comprised of either β-D-amicetose (48) or β-D-olivose (53) attached α-(1′→4′) to either α-D-amosamine (39) or N-demethyl-α-D-amosamine (39). Amicetin inhibits the bacterial peptidyl transferase by binding 23S-like rRNA.⁹⁰⁰ Amicetins were also found to be active against herpes virus 1 and poliovirus⁹⁰¹ cytosaminomycins have been noted for anticoccidial activity.^898,902

Mildiomycins from Streptoverticillium are a group of cytosine N-1-glycosidic peptidyl nucleosides bearing C-2′-C-3′ unsaturated C-4′-N-acyl-β-D-hepturonic acid derivatives (241 and 242) (Fig. 40).^903–905 These molecules have activity against powdery mildews and function as protein synthesis inhibitors via binding the ribosomal large subunit.⁹⁰⁶ The related sugar 240a and corresponding esters are present in the antibacterial arginomycin and related 10381 series from Streptomyces,^907,908 while 240b or its hydrated congener 243 exists as part of the antifungal blasticidins from Streptomyces. Cytosylglucuronic acid bearing a N1-2′-deoxy-β-D-glucuronic acid (249) was isolated from the same strain while cytosyl-N-1-glycosides bearing unique 4′-amino-, keto-, or oxime-functionality (244a, 244b, 245, and 246), were also isolated from bioconversion experiments with this same blasticidin producer.^909–913 Additional studies, in conjunction with enzyme inhibitors, yielded an even extended glycoside diversity to include β-D-glucose (104), 261–265, 136, 72 and 243 attached to pentopyranines.^{910,913–915} Hikizimycin, also referred to as anthelmycin, from Streptomyces is a cytosyl-N-1-β-D-dissacharide (III), the latter of which is comprised of 3′-amino-3′-deoxy-β-D-glucose (kanosamine, 250) linked β-(1′→6′) to an unusual 4′-amino-undecose (hikosamine, 256) (Fig. 40).^916–918 Hikizimycin displays antiparasitic activities and inhibits protein synthesis.⁹¹⁹ The Bacillus metabolites bagougeramines A and B both contain modified cytosyl-N-1-4′-amino-4′-deoxy-β-D-glucuronamides (251, appendage iv) and function as antimicrobials.⁹²⁰ Sugar 251 was also found to be present in mitaimeisu (appendage v),⁹²¹ gougerotin (appendage vi),⁹²² and possibly ningnanmycin;⁹²³ although in the latter case the C-3′-stereochemistry remains undefined (247). Albomycins from Streptomyces are siderophore antibiotics similar to other sideromycins (Section 15.4) are composed of iron chelating portion in addition to a 6′-amino-6′-deoxy-4′-thio-heptofuranose (315) uronic acid moiety (Fig. 40).^924,925 Dapiramicins A and B from Micromonospora are N-glycosidic pyrrolopyrimidines bearing 6′-deoxy-α-D-glucose (215) or β-D-glucose (104), respectively, each capped (β1→4) with 4′-methyl-β-D-glucose (104).^926,927 The α-configured dapiramicin A with disaccharide IV possessed significantly higher activity against the plant disease, sheath blight than the analog bearing V.

Fig. 40.

Cytosine-derived aglycons and associated sugars.

13.2 Purine-derived nucleosides

13.2.1 Adenine-derived nucleosides

There are 32 adenine-based glycosides reported where N-glycosylation is restricted to the adenosyl N-9 and C-6-amine. The adenosyl N-9-amino-pentose 3′-amino-3′-deoxy-α-L-ribose (253) was found in puromycin⁹²⁸ and A-201-A, C, D, and E,^929,930 from Streptomyces (Fig. 41) where the 3′-N-tyrosyl moiety contains additional glycosylation in the A-201 series (335 in A, C, and D; 336 in E – capped with 3′,4′-di-O-methylated α-L-rhamnose 46 in A, C, and E).

Fig. 41.

Purine-derived aglycons and associated sugars.

Sinefungins from Streptomyces are a group of adenosyl-N-9-glycosidic antileshmanial and antitrypanosomal metabolites signified by an unusual ribose-derived 10-carbon sugar, likely formed via a carbon–carbon bond-forming reaction between adenosine and an amino acid.^931–933 Sugar 337 and its amide derivative 339 are present in sinefungin and ureidosinefungin, respectively. Sugar 339 is also present in sinefungin VA with appendage iii attached to the terminal amino group (Fig. 41). Sugar 340 is a cyclic version of 339 and is present in cyclosinefungin. Sugars 252 and 338 are 4′,5′-unsaturated derivatives of 337 and 339, respectively, and are present in other sinefungin derivatives. Dehydrosinefungin V and KSA-9432 contain the same sugar 338 with appendages i and ii, respectively (Fig. 41). These metabolites function as S-adenosylmethionine (AdoMet) mimetics and methyltransferase inhibitors where derivatives with unsaturated sugars were found to generally be less potent.⁹³⁴

Herbicidins are a group of herbicidal compounds isolated from Streptomyces that contain an interesting tricyclic furanopyranoside (254) attached to the adenosyl N-9 (Fig. 41).^935–939 The antitumor compound septacidin from Streptomyces contains a C-4′-amino-modified 4′-amino-4′-deoxy-β-L-glucoheptose (257) connected to C-6 amino group of adenine (Fig. 41, appendage vii). Replacement of the natural sugar with 4′-amino-4′-deoxy- or 4′-amino-4′,6′-dideoxy-L-glucose did not affect activity.⁹⁴⁰ C-4′-amino-modified analogs of the C-2′-epimer (4′-amino-4′-deoxy-β-L-mannoheptose, 255) are found in the antitumor compounds spicamycin and anicemycin from Streptomyces.^941,942 Agrocin 84 is a diglycoside structurally related to sugar phosphate opines used to control crown gall disease in plants.⁹⁴³ Agrocin 84 contains a C-5′-modifed adenosyl-N-3′-deoxy-β-D-arabinofuranose (259) and a corresponding β-D-glucofuranoside (258) linked to the adenosyl C-6-amine via a phosphoramidate bond (Fig. 41 appendage ix). The β-L-gulosamine (260) in the pseudodisaccharide III, present in mitocides C-8030 C, D, and E from Streptomyces, is connected to the C-5′ of the ribose through a cyclohexane ring.⁹⁴⁴ Anhydrothuriniensin isolated from Bacillus contains a disaccharide IV composed of α-D-glucose (18) linked to the β-D-ribose sugar of adenosine.^945,946 Griseolic acids isolated from a Streptomyces species, contain four unique nine carbon bicyclic sugars 341–344 (Fig. 41).^947,948 They were found to inhibit cyclic nucleotide phosphodiesterase.⁹⁴⁹

13.2.2. Guanine and other miscellaneous nucleosides

The sugar α-D-mannose (160) is present in the chitin synthase inhibitor guanofosfocin A from Streptomyces, the only guanine-based glycoside reported.⁹⁵⁰ In this unique metabolite, the mannose serves to bridge the base C-8 and ribose C-5 of guanosine diphosphate (via connections at C-1′ and C-3′, respectively) (Fig. 41).

Miharamycins A and B,⁹⁵¹ amipurimycin^952,953 from Streptomyces belong to atypical purines. All are aminopurine N-9-glycosides where miharamycins contain modified bicyclic D-sugars (238) and amipurimycins the unusual modified D-sugar 239 (Fig. 41).

14. Orthoester glycosides

Orthoester glycosides are a group of metabolites that are characterized by the presence of one or more of acid sensitive orthoester glycosides. A total of 50 orthoester glycoside-containing bacterial metabolites have been reported, 47 of which fall within two major subclasses – orsellinic acids (subclass I) and aminocyclitols (subclass II). Three additional orthoester glycoside-containing outliers can be found within the macrolide (kanglemycin A and swalpamycin, Section 12) and the polyene (polyene SE-73, Section 11) sections.

14.1. Orsellinic acid derivatives

Members of the orsellinic acid subclass of the orthoester glycosides have attracted attention for both their exciting bioactivity and unprecedented oligosaccharide-based structural diversity. Members of this class possess potent activity against antibiotic-resistant Staphylococcus and Enterococci and include the everninomicins, avilamycins, and flambamycin.⁹⁵⁴ These unique metabolites function via inhibiting bacterial protein synthesis through binding at a unique site of the bacterial 50S ribosomal subunit.^955,956 One member of this family, ziracin, advanced to phase III clinical studies but was ultimately discontinued due to side effects and poor pharmacokinetic properties.⁹⁵⁷

Sixteen avilamycins from Streptomyces contain a signature chloroisoeverninic C-4-heptasaccharide ester.^958–960 Additional avilamycin-derived gavibamycins were generated through pathway engineering.^961–964 The heptasaccharide moiety I of avilamycin A (Fig. 42), is composed of two units of β-D-olivose (A and B, 53), 2′-deoxy-β-D-evalose (C, 192), 4′-O-methyl-β-D-fucose (D1, 270), 2′,6′-di-O-methyl-β-D-mannose (E, 156), 2′-O-isobutyryl-α-L-lyxose (F, 111), and the C-4′-branched sugar methyleurekanate (G1, 272). Subtle variants where 272 has been substituted with 270 (β-D-fucose, avilamycin A1 and gavibamycin O), 273 (gavibamycins A3, B3, J3, K3, L3, E3, H3, I3) or 274 (avilamycin L) have also been reported. In addition, analogs where β-D-fucose (D1, 270) has been replaced by 4′-O-methyl-β-D-arabinose (D2, 265; avilamycin M) or β-D-galactose (D3, 161; avilamycin K) are also known (Fig. 42).

Fig. 42.

Orsellinic acid aglycons and associated sugars.

The closely related everninomicins^965–968 from Micromonospora include both ziracin (evernimicin) and a series of SCH-compounds identified by Schering-Plough.^969–971 Everninomicins are distinguished by an additional sugar (C-3′-nitrosugar α-L-evernitrose, H1, 127 or aminosugar H2, 280) attached to C-3′ of sugar A to provide octasaccharide II. Analogs that contain reduced nitro sugar such as the hydroxylamino (H3) or nitroso (H4) derivatives have also been obtained by reduction during structure elucidation or derivatization and have shown comparable antibacterial activity.⁹⁶⁷ A second delineation stems from the replacement of the avilamycin sugar G (β-D-methyleurekanate) by β-D-eurekanate 265 (everninomicin C, ziracin, SCH49088, SCH58775) (Fig. 42) or the C-4′-branched sugar 271 (everninomicins B and D, 13385-1).⁹⁷² In some everninomicins, 2′-deoxy-β-D-evalose (C1, 192) is replaced by 6′-deoxy-3′-C-methyl-β-D-mannose (C2, 268). The hydrolytic products (sporacuracins and Sch-58777 which contain only sugars E, F, and G)⁹⁷³ lack antibacterial activity⁹⁷⁰ while removal of the nitro sugar has no major effect.^958,974

14.2. Aminocyclitol derivatives

Members in this class are also considered aminoglycosides (Section 1) and include destomycins,^975–977 hygromycin B,⁹⁷⁸ RH5012 C,⁹⁷⁹ and SS-56-C⁹⁸⁰ from different bacteria including Streptomyces, Streptoverticillium, and Saccaropolyspora (Fig. 43). The aglycon in this subclass contains an aminocyclitol moiety derived from D-streptamine (hyosamine, P7) bearing a C-5 glycoside comprised of either β-L-talose (144) or β-D-mannose (156). The signature orthoester glycoside linkage occurs between the C-2′/C-3′ and the anomeric carbon of the 7-membered amino sugar, destomic acid 275 (Fig. 43). It is worth noting that analogs which lack destomic acid (and thus, the orthoester linkage) are inactive.

Fig. 43.

Sugars associated with orthoester aminocyclitols.

15. Peptides

Glycosylated peptides represent a large and clinically-important group of bacterial metabolites and its members invoke a broad range of activities particularly relevant to anti-cancer and anti-infective development. For the scope of this discussion, glycosylated peptides have divided into distinct sections covering bleomycins, vancomycins, mannopeptimycins, salmochelins and salmycins, linocosamides, thiazoles, lipoglycopeptides, miscellaneous antiinfective peptides and miscellaneous (other) members.

15.1. Bleomycins

Bleomycins are a group of cytotoxic glycopeptides from Streptomyces that bind both DNA and a metal [typically Cu(I) or Fe(II)] and invoke oxidative DNA strand cleavage.^981–986 While members of this family (Blenoxane^®) have been used with great success clinically to treat malignant lymphomas,^478,986 dose-dependent pulmonary fibrosis and drug resistance restrict broader application. In conjunction with the structurally-related metabolites phleomycins,^987,988 tallysomycins,^989,990 zorbamycin,^991,992 cleomycins,^993,994 LL-BO 1208,⁹⁹⁵ and platomycin,⁹⁹⁶ the total number of glycopeptides in this class is 69.⁹⁸⁵ Structure divergence among members derive primary from subtle variations within the peptide linker, oxidative state of the bithiazole (BTT), the terminal substituted amide^{58,990,997,998} and, the presence (as in tallysomycins) or absence of a second point of glycosylation (Fig. 44). All members contain a conserved O-disaccharyl-substituted β-hydroxyhistidine, the disaccharide composition of which is 3′-O-carbamoyl-α-D-mannopyranosyl-(1′→2′)-α-L-gulose (I) in most [the exception being 3′-O-carbamoyl-α-D-mannopyranosyl-(1′→2′)-6′-deoxy-α-L-gulose in zorbamycin, II].⁹⁹⁹ Tallysomycins differ from other members via an additional O-glycosylcarbinolamide bearing 4′-amino-4′,6′-dideoxy-α-L-talose (279) at C-13 of the bithiazole moiety (Fig. 44). Zorbamycin-bleomycin disaccharyl hybrids have also been constructed via strain engineering.¹⁰⁰⁰ The precise role of the disaccharide remains controversial and hypotheses include improving DNA affinity and cleavage^{985,1000,1001} as well as extracellular recognition and uptake.^1002,1003 In support of the latter, the disaccharide moiety was demonstrated to target tumor cells possibly by a specific ATP-dependent uptake mechanism.^1002,1003

Fig. 44.

Bleomycin aglycons and associated sugars. Platomycin structure is based on partial structure reported.

15.2. Vancomycins

Vancomycins are cyclic glycopeptides comprised of seven amino acids, the heptapeptide core of which presents two 16-membered and one 12-membered ring. Members are classified into four different subclasses based on the heptapeptide core and cumulatively, there are 133 corresponding bacterial glycopeptide members isolated from different bacteria including Streptomyces, Nocardia, Pseudonocardia, Amycolatopsis, Actinoplanes, Kibdelosporangium, Micropolyspora, Saccharothrix, Actinomadura.¹⁰⁰⁴ Type I glycopeptides contain two aliphatic (residues 1 and 3) and five aromatic amino acids, while types II–IV contain seven aromatic amino acids (Fig. 45). Type III differs from type II by an additional ether-bridged ring system between residues 1 and 3 while type IV (generally referred as lipoglycopeptides) differs from type III via N-acyl-substitution of appended sugars. Other compounds that are structurally related to vancomycins but lack the sugar include the complestatins,^1005–1007 chloropeptins,¹⁰⁰⁸ and A-47934.¹⁰⁰⁹ Each of the previous types, with a particular focus upon glycosylation, is further elaborated herein.

Fig. 45.

Aglycons of the five types of vancomycin. The ether bridge in types III and IV is highlighted in red. Locataions of attachment to most sugars in CWI-785 series is not determined.

Vancomycin from Amycolatopsis is the prototypical member of the type I sublcass. First discovered in the mid-fifties, the structure of this metabolite was not elucidated until 30 years later.^1010–1012 Vancomycin is a potent antibiotic to treat life-threatening Gram-positive infections and inhibits bacterial cell wall biosynthesis by binding peptidoglycan L-Lys-D-Ala-D-Ala.^1013–1015 In addition to vancomycin, there are 37 glycosylated type I metabolites. The predominate site of O-glycosylation within type I–IV members is the phenolic hydroxyl of residue 4 (hydroxyphenylglycine, HPG-4, Fig. 45). To a lesser extent, O-glycosylation of the substituted β-hydroxy-L-tyrosine (Tyr-6) is also observed among type I–IV members. HPG-4 O-glycosylation is comprised of a β-D-glucose (104, devancosamine vancomycin, M43C, most balhimycins, and chloro-orienticins B and E) or an α-L-sugar-(1′→2′)-β-D-glucose disaccharide (I–IV) where the L-sugar varies among members and includes: α-L-vancosamine (118, vancomycins)^1016–1018 4′-oxo-α-L-vancosamine (281, balhimycin V, A-83850);^1019–1021 4′-epi-α-L-vancosamine (280, eremomycin, orienticins, and chloro-orienticins A and D)¹⁰²² and its N-carboxymethyl derivative (280, eremomycin B);¹⁰²³ or α-L-rhamnose (46, decaplanin, A-42867, MM 47761, and MM 49721) (Fig. 46).¹⁰²⁴ Sugars employed for residue 6 tyrosine (Tyr-6) β-hydroxy O-glycosylation within type I members include: ureido-α-L-vancosamine (282, ureidobalhimycin);^1019–1021 4′-oxo-α-L-vancosamine (281, balhimycins);¹⁰²¹ 4′-epi-α-L-vancosamine (280; eremomycin,¹⁰²² orienticins, A, C, D, chloro-orienticins, decaplanin, MM 47761, MM 49721¹⁰²⁵); α-L-olivose (184, orienticin B);^1026,1027 or α-L-vancosamine (118, MM 49727, A-42867) (Fig. 46).¹⁰²⁴ OA-7653^1028,1029 from S. hygroscopicus is unique among type I members as it contains α-D-glucose (18) attached to the residue 6 tyrosine-β-hydroxyl but, similar to chloro-orienticins C and F, lacks residue 4 phenolic hydroxyl glycosylation. Importantly, the broad substrate specificity of the residue 4 phenolic glucosyltransferase GtfE has been exploited to generate a wide range of differentially glycosylated analogs.^1030–1032

Fig. 46.

Sugars associated with vancomycins.

There are 21 metabolites classified as type II including avoparcins,^1033,1034 galacardins,¹⁰³⁵ helvecardins,¹⁰³⁶ chloropolysporins,¹⁰³⁷ and actinoidins^1038–1041 from Streptomyces, Nocardia, Pseudonocardia, Micropolyspora, Saccharothrix, and Nonomuraea. Variant glycosylation is observed among type II metabolites with heavily glycosylated members containing maximally seven sugars at five glycosylation positions (galacardins) while sparsely glycosylated members contain just a single monosaccharide (demannosyl-A-40926). Of these, only avoparcins, galacardins, and actinoidins contain the residue 4 phenolic β-D-glucose (104, chloropolysporins and deristosaminyl-avoparcins) or α-L-sugar-(1′→2′)-β-D-glucose disaccharyl signature (IV–VI) typically observed among type I members where the terminating sugar can be α-L-rhamnose (46; actinoidin A2), α-L-ristosamine (45; avoparcins, galacardins and helvecardins), or α-L-actinosamine (124; actinoidins A and B, MM 47766, MM 47767, MM 55256, MM 55260) (Fig. 46). Like type I members, residue 6 tyrosine-β-hydroxyl O-glycosylation is also prevalent among type II members and includes α-L-ristosamine (45; avoparcins, galacardins, helvecardins, chloropolysporins) or 4′-O-methyl-α-L-actinosamine (124, actinoidins, MM 47766, MM 47767, MM 55256, MM 55260, MM 55261). Distinct from type I members, O-glycosylation of the residue 2 tyrosine-β-hydroxyl and/or phenolic hydroxyl of residues 1, 3 and/or 7 is also observed among type II members. Type II member residue 1 phenolic hydroxyl O-glycosylation employs two sugars α-L-rhamnose (46, chloropolysporins, helvecardin B, avoparcins) or a disaccharide α-L-rhamnosyl-(1′→4′)-α-D-galactose (VII, galacardins), while a single sugar, α-D-mannose (160), is observed for residue 2 tyrosine-β-hydroxyl O-glycosylation (avoparcins, galacardins, chloropolysporins, helvecardin A) (Fig. 46). An α-D-mannose (160) or β D-mannose (156) are observed at residue 7 phenolic O-glycosylation (actinoidins, MM 47766, MM 47767, MM 55260). Finally, residue 3 phenolic O-glycosides are comprised of α-D-galactose (286, avoparcins and galacardin A). With respect to activity/use, the use of avoparcins as animal growth promoters was banned due to reports of increased vancomycin resistance¹⁰⁴² while helvecardins were demonstrated to inhibit Neisseria gonorrhoeae in addition to Gram positive bacteria.¹⁰³⁶

There are 23 metabolites classified as type III including ristocetins,^1043–1046 A41030 C, F, and G,¹⁰⁴⁷ actaplanins,^1048–1051 UK-69542, and UK-68,597¹⁰⁵² from Actinoplanes, Nocardia, and Streptomyces species. Variant glycosylation is observed among type III metabolites with heavily glycosylated members containing maximally six sugars at three glycosylation positions (ristocetin A) and sparsely glycosylated members containing just single monosaccharides (A41030C). O-glycosylation of the residues 1, 3, 4, and/or 7 phenolic hydroxyls and/or the residue 6 tyrosine β-hydroxyl is observed among type II metabolites. Residue 1 phenolic hydroxyl O-glycosylation is only observed among A41030 metabolites with β-D-galactose (161) or a β-D-galactosyl-(1′→4′)-β-D-galactose disaccharide (VIII). Likewise, only a single sugar is observed [D-mannose, α-160, β-156] as residue 3 (actaplanins) and/or residue 7 (actaplanins, ristocetin A) phenolic hydroxyl O-glycosyl substituents. Wider variation is observed among the residue 4 phenolic hydroxyl O-glycosylation patterns. For example, actaplanins B2, C3, G, O, and brominated actaplanins contain the single sugar β-D-glucose (104) while other type III metabolites contain D-hexosyl-(1′→2′)-β-D-glucose (IX, X, I) disaccharyl substitution reminiscent of type I/II metabolites where the terminal sugar is either D-mannose (α-160, β-156) or α-L-vancosamine (118, UK-68597). Other observed disaccharides at this position include α-L-rhamnosyl-(1′→4′)-β-D-glucose (XI, actaplanins) and α-L-rhamnosyl-(1′→6′)-β-D-glucose (XII, actaplanins) (Fig. 46). Alternatively, in ristocetin A the α-D-mannosyl-(1′→2′)-β-D-glucose disaccharide is further modified at the glucose C-6′ by α-L-rhamnose (46) and at the mannose C-2′ by α-D-arabinose (204), forming tetrasaccharide XIII. Like type I/II metabolites, residue 6 tyrosine β-hydroxyl O-glycosylation in type III is observed to engage uniquely functionalized sugars including α-L-ristosamine (45; ristocetins and actaplanins) and the 2′,3′,6′-trideoxy-α-L-hexose 44 of UK-69542 (Fig. 46). Regarding activity/use, ristocetins were approved for clinical use but were subsequently removed due to thrombocytopenia and platelet agglutination.^1043–1045 While the corresponding mechanism remains unclear, ristocetin causes von Willebrand factor to bind the platelet receptor glycoprotein Ib (GpIb), and is now used solely in the context of in vitro assays for the diagnosis of conditions such as von Willebrand disease (vWD) and Bernard–Soulier syndrome. Semisynthetic analogs, including those deriving from differential glycosylation, have revealed derivatives with improved antibacterial/antiviral potency, and/or less toxicity.

Type IV is the largest subclass with 46 members including teicoplanins,^1053–1056 RS1-4,¹⁰⁵⁴ kibdelins (AAD-609),¹⁰⁵⁷ aridicins,¹⁰⁵⁸ A-40926 analogs, parvodicins,^1059–1061 and MM 49728, MM 55266, and MM 55268¹⁰⁶² from Actinoplanes, Actinomadura, Kibdelosporangium and Amycolatopsis. CWI-785 A-C (symonicins) are comprised of a similar aglycon where methionine has replaced dihydroxyphenylglycine (residue 3).¹⁰⁶³ O-glycosylation of the residues 4, 5, and/or 7 phenolic hydroxyls and/or the residue 6 tyrosine β-hydroxyl is observed among type IV metabolites. Residue 4 phenolic O-glycosylation has been observed with various monosaccharides including N-acyl-2′-amino-2′-deoxy-β-D-glucosamine (153; teicoplanins, RS1-4, kibdelins, AAD-609), N-acyl-2′-amino-2′-deoxy-β-D-glucuronic acid (283, aridicins, parvodicins, A-40926, A-84575A), β-D-mannose (156; MM 49728, MM 55266, MM 55268, MM 56597), or a disaccharide β-D-mannosyl-(1′→2′)-β-D-glucose (X, A-39893) (Fig. 46). It is noteworthy that teicoplanins (approved an an antibacterial, Targocid^®) and RS1-4 isolated from the same strain differ mainly in their sugar 2′-N-acyl substitution and this served as inspiration for the clinically approved semi-synthetic analogs such as dalbavancin (Dalvance^™). MM 49728, MM 56597, MM 56598, MM 55266 and MM 55268 are the only type IV members that contain residue 5 phenolic O-glycosylation (β-D-glucose, 104) while some variation in the sugars employed for residue 6 tyrosine β-hydroxyl O-glycosylation is observed [N-acyl-β-D-glucosamine (153), teicoplanins; 2′-amino-2′-deoxy-α-D-glucuronic acid (285), MM 49728, MM 55266, and MM 55268; α-L-ristosamine (45), CWI-785 A-C (symonicins), A-39893]. Teicoplanins, kibdelins (with the exception of AAD-609D), and aridicins also carry an α-D-mannose (160) as a residue 7 phenolic hydroxyl modification. CWI-785 A-C (symonicins) also contain β-D-glucose (104), α and β-D-mannose (160 and 156, respectively), and α-L-rhamnose (46), the position of attachment for which has not been confirmed.¹⁰⁶³

15.3. Mannopeptimycins

Mannopeptimycins α-ε are hexapeptides made from D-tyrosine(Tyr-1), β-methylphenylalanine (MePhe-2), glycine (Gly-3), L-serine (Ser-4), and 2 units of β-hydroxyenduracididines (HyEnd-5 and HyEnd-6) (Fig. 47).¹⁰⁶⁴ All members are N-glycosylated with α-D-mannose (160) at the N-5-terminus of HyEnd-5. In addition, all members except mannopeptimycin β contain a common α-D-mannosyl-(1′→4′)-α-D-mannose (disaccharide I) appended to Tyr-1 phenolic hydroxyl group (C-6). Analogs γ, δ, and ε contain a C-2′, C-3′, C-4′-isovaleryl (Fig. 47, appendage i) modification of the terminal mannose (160), respectively. Like vancomycins, mannopeptins inhibit bacterial cell wall biosynthesis and display potent Gram positive antibacterial activity. The presence of the isovaleryl moiety (i) is important for antibacterial potency possibly by virtue of improving cell membrane penetration.

Fig. 47.

Mannopeptimycin, salmochelin, and salmomycin aglycons and associated sugars.

15.4. Salmochelins and salmycins

Salmochelins are glycosides of the catecholate-type enterobactins from Salmonella which function as siderophores for metal sequestration.^1065,1066 Four salmochelins have been characterized including SX (monomer), S1 (dimer), S2 (linear trimer) and S4 (cyclic trimer) where each monomeric unit is comprised of a L-serine (Ser) attached to a dihydroxybenzamide (Bnz). Salmochelins exist as C-glucosides (β-D-glucose, 104) of the dihydroxybenzamide core structure (Fig. 47). Salmycins, hydroxamate-containing siderophores from Streptomyces violaceus, are glycosylated danoxamins at the C-1 position with disaccharides II or III.^1067,1068 Disaccharide II present in salmycins A and D consists of a unique 6′-methylamino-D-heptopyranose (288) attached at the C-2′ position to the rare 2′-hydroxyimino-α-D-glucose (289). Disaccharide III present in salmycins B and C contains the α-D-keto sugar 2′-oxo-α-D-glucose (287) in place of the hydroxylimino sugar (289) (Fig. 47). This disaccharide is attached to the aglycon via an ester bond to C-6′ of the modified glucosyl unit. Salmycins were found to be strong inhibitors of the growth of Staphylococci and Streptococci and are therefore considered siderophore antibiotics (sideromycins).

15.5. Lincosamides

Lincosamides from Streptomyces are a group of clinically important antibacterials that target Gram positive and anaerobic bacteria as well as some protozoa through inhibition of protein synthesis. Including the lincomycins first discovered in 1963 (which serve as the basis for the semi-synthetic drug clindamycin, Dalacin^®), 29 naturally-occurring members have been reported from Streptomyces species including celesticetins and U-57930 analogs.^1069–1074 Their core structure is composed of a unique thioglycoside with variable substitution of the anomeric thioether and the 6′-prolylamide (Fig. 48). The anomeric stereochemistry of the central sugar, 6′-amino-6′,8′-dideoxy-1′-thio-D-erythro-α-D-galactose (methylthio-lincosamide, 290) (Fig. 48) is critical for antibacterial activity and oxidation of the thioether (to a sulfoxide) is also detrimental to function.^1075,1076 Additional derivatives have been obtained via precursor-directed feeding^1077–1081 including ribonucleotide derivatives where the proline has been replaced with a 4-ethyl-piperidine moiety.¹⁰⁸²

Fig. 48.

Lincosamide and thiazolyl peptide aglycons and associated sugars. Indole moiety in red is absent or replaced by quinoline in different members.

15.6. Thiazoles

Thiazolyl peptides contain one or more signature thiazoles embedded within their macrocyclic architecture. In spite of their potent antibacterial activity members of this family have not advanced to the clinic due to poor biodistribution and pharmacokinetic properties. Currently 21 thiazole glycosides have been reported from different Actinomycetes including Micromonospora, Amycolatopsis, Nocardia, Actinoplanes, and Amycolata. Regiospecificity of glycosylation among the thiazomycin-type members (which includes thiazomycins,^1083,1084 nocathiacins,^1085–1087 glycothiohexides,^1088,1089 philipimycins¹⁰⁹⁰ and S-54832-A series¹⁰⁹¹) is restricted to the hydroxyl group of a modified glutamate residue (Glu), the C-4 of a thiazole ring (Thz-3) or a C-3 phenolic substitution of the unique 2,5,6-trithiazolyl-3-hydroxypyridine (Pyr) (Fig. 48). In contrast, SCH 40832 glycosylation occurs at the Thr-1 side chain hydroxyl (disaccharide II composed of two units of β-D-olivose 53 connected 1′→3′)¹⁰⁹² (Fig. 48). The sugars found within thiazomycins, nocathiacins, and glycothiohexides include 2′,4′-dideoxy-4′-dimethylamino-3′-C-methyl-α-L-fucose (292a), oxazolidine-substituted 2′,4′-dideoxy-4′-amino-3′-C-methyl-α-L-fucose (292b), β-L-chromose 186, 4′-deoxy-2′,3′-di-O-methyl-β-D-glucose 291 or β-D-quinovose (140) (Fig. 48). The related philipimycin I contains a trisaccharide [trisaccharide I comprised of 2′,3′-dimethoxy-α-L-rhamnosyl-(1′→4′)-β-D-amicetosyl-(1′→4′)-2′,3′-dimethoxy-α-L-rhamnose].¹⁰⁹⁰ The degradation product of philipimycin I (philipimycin II) contains only the monosaccharide 2′,3′-dimethoxy-α-L-rhamnose (46). In general, thiazole glycosylation was found to improve both solubility and antibacterial potency.

15.7. Lipoglycopeptides

The lipoglycopeptides presented herein are macrocyclic peptides bearing one or more signature lipophilic side chains and are separated for discussion by glycosides of the peptide core backbone and glycosides of lipid side chains. While most members in this group display antifungal activity (hassillidins from Hassallia,^1093,1094 occidiofungins¹⁰⁹⁵ and burkholdine¹⁰⁹⁶ from Burkholderia, cepacidines from Pseudomonas,^1097,1098 herbicolins from Erwinia^1099,1100 and W-10β from Aeromonas¹¹⁰¹), the ramoplanins from Actinoplanes stand out as agents recently approved by the FDA to treat C. difficile (Fig. 49).^1102,1103 Ramoplanins are phenolic O-disaccharyl-substituted metabolites, the disaccharide I of which is comprised of two α-D-mannose (160) monomers. Ramoplanins display broad antibacterial activity via inhibition of cell wall biosynthesis however; serum instability restricts current use to oral applications. The cyclic lipopeptide hassallidins are also peptide core O-mannosides, both containing N-methylthreonine (MeThr-9) C-3-O-β-D-mannose (156) and hassallidin B having an additional threonine (Thr-2) C-3-O-α-L-rhamnose (46) (Fig. 49).^1093,1094 Both the mono and diglycosylated analogs of hassallidins presented comparable antifungal activity against Aspergillus and Candida. Finally, W-10β isolated from Aeromonas is a threonine residue (Thr-1) C-3-O-β-D-glucoside (104) with antifungal activity (Fig. 49).¹¹⁰¹

Fig. 49.

Lipoglycopeptide aglycons and associated sugars. Only partial structures of burkholdine, cepacidins, and herbicolin are shown.

The octapeptides occidiofungins¹⁰⁹⁵ and burkholdine¹⁰⁹⁶ also possess broad antifungal properties and display both core peptide and side chain O-glycosylation. Occidiofungins are C-3-O-β-D-xylosides (136) of a deaminated lysyl monomer (Fig. 49) while burkholdines are C-7-O-D-xylosides (136 or 293, anomeric stereochemistry undetermined) of the appended lipid side chain. Importantly, the corresponding glycoside displayed 4–8 fold increased antifungal potency compared to the aglycon burkholdine.¹⁰⁹⁷ Lipid side chain O-D-xylosyl modification (136 or 293, anomeric stereochemistry undetermined) was also observed in the antifungal metabolites cepacidines A₁ and A₂ (Fig. 49), where glycosylation occurs at C-5 of the 5,7-dihydroxy-3,9-diaminooctadecanoic acid.^1097,1098 Finally, the antifungal lipodepsinonapeptide herbicolin A also displays lipid side chain O-glycosylation with α-D-glucose (18) found at the C-3 position of the hydroxytetradecanoic acid residue.^1099,1100

15.8. Miscellaneous anti-infective peptides

Miscellaneous glycopeptides with reported antibacterial and antifungal activities that fall outside the scope of those previously discussed are combined within this subsection, the predominant representation of which are polyamine peptides. The first group of compounds are comprised of an acylated spermidine (glycocinnaspermicin D and cinodines from Nocardia)^1104,1105 or 3-aminopropanamidine (coumamidines from Saccharopolyspora)^846,1105 conjugated to a cinnamoic acid moiety, the latter of which is glycosylated at C-7 with unique di- or trisaccharides (Fig. 50). The corresponding disaccharide I and trisaccharides II–IV are comprised of the core sugars 2′,4′-diamino-2′,4′,6′-trideoxy-α-D-glucose (295), 2′-amino-2′-deoxy-β-D-xylose (173), 2′-amino-2′-deoxy-α-D-xylose (24) and 2′,3′-diamino-2′,3′-dideoxy-α-L-pentose (296a–296c) (Fig. 50). Unique features among this series include the high degree of C-2′, C-3′ and/or C-4′ carbamidyl/guanidinyl modification of the corresponding sugar monomers and the carbamide-glycosyl linkage exclusive to this family of metabolites. These compounds exhibited broad spectrum antibacterial activity. Other polyamine antibacterial compounds include the cyclitol antibiotics LL-BM-123α and LL-BM-782 series from Nocardia (Fig. 50). They are Gram negative bacterial growth inhibitors that contain a pseudotri- and pseudodisaccharide, respectively (V and VI).^1106,1107 The pseudotrisaccharide V in LL-BM-123α consists of an arginine N-linked to the C-2′ of α-D-glucosaminyl-(1′→4′)-β-D-mannose conjugated to a modified myoinosamine (P20). The corresponding sugar of pseudodisaccharide VI in LL-BM-782 series is 3′-guanidino-β-mannose (42) which is conjugated to the aglycon via a modified myoinosamine (P12). Both LL-BM-123α and LL-BM-782 compounds displayed in vivo efficacy against Gram negative bacteria where LL-BM-782 was more potent but also more cytotoxic.

Fig. 50.

Miscellaneous antiinfective peptide aglycons and associated sugars.

Non-polyamine metabolites include SQ 28504 and SQ 28546 from Chromobacterium,^964,965 formadicins from Flexibacter,^1108,1109 and theopalauamide and theonegramide from bacterial symbionts of Theonella.^1110–1112 SQ 28504 and SQ 28546 are C-4 bulgecinine (Blg-1) and C-3 threonine (Thr-3) di-O-glycosides [N-acetyl-β-D-glucosamine (153)] where the Blg-1-appended sugar is C-4′-O-sulfated (Fig. 50). Both compounds potentiate the activity of β-lactam antibiotics. Formadicins are β-lactam antibiotics, two of which (formadicins A and B) are C-9-O-β-D-glucuronidates (72) (Fig. 50). Glycosylation is notably rare among naturally occurring β-lactams and these unique metabolites displayed antibacterial activity againts Pseudomonas and Proteus species where glyco-sylation contributed to reduced potency. Finally, the anti-fungals theopalauamide and theonegramide are histidine (His) N-3 N-glycosides of β-D-galactose (161) and a β-D-arabinose (294), respectively.^1110–1112

15.9. Other peptides

Miscellaneous peptides that lack anti-infective related activity are summarized herein. These include aeruginosides,¹¹¹³ aeruginosins,¹¹¹⁴ muraceins,¹¹¹⁵ enkastines,¹¹¹⁶ banyasides,¹¹¹⁷ phosphoramidon,¹¹¹⁸ GlcNAc-TA,¹¹¹⁹ mycothiols,^47,1120 bacillithiols,¹¹²¹ and alanopine¹¹²² from Streptomyces, Nostoc, Nocardia, Planktothrix, Bacillus, Staphylococcus Providencia, Proteus and Oscillatoria (Fig. 51). Aeruginosides and aeruginosins share a common peptide skeleton [composed of 2-carboxy-6-hydroxyoctahydroindole (Choi), leucine (Leu) or hydroxyleucine (HLeu), phenyl lactic acid (Plac), and agmatin (Agm) or 1-amino-2-(N-amidino-Δ³-pyrrolinyl)ethyl (Aeap)], a common sugar (α-D-xylose, 293) but distinct regioselectivity of glycosylation (aeruginosides, Choi C-6; aeruginosins, HLeu C-3; Fig. 51).^1113,1114 Banyaside A and B are comprised of 1-amidino-3-(2-aminoethyl)-3-pyrroline (Aaep), an azabicyclononane (Abn) C-7-O-α-D-glucoside (18), D-leucine (Leu), and a 2-O-methylglyceric acid-3-O-sulfate (MGAS).¹¹¹⁷ They differ by subtle modifications of the appended glucose and, like aeruginosins, are potent inhibitors of trypsin and thrombin.

Fig. 51.

Miscellaneous peptide aglycons and associated sugars.

Muraceins are a group of N-acetylmuramyl peptides composed of alanine (Ala), glutamic acid (Glu), diaminopimelic acid (Dap), and, in the case of muracein C, serine (Ser).¹¹¹⁵ Glycosylation of muraceins is afforded via an amide bond between the N-terminus Ala α-amine and the C3′-side chain carboxylate of N-acetyl muramic acid (153a, Fig. 51). They inhibit angiotensin-converting enzyme (ACE) and have been put forth as potential antihypertensives.

Enkastines I–IV are dipeptides composed of an N-terminal hydrophobic amino acid [isoleucine (Ile) or valine (Val)] and an acid amino acid [aspartic acid (Asp) or glutamic acid (Glu)]. All exist as N-terminal N-glycosides of 1′-deoxyfructopyranose (201) and inhibit enkephalinase endopeptidase.¹¹¹⁶ Phosphoramidon, another dipeptide endopeptidase inhibitor, consists of a C-terminal tryptophan (Trp) and and N-terminal leucine (Leu), the N-terminal amine of which is uniquely connected to α-L-rhamnose (46) via a phosphoramidate bridge.¹¹¹⁸ GlcNAc-TA is a prenylated indolactam C-14-O-glycoside (N-acetyl-β-D-glucosamine, 153).¹¹¹⁹ This is the only naturally-occurring teleocidin glycoside and is a small molecule activator of protein kinase C and induces the release of excitatory neuropeptides.

Examples of amino acid glycosides include mycothiols,^47,1120 bacillithiol,¹¹²¹ and alanopine. Mycothiols are constructed of N-acetyl or N-propionyl cysteine (Cys) modified at C-1 with pseudodisaccharide I (composed of α-D-glucosamine 5 and D-myoinositol P12). In a similar manner, bacillithiol is a 1′-(2-butanedioic acid)-α-D-glucosamine (5)-conjugated L-cysteine. Both mycothiols and bacillithiol protect against oxidative stress.¹¹²³ Alanopine, a component of bacterial lipopolysaccharide, is an N-carboxyethylalanine (CEAla) attached to 4′-amino-4′-deoxy-D-quinovose (α, 39; β, 202), the anomeric configuration for which was not established.¹¹²²

16. Phenazines

Phenazines comprise a large group of nitrogen-containing heterocyclic bacterial metabolites that display a range of activities including antibacterial, antitumor, antimalaria, and antiparasitic functions.¹¹²⁴ Over 140 of different phenazine metabolites have been reported from bacteria, only 13 of which are glycosylated (Fig. 52).¹⁸ Regiospecificity of O-glycosylation among these metabolites is restricted to the 4- and 6-positions of the phenazine ring as well as the less common glycosyl esters which occur at a phenazine 6-carboxylic acid. Only three sugars were observed among the corresponding glycosides where α-L-quinovose (194) and β-L-quinovose (297), a sugar common to saponin glycosides,¹¹²⁵ were found as sugars with free reducing termini attached as sugar 2′/or 3′-esters of the phenazine C6-carboxylic acid [phenazine α-L-quinovose 2′-ester (194, C-6), phenazine α-L-quinovose 3′-ester (194, C-6), phenazine β-L-quinovose 2′-ester (297, C-6) and phenazine β-L-quinovose 3′-ester (297, C-6)] in metabolites of a marine Streptomyces.¹¹²⁶ The remaining glycosylated phenazines (aestivophoenins A-C,^1127,1128 izuminosides A-C,¹¹²⁹ and phenazoviridin^1130–1132 from Streptomyces) are mono-glycosylated with α-L-rhamnose (46) at the phenazine C-4- or C-6-position, or as anomeric glycosyl esters of the phenazine 6-carboxylic acid similar to that described above (Fig. 52). Izuminosides B and C were reported to have a synergistic activity in sensitizing tumor necrosis factor-related apoptosis-inducing ligand “TNF-RAIL”-resistance AGS cells.¹¹²⁹

Fig. 52.

Phenazine aglycons and associated sugars.

17. Piericidins and pyranones

Piericidins are prenylated polyoxypyridines with variant functions that include general cytotoxicity and insecticidal activity. More than 38 naturally occurring piericidins and closely related pyranones have been reported from Streptomyces, 11 of which are piericidin C-3a or C-10 O-monoglycosides (Fig. 53).¹⁸ Three different monosaccharides have been observed among glycoconjugates of this class including β-D-glucose (104), α-L-rhamnose (46) and 2′-epi-β-D-fucose (also known as 6′-deoxy-β-D-talose, 298). Of these, β-D-glucose (104) is the most predominant, found in 6 of the 11 naturally-occurring members of this class including glucopiericidins A-C,^{236,1133–1135} 13-hydroxyglucopiericidin A,¹¹³⁶ and glucopiericidinols A₁ and A₂.¹¹³⁴ In contrast, the C-3a-appended α-L-rhamnose (46) was only observed in 3a-rhamnopiericidin A1 (SN-198-C),^1137,1138 and 7-demethyl-3a-rhamnopiericidin A1 (SN-l98-B),¹¹³⁹ while the C-3a attached 2′-epi-β-D-fucose (298) was observed only in 3-deoxytalopiericidin A1.¹¹⁴⁰ Additionally, the two glucopyranone (PM-050511 and PM-060431) metabolites of a marine Streptomyces display C-10-O-β-D-glucosyl (104) regiospecificity reminiscent of corresponding glucopiericidin.¹¹⁴¹ Importantly, glycosylation generally correlates with improved antimicrobial properties and reduced general in vivo toxicity.^{1133–1135,1137,1139–1141}

Fig. 53.

Piericidin and pyranone aglycons and associated sugars.

18. Pluramycins

The 4H-anthra[1,2-b]pyran-4,7,12-trione core is a characteristic feature of the pluramycin antitumor antibiotics produced by Streptomyces, Actinomyces, Actinomadura and Saccharotrix.^1142,1143 To date, 45 pluramycin glycosides have been reported (Fig. 54) and a unique signature of most pluramycin members is the presence of C-8- and/or C-10-C-glycosides. C-2- and/or C-5-side chain O- and/or C-glycosylation has also been observed and two metabolites (clecarmycins)¹¹⁴⁴ distinctly contain C-9-C-glycoside substitution. Those members that present the prototypical C-8-/C-10-C-glycosyl archetype include hedamycin,^1145–1148 DC92-B and DC92-D,^1149,1150 kidamycin and epoxy-kidamycin (largomycin FII chromophore component 4),¹¹⁵¹ PD 121222,¹¹⁵² ankinomycin,^{1145,1146,1153–1155} altromycins,^1156–1159 saptomycins,^1160–1162 neopluramcin and pluramycin A,^1163–1165 pluraflavins,¹¹⁶⁶ A51493A,¹¹⁶⁷ rubiflavins¹¹⁶⁸ and chromoxymycin,^1169–1171 the latter of which has a uniquely modified aglycon core at C-12. Eighteen monosaccharide units are represent among pluramycin glycosides where N-alkyl aminosugars are a predominate feature known to be critical for pluramycin metabolite DNA interaction and cytotoxicity.^{940,1153,1168,1172–1177}

Fig. 54.

Pluramycin aglycons and associated sugars.

Among the sugars found as C-8-C-glycosides, β-D-anglosamine (59) is most common with 3′-epi-α-D-anglosamine (69; SS 21020B) and Δ^1,2-D-anglosamine (306; DC92-D) also observed. Of these, the C-1′/C-2′-unsaturated Δ^1,2-D-anglosamine (306; DC92-D) is perhaps the most unique. For C-10-C-glycosides, N,N-dimethyl-α-L vancosamine (118c) is the chief sugar where X-ray analyses revealed this sugar to adopt a boat confirmation.^1178–1180 Other monosaccharyl C-10-C-glycosides include N,N-dimethyl-β-L-vancosamine (299; SS 21020B, SS 21020C, isokidamycin), N,N-dimethyl-α-D-vancosamine (304; chromoxymycin), N,N-dimethyl-3′-epi-5′-hydroxy-β-D-vancosamine (302; DC-92B, DC-92D), α-L-vancosamine (118a; altromycin G) and N,N-dimethyl-β-D-vancosamine (301; saptomycin B) and corresponding disaccharides have also been reported (altromycins, disaccharides II, III, IV; pluraflavins, disaccharide I). It is important to note that the C-10 N,N-dimethyl-α-D-vancosamine (304; chromoxymycin) was not observed in any other pluramycin member, potentially calling into question this assignment. Finally, altromycins A-G also present C-glycosylation of the C-5 side chain (2′,6′-didexoy-3′-methoxy-α-D-altrose, 129; 3′-methoxy-6′-deoxy-α-D-altrose, 307) while clecarmycins contain C9-C-glycosides of β-D-kerriose (55). While the genes encoding for the unique C-8-/C-10-C-glycosyltransferases have been identified,¹¹⁸¹ they remain resistant to biochemical study.

19. Polyethers

Polyether ionophores represent a large group of natural occurring biologically active compounds produced mainly by Streptomyces.^18,1182 This family of secondary metabolite contributes to the transport of metal cations across cell membranes and members also display a broad spectrum of activities including antibacterial, antifungal, antiviral, antiparasitic, and cancer cell line cytotoxicity. To date, 52 glycosylated bacterial polyethers have reported, subdivided herein into four sub-classes I–IV according to their aglycon core structural conservation (Fig. 55). Despite the rather large group of glycosylated analogs observed, the glycosides found are restricted to the use only eight distinct sugars (Fig. 55). The sugar 4′-O-methyl-β-D-amicetose (48b) represents the most predominant sugar, found in 34 of the 52 glycosylated bacterial polyethers. Exceptions to the use of 48b are noted herein.

Fig. 55.

Polyethers aglycons and associated sugars.

In sub-class I, 29 O-glycosylated polyethers were found with variant glycosyl regiospecificity (C5, C6, C15, C18, C22 and C27) including; antibiotics 6016,^1183–1186 A204-A [α-D-amicetose (308)],¹¹⁸⁷ carriomycin,^1188,1189 K-41-A,^1190,1191 K-41-B,¹¹⁹² X-14868A (maduramicin α),^1193–1196 X-14868C~D,¹¹⁹³ etheromycin,^1189,1197 >UK-58852, nanhumycin, RP-37454, A80789, CP-120509 [α-D-amicetose (308)],¹¹⁹⁸ CP-91243,¹¹⁹⁹ CP-91244,¹¹⁹⁹ CP-96797,¹²⁰⁰ octacyclomycin,¹²⁰¹ CP-82009,¹²⁰² semduramicin [α-D-amicetose (308)],¹²⁰³ UK-58,852-methylketal,¹²⁰⁴ A-13001-C¹¹⁹⁶ and SF-2361,¹²⁰⁵ along with the septamycin analogs.^1206,1207 Of these, β-D-amicetose (48a) was observed in CP-91243 and CP-91244.¹¹⁹⁹ In addition, 4′-O-methyl-α-L-amicetose (97) was observed only in etheromycin (CP-38295) while X-14868A through D¹¹⁹³ contain β-L-olivose (100) connected at the same position (C-22) of the aglycon. Sub-class II, contains the four glycosylated polyethers 26-O-β-D-glucopyranosylmonensin (β-D-glucose, 104) and the α-D-amicetose (308)-bearing CP-47433 and CP-47434, and CP-82996.¹²⁰⁸ Alternatively, sub-class III members (16 compounds) contain 4′-O-methyl-β-D-amicetose (48b) connected at five different positions of the aglycon (C-11, C-15, 20-CH₂, and C-27) as exemplified by A-130-B,¹²⁰⁹ A-130-C,¹²⁰⁹ X-14934A, moyukamycin,¹²¹⁰ lenoremycin (A-130),^1211,1212 leuseramycin,¹²¹³ endusamycin,¹²¹⁴ CP-60993,¹²¹⁵ CP-80219,¹²¹⁶ nangchangmycin^1217–1219 and dianemycin analogs.^{1212,1220,1221} All subclass VI analogs are C-14-O-glycosides of 4′-O-methyl-β-D-amicetose (48b), including, 6270B,¹²²² portmicin¹²²³ and CP-84657.¹²²⁴

20. Pterins

Pterins are characterized by the presence of a pteridine or a tetrahydropterine ring and have been proposed to prevent the bleaching of photosynthetic pigments due to irradiation in bacteria.¹²²⁵ Fifteen glycosylated metabolites of this class were reported predominately from cyanobacteria, green sulfur bacteria or methanogens where, in all cases, regioselectivity of glycosylation was observed to occur at variable positions upon side chains appended at C-6 of the pterin ring structure (Fig. 56). Cyanopterin^1226,1227 and monapterin, representing members that display O-glycosylation closest to the ring, contains a C-9 4′-O-methyl-α-D-glucuronic acid-(1′→6′)-β-D-galactose disaccharide (I) and β-D-glucose (104), respectively. Limipterin, tetrahydrolimipterin, tepidopterin and biopterin glucoside represent C-10-O-glycosides, all which contain N-acetyl-β-D-glucosamine (153) with one exception (biopterin).^{1225,1228,1229} The latter, incorrectly labeled 2′-O-(α-D-glucopyranosyl)biopterin, actually contains α-D-altrose (163). Solfapterin, the only C-11-O-glycoside within this series, contains α-D-glucosamine (5).¹²³⁰ Finally, tatiopterins, thermopterin, methanopterin, sarcinapterin, and tetrahydromethanopterin all contain uniquely functionalized anilines at C-9 that terminate with an O-glycosidic 5′-phospho-α-D-ribose (231) (Fig. 56).^1231–1233 In some cases, the corresponding 5′-phosphate is further modified with α-hydroxyglutaric moiety via a phosphodiester bond (Fig. 56, appendages i, ii, or iii).

Fig. 56.

Pterin aglycons and associated sugars.

21. Streptothricins

Streptothricins are N-glycosides composed of streptolidine or streptolidine lactam and a sugar (typically, β-D-gulosamine 320) attached to the C-2 exocyclic amino group, the latter of which (320) is C-2′-amino modified via a β-lysine derived moiety (Fig. 57). The streptolidine lactam is typically trans but a few cis conformers were recently reported.^1234,1235 Including the first member discovered (streptothricin F in 1942),^1236–1238 45 analogs have been reported as metabolites of Streptomyces and Bacillus including streptothricins, racemomycins, albothricin, glycinothricin, lavenothricins, boseimycin, and sclerothricin.^{1234,1235,1239–1251} These metabolites act as broad spectrum antibiotics through inhibition of protein synthesis and lavendothricins B and C have also been reported as calcium channel blockers.^1252,1253 However, apparent renal toxicity has hampered further development.^1254,1255 As mentioned, the core sugar employed among these metabolites is β-D-gulosamine (320) where 43 out of the 45 derivatives contain subtle modifications of the C-2′ amino group and/or C-4′ and/or C-6′-carbamoylation and are contributors to activity modulation.^{1246,1250,1251} The two exceptions include streptothricin F (which carries a corresponding C-2′, C-6′-modified α-D-gulosamine, 189)¹²³⁵ and racemomycin O (which alternatively contains a corresponding C-2′,C-4′,C-5′-modifed β-D-glucosamine 153).¹²⁵⁶ Sugar C-4′ carbomylation favors activity while cleavage of streptolidine lactam amide bond reduces activity and toxicity.^1251,1257

Fig. 57.

Streptothricin aglycons and associated sugars.

22. Terpenes and sterols

While prominent among plant metabolites, terpenes are relatively rare in the context of bacterial natural products with only 41 glycosylated terpenoids reported from bacteria to date. These are discussed below in the context of standard terpene classification.

22.1. Sesquiterpenes and diterpenes

Sesquiterpenes are comprised of three isoprene units and only one, the mildly cytotoxic deoxypentalenylglucuron (containing an aglycon exocyclic C-6 acid anomeric ester of β-D-glucuronic acid 72), has been reported from various Streptomyces.¹²⁵⁸

Diterpenes are comprised of four isoprene units and 16 glycosylated diterpenoids have been reported as metabolites of Strepomyces and Nocardia including the phenalinolactones,¹²⁵⁹ lucensimycins,¹²⁶⁰ brasillicardins,^1261,1262 platensimycins,¹²⁶³ and platencins.^1264,1265 Phenalinolactones, C-4 glycosides of a methylated α-L-amicetose (97) (Fig. 58), display moderate activity against Gram positive bacteria.¹²⁵⁹ In contrast, brasillicardins are C-2 (α-L-rhamnose 46) glycosides which, in brasillicardins A and B, are further modified at the rhamnose C-3′ with N-acetyl-β-D-glucosamine (153).^1261,1262 The latter sugar is believed to be essential for the immunosuppressive activity of brasillicardin A. Two of the seven lucensimycins from Streptomyces are glycosides,¹²⁶⁰ attached to the modified cysteine side chain via an amide bond between the cysteine C-1′ carboxylate and an aminosugar (2′-amino-2′-deoxy-α-L-idose, 316) amine. The anomeric position of the latter forms a α-glycosidic bond with the C-1′ of myo-inositol (P12) to form pseudodisaccharide II (Fig. 58) and it is important to point out that some confusion exists in the literature regarding the structure of pseudodisaccharide II.¹¹³¹ Platensimycin B₄ and its methyl ester¹²⁶³ and platencins A9, A10 and A12–A15^1264,1265 are the known glycosylated derivatives of the platensimycins and platencins. Members of this class were found to be potent inhibitors of mammalian and bacterial fatty acid synthases with potential relevance for the treatment of metabolic disease and bacterial infections.¹²⁶⁶ All glycosylated analogs exist as 4′-β-D-glucosides (104) of the corresponding 3-amino-2,4-dihydroxybenzoic acid moiety (Fig. 58) and display attenuated inhibition of FAS.¹²⁶³

Fig. 58.

Sesquiterpene and diterpene aglycons and associated sugars.

22.2. Triterpenes

Triterpenes consist of six isoprene units and are hypothesized to function as cell membrane constituents in bacteria. Cholesteryl glucosides with hemolytic activity have been reported as metabolites of Helicobacter and Streptomyces (Fig. 59).^1267,1268 The former are C-3-α-glycosides bearing C-6′-modified α-D-glucose (18) while corresponding metabolites from Streptomyces are β-glucosides (104).¹²⁶⁸ Hopanoids are pentacyclic triterpenoids thought to function as membrane stabilizers in bacteria to enable adaptation to extreme environments and are occasionally used as biomarkers for bacterial chemical taxonomy.^1269–1276 Bacteriohopanepolyols are hopanes characterized by a signature C-21-polyol side chain where C-36-glycosylated members have been characterized from different bacterial species including Zymomonas, Synechocystis, Synechococcus, Prochlorothrix, Acetobacter, Pseudomonas, Ralstonia, and Burkholderia. Nine different monosaccharides have been observed among this group including 2′-N-methyl-β-D-glucosamine (153), 6′-amino-β-D-glucose (318), β-D-glucose (104), 2′-amino-β-L-glucose (260), β-D-galacturonic acid (172), α-D-glucuronic acid (154), α-D-altropyranouronic acid (317), pseudosugar P23, and the unusual sugar 3,5-anhydrogalactouronic acid (321) (Fig. 59). MK800-62F1, a gammacerane and inhibitor of apotosis isolated from Streptomyces, is a C-22-glycoside of α-L-arabinose (265).¹²⁷⁷ It should also be noted that glycosylated bacterial carotenoids are briefly discussed in Section 11.

Fig. 59.

Triterpenes aglycons and associated sugars.

23. Miscellaneous

23.1. Trioxacarcin analogs

Trioxacarcins and LL-D49194 analogs are metabolites from Streptomyces that broadly inhibit the growth of cultured bacterial and eukaryotic cells.^1278–1281 To date, 16 glycosylated trioxacarcin metabolites have been reported, glycosylated at C-4-, C-13- and/or C-16 of the aglycon (Fig. 60). Three branched-chain sugar monomers are represented among this group including α-L-trioxacarcinose A (4′-epi-α-L-mycarose; 122), α-L-trioxacarcinose B (103), and the corresponding keto-reduced trioxacarcinose B (323).¹⁸ The C-4-O-α-L-trioxacarcinose A moiety (122) is common to all trioxacarcin glycosides, including the monoglycosidic trioxacarcins A1 (DC-45-A1) and E, the diglycosidic trioxacarcins (A, B, D, F), gutingimycin,¹²⁸² and the triglycosidic LL-D49194 (α1, β1, β2, e and w1).¹²⁸³ Additional C16-O-glycosylation with a mono-(122) or disaccharide (fragment I) side chain was observed in the LL-D49194 analogs while additional C-13-O-glycosylation with α-L-trioxacarcinose B (103) or its corresponding keto-reduced trioxacarcinose B (323), was observed in trioxacarcins A–D, F and gutingimycin. Sugar 323 of trioxacarcin C and its keto-form 103 have also been found appended to the anthracycline antibiotics isoquinocycline A and kosinostatin (Section 4.1), respectively.^188,251

Fig. 60.

Trioxacarcins and LL-D49194 aglycons and associated sugars.

23.2. Polycyclic xanthones and related analogs

Polycyclic xanthone natural products are a family of diverse polyketides characterized by highly oxygenated, angular hexacyclic frameworks, many of which are glycosylated. As described herein, C-11-O-glycosylation predominates but select members representing C-4-, C-6-, C-9- and C-10-O-glycosylation and C-11-N-glycosylation have also been characterized (Fig. 61). Among the C-11-glycosyl series, kibdelones A-C rhamnosides and isokibdelone A rhamnoside from Kibdelosporangium all exist as C-11-O-α-L-rhamnosides (46).^1284,1285 Interestingly, kibdelones display potent anticancer, antibacterial and nematocidal activity while isokibdelones lack activity.^1284,1285 Neocitreamicin II from Nocardia is a C-11-O-glycoside bearing 4′-acetylated-β-D-oliose (47).¹²⁸⁶ Both neocitreamicin II and its aglycon neocitreamicin I display equal in vitro potency against Gram-positive bacteria including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecalis.¹²⁸⁶ FDA-594,^1287,1288 MSO 901809¹²⁸⁹ and BE-13793X,¹²⁹⁰ from Streptomyces are three O-trisaccharyl-appended antitumor antibiotics. MSO 901809 and FD-594 contain the same trisaccharide [β-D-oleandrosyl-(1′→4′)-O-β-D-olivosyl-(1′→4′)-O-β-D-olivose, trisaccharide V] connected at C-9 and C-11 of the xanthone backbone, respectively. A similar C-11-trisaccharide is found in BE-13793X which contains a distinct internal β-D-olivosyl-(1′→3′)-O-β-D-olivose connection [β-D-oleandrosyl-(1′→4′)-O-β-D-olivosyl-(1′→3′)-O-β-D-olivose, trisaccharide VI]. Antibacterials/antimycoplasmodials SF2446A1, A2, B1 and B2 from Streptomyces along with the non-selective cytotoxin arenimycin from Salinospora also reflect C-11 glycosyl regiospecificity reminiscent of the kibdelones but are the only N-glycosidic members of this group, all bearing 2′,4′-di-O-methyl-β-L-rhamnose (322).^1291–1293

Fig. 61.

Polycyclic xanthone aglycons and associated sugars.

Distinct from the C-11 glycosyl series, kigamicins A–E are antitumor antibiotics from Amicolatopsis that possess a fused octcyclic ring system comprised of seven six membered rings and an oxazolidine.^1294–1296 Kigamicins are C-10-O-glycosides bearing a mono- to tetrasaccharide comprised of β-D-amicetose (48) and β-D-oleandrose (53b).^1294–1296 The C-6-O-glycosyl regiospecificity of benaphthamycin-β-D-glucoside (104) from Streptomyces is unique from other members in this class. Likewise, the C4-O-glycosyl regiospecificity of IB-00208 (β-D-quinovose, 140) from Actinomadura is unique from other members in this class.^{236,1297,1298} IB-00208 displays both in vitro cytotoxicity against various tumor cell lines (P-388, A-549, HT-29 and SK-MEL-28) and activity against Gram-positive bacteria.^{236,1297,1298}

23.3. Paulomycins related compounds

This class of compounds share a 2-amino-5-hydroxy-3,6-dioxocyclohex-1-enecarboxylate-based aglycon. Twenty analogs belong to this class and include ten paulomycins, two paldimycins, two 273a₂ analogs, two paulomenols, U-77,802 and U-77,803, and two senfolomycins from Streptomyces.^1299–1303 These metabolites carry a C-5-C-glycoside which, in most cases, is α-L-paulomycosyl-(1′→3′)-α-D-allose (disaccharide I, Fig. 62). Exceptions to this include paulomycin E and senfolomycins which differ via subtle variations of the disaccharide C-4′-branched α-L-paulomycose (328) monomer (ketosugar 329 in disaccharide II, paulomycin E; ketosugar 103 in disaccharide III, senfolomycin A; epimer 323, disaccharide IV, senfolomycin A). Modification of the disaccharide core [C-6′ of α-D-allose (162), C-3′ (328 and 329) and/or C-7′ (328) of the terminal branched sugar] further differentiates members as illustrated in Fig. 62. Members of this family display potent Gram positive antibacterial activity.

Fig. 62.

Paulomycins and associated sugars.

23.4. Phenyls and related compounds

Metabolites that contain a phenyl ring and do not belong to one of the previous classes are consolidated within this section which contains 10 structurally/functionally diverse glycosides. Myxotyrasoides, moderately cytotoxic N-acyl tyrosine derived metabolites from Myxococcus,⁶¹⁴ contain a phenolic-O-α-L-rhamnose (46) (Fig. 63). Myxotyrasoides, along with chivosazoles and sorangiosids (Section 12), are among the only glycosylated secondary metabolites reported from myxobacteria.

Fig. 63.

Phenyl glycosides and associated sugars.

Sibiromycin and sibanomicin from Streptosporangium and Micromonospora, respectively, are C3-O-glycosides (α-L-sibirosamine, 327) of modified pyrrolo[1,4]benzodiazepines.^1304–1306 Both are cytotoxic and sibiromycin binds the minor groove of DNA.¹³⁰⁷ Two C-6-O-2′,4′-di-O-methyl-β-D-xylosyl (136) quinoline alkaloids were isolated from the cyanobacterium Lyngbya^1308,1309 and the same sugar was also found appended at C-4 of malyngamide J cyclohexanone core (a cytotoxin).¹³¹⁰ Terfestatin A, a specific inhibitor of auxin signaling in Arabidopsis, is a C-3-O-β-D-glucoside (104).¹³¹¹

Saccharomicins are O-heptadecaglycosides of N-(m,p-dihydroxycinnamoyl) taurine from Saccharothrix.¹³¹² Common monosaccharide units of the saccharomicin oligosaccharide include β-D-fucose (270), 2′-sulfate-β-D-fucose (270), the C-3′-branched β-D-saccharosamine (269), α-L-rhamnose (46), 4′-epi-α-L-vancosamine (280), and α-L-digitoxose (68). Saccharomicins A and B differ via a single tenth sugar residue (α-L-rhamnose 46 or α-L-digitoxose 68, respectively). Both compounds display potent Gram-positive and Gram-negative antibacterial activity where the primary cellular target put forth was the bacterial membrane. While a new potential antibacterial class, their narrow therapeutic index has prohibited further development.

23.5. Heterocycles

23.5.1. Oxazoles

Fourteen oxazole glycosides have been reported from Streptomyces. Of these, seven are allosamidins (chitinase inhibitors) distinguished by the fused bicyclic pentanoidoxazolidine aglycon.^1313–1315 Members contain a C-5-O-β-D-N-acetyl-β-D-allosaminyl-β-(1′→4′)-N-acetyl-β-D-allosamine (disaccharide I, monomer 326) or the C-6′-O-methyl variant (Fig. 64). Two related glucoallosamidins exist in which the proximal monomer (326) has been replaced by N-acetyl-β-D-glucosamine (153). N-glucosides trehazolin and trehalostatin share a similar bicyclic aglycon (trehazolamine) but carry a N-7-α-D-glucose (18).^1316,1317 Both function as specific irreversible inhibitors of trehalase, with trehazolin as the most potent.^1316,1318 One tricyclic streptazolin C-5-O-β-D-xyloside (136) was reported where the glycoside displayed notably higher in vitro anticancer cytotoxicity than non-glycosylated comparators.¹³¹⁹ Finally, two glycosylated cyclocarbamides (SB-253514 and SB-315021 from Pseudomonas) both exist as C-14-O-α-L-rhamnosides (46).¹³²⁰ Both metabolites inhibit lipoprotein-associated phospholipase A₂ and exhibit anti-inflammatory properties.

Fig. 64.

Oxazole aglycons and associated sugars.

23.5.2. Porphyrins

Porphyrins are heterocyclic tetrapyrrole-based macrocyclic metabolites (Fig. 65). Of the 27 porphyrins reported from bacteria, seven tolyporphins from Tolypothrix exist as C17-C-glycosides or C-7,C-17-di-C-glycosides. While 3′,6′-dideoxy-β-D-xylohexopyranose (313) was initially reported as the corresponding monosaccharide, a tolyporphin A structure revision by Moore and coworkers implicated 3′,6′-dideoxy-α-D-xylo-hexopyranose (311)¹³²¹ and may call into question the anomeric configuration of other reported members. Tolyporphins inhibit P-glycoprotein^1321–1324 and can photosensitize tumor cells.¹³²⁵

Fig. 65.

Porphyrin aglycons and associated sugars.

23.5.3. Other heterocycles

Eighteen other bacterial heterocyclic glycosides have been reported. Of these, the macrolactam tetracyclic tetrapetalones A-D from Streptomyces were discovered via a lipooxygenase inhibition guided-assay.^1326,1327 They are C-9-β-D-rhodinosides (56) (Fig. 66). The corresponding enantiomer, β-L-rhodinose (169), is appended to C12 of the C2-heptenylidene side chain in hatomamicin, a cytotoxic Saccharopolyspora metabolite.¹³²⁸ Gualaymycin from Streptomyces is a pyrrolidinyl butanoate metabolite bearing a C2-C-disaccharyl moiety [disaccharide I; 2′-deoxy-2′-amino-β-D-gulosyl-(1′→4′)-α-D-galactose]. These metabolites are superior to dicofol as acaricidals.¹³²⁹ A phenoxazine N-β-D-glucosidic (104), questiomycin from Microbispora, was reported to have both antibacterial and cytotoxic activity where N-glucosylation was found to suppress bioactivity.¹³³⁰ Pyralomicins are chlorinated benzopyranopyrrole N-(pseudo)-glycosides produced by Nonomuraea.^1331–1333 Four derivatives 1a–d contain the pseudosugar 1′-epi-valienol (P26) proposed to arise from D-seduheptulose-7-phosphate¹³³⁴ while three others (2a–c) are N-β-D-glucosides (104). Pyralomicins display Gram-positive antibacterial activity where pseudosugar-based derivatives are superior to the corresponding glucosides.¹³³¹ Pyrazomycins A and B from Streptomyces are pyrazole C-3-C-glycosides bearing β-D-ribose (25) and α-D-ribose (231), respectively. Two rings of the pentacyclic red pigment rubrolone from Streptomyces derive from a unique C-4-O-C1′/C-3-C-2′ fusion with β-D-fucose (270) (Fig. 66).¹³³⁵

Fig. 66.

Other heterocycle aglycons and associated sugars. Pink fragment highlights differences between tetrapetalones. Red fragment highlights sugar in rubrolone.

23.6. Additional simple aglycons

Compounds combined with this collection include 44 metabolites. While some are bacterial metabolites, most derive from precursor directed biosynthesis by feeding Streptomyces GT61150 a range of putative aglycons including benzoyls, pyrrolycarbonyls, furanylcarbonyls, phenylacetyls, thiophenylcarbonyls, indolylcarbonyls, pyridylcarbonyls, phenyls, and isovaleryls (Fig. 67) to afford the corresponding α-L-rhamnosides (46) or 6′-deoxy-α-L-talosides (125).^{590,1336–1339} All corresponding products were monosaccharide-substituted with one exception, benzoyl dirhamnoside bearing both a phenolic hydroxyl- and ester-linked α-L-rhamnose (46) (Fig. 67). Rhamnosylated lactones analogs were found to inhibit the 3α-hydroxysteroid dehydrogenase.⁵⁹⁰

Fig. 67.

Aglycons with simple moieties and associated sugars.

Additional metabolites include menisdaurin,¹³⁴⁰ byelyankacin,¹³⁴¹ and two CKD-711 derivatives^1341–1343 from Streptomyces or Enterobacter. Menisdaurin is a C-3-β-D-glucoside (104) while byelyankacin is a C-1-α-L-rhamnoside (46) and potent tyrosinase inhibitor.¹³⁴¹ CKD-711 and CKD-711a are epoxycyclohexanol C-1-N-glycosides bearing either a pentasaccharide I or a trisaccharide II (Fig. 67). Both oligosaccharides are comprised of one or two α-D-glucosyl-(1′→4′)-α-D-glucose units “18-(1′→4′)-18” terminating with a C-4-O-4′-deoxy-4′-amino-β-L-glucose (36) and inhibited α-glucosidase with the trisaccharyl derivative as the most potent. Both compounds also selectively inhibited the growth of the Gram negative bacteria Comamonas terrigena but displayed no other antibacterial/antifungal effects.¹³⁴²

24. Conclusions

This cumulative analysis reveals a relatively high degree of natural product family-specific conservation in both glycosyl regiospecificity and the specific sugars employed as well as a moderate degree of crossover among natural product families in the use of less functionalized ‘common’ sugars (e.g., standard metabolic sugars and simple corresponding deoxysugars). This likely parallels a higher prevalence (and therefore a higher likelihood of lateral gene transfer) of the core genes encoding some of the common sugar functionalization chemistry in both primary (cell wall) and secondary metabolism where the more complex natural product family-specific sugar functionalization may have derived via divergent evolution. No clear overall general trends relating glycosylation to biological activity were observed (i.e., glycosylation does not always improve or suppress biological activity) however, within a number of natural product subfamilies such trends were apparent. While this points to a lack of a predictive model to apply glycosylation as a medicinal chemistry tool for natural product optimization, it suggests efficient glycodiversification strategies (chemoenzymatic,^9,1344 chemoselective¹⁰ and/or pathway engineering¹³⁴⁵) to offer unique opportunities for improving and/or broadening natural product therapeutic potential. While notable advances have been made toward understanding the fundamental mechanisms and structural biology of enzymes involved in natural product glycosylation,^1346–1348 this review also exposes a number of uniquely functionalized sugars and glycosidic connections as a basis for the future discovery of unprecedented enzymatic chemistry.

Biographies

Sherif I. Elshahawi

Sherif received his BS degree in Pharmaceutical Sciences (2000) from Cairo University, a Master’s degree in Pharmacognosy (2006) from the University of Mississippi and a PhD degree in Biochemistry and Molecular Biology (2012) from the School of Medicine at Oregon Health & Science University with Professor Margo Haygood. He is currently a postdoctoral scholar in the Center for Pharmaceutical Research and Innovation at University of Kentucky College of Pharmacy. Dr Elshahawi’s research interests include the isolation and phylogenetic analyses of microorganisms from unusual environments and the biosynthesis, isolation, bioassay and resistance of bioactive secondary metabolites.

Khaled A. Shaaban

Dr Khaled A. Shaaban obtained his BS degree in Chemistry (2000) from Mansoura University (Egypt) and MS (2005) and PhD degrees in Organic Chemistry (2009) from the University of Göttingen (Germany) with Professor Hartmut Laatsch. He subsequently held postdoctoral appointments at the University of Kentucky (Professor Jürgen Rohr, 2009–2011) and The Scripps Research Institute (TSRI Florida, Professor Ben Shen, 2011–2012) prior to joining the Center for Pharmaceutical Research and Innovation at University of Kentucky College of Pharmacy as a Research Scientist. His major research interests include isolation of bacterial strains, screening, scale-up fermentation, isolation and structure elucidation of novel bioactive microbial natural products.

Madan K. Kharel

Dr Kharel obtained his BS (1998) and MS (2000) from the Tribhuvan University (Nepal), a PhD in biochemistry from the Sun Moon University (South Korea, 2007) and a PhD in pharmaceutical sciences from the University of Kentucky (2010, Professor Jürgen Rohr). Dr Kharel served as a research faculty in the University of Kentucky Center for Pharmaceutical Research and Innovation (2012–2013) and currently is an Assistant Professor of Medicinal Chemistry, Department of Pharmaceutical Sciences, University of Maryland Eastern Shore. His research interests include elucidation of function of novel enzymes, enzymatic synthesis of natural products, and isolation of new bioactive microbial natural products.

Jon S. Thorson

Jon received his BA from Augsburg College (1986), a PhD from the University of Minnesota (1993, Professor Hung-wen Liu) and was a Helen Hay Whitney postdoctoral fellow at the University of California, Berkeley (1993–1996, Professor Peter Schultz). He has served on the faculty of the Memorial Sloan-Kettering Cancer Center and the Sloan-Kettering Division, Joan and Sanford I. Weill Graduate School of Medical Sciences, Cornell University (1996–2001), the University of Wisconsin –Madison School of Pharmacy (2001–2011), and the University of Kentucky College of Pharmacy (2011–present) where he is also the director of the Center for Pharmaceutical Research and Innovation. His research interests include bioprospecting, biocatalysis and biosynthesis.