Synthetic Studies Towards Mannopeptimycin-E

Abstract

The enantioselective synthesis of the C-4′ acylated 1,4-α,α-manno,manno-disaccharide fragment of mannopeptimycin-E has been achieved in 7 steps from D-tyrosine. The route relies upon diastereoselective palladium-catalyzed glycosylation, diastereoselective reduction and diastereoselective bis-dihydroxylation. The efficiency of the synthesis is demonstrated by the high overall yield (37%) and the preparation of various analogues.

The continuing emergence of bacterial resistance to traditional antibiotics has inspired a never-ending search for new antibiotics.¹ The five mannopeptimycins (1a-e) were isolated from the fermentation broths of Streptomyces hygroscopicus LL-AC98 and related mutant strains.² The key structural features of the mannopeptimycins are a cyclic hexapeptide core with alternating D- and L-amino acids, three of which are rare. Two of the amino acids (β-D-hydroxyenuricididine and D-tyrosine) are glycosylated with mannose sugars. The glycosylated amino acids are an N-glycosylated β-hydroxyenuricididine with an α-mannose, and an O-glycosylated tyrosine with a α-(1,4-linked)-bis-manno-pyranosyl-pyranoside.

The unique structure and unprecedented biological activity have inspired both biological^2,3 and synthetic studies⁴ from labs at Wyeth pharmaceuticals. Among the mannopeptimycins, mannopeptimycin-E (1e, Scheme 1) was reported as the most active member against methicillin-resistant staphylococci and vancomycin-resistant enterococci (Table 1).⁵

Scheme 1.

Structure of mannopeptimycin E 1e

Table 1.

Activities of the mannopeptimycins⁵

mannopeptimycin A-E	MIC range (μg/mL)
	MRSAa	Enterococus faeciumb
	>128	>128
1b R= H	64-128	32->128
	8	16-64
	8	8-64
	4	4-32

^amethicillin-resistant S. aureus.

^ba range of activities vs 4 lines of vancomycin-resistant.

^ci-val = i-valerate

A particularly interesting aspect of the SAR for the mannopeptimycins is how the specific placement of the isovalerate group on the bis-manno-disaccharide correlates with its antibacterial activity. It has been shown that C-4 isovalerate substitution on the terminal mannose leads to a substantial increase in antibacterial potency. For instance, mannopeptimycins-C and -D, which have C-2 and C-3 isovalerate groups respectively, have reduced activity, whereas mannnopeptimycins-A and -B, which lack isovalerate substitution have even lower activity (Table 1).⁵

Although the total synthesis of mannopeptimycin has not been reported, Wang et al. have reported a synthesis of cyclic peptides related to mannopeptimycin having a C-4/C-6 acetal as an isovalerate substitute.^4a This work also confirmed the importance of the C-4 isovaleryl group for antibiotic activity. The critical role isovalerate substitution has on the antibacterial activity of the mannopeptimycin-E inspired us to pursue a synthesis of an appropriate O-glycosylated D-tyrosine with C-4 isovalerate substitution (e.g. 2a and 2b, Scheme 1).⁶ In addition to our desire to synthesize and test the mannopeptimycin analogues 2a and 2b, we felt that the synthesis of 2a would serve as part of a model study for our synthesis of the natural product. In addition, the preparation of 3b, a fully protected bis-glycosylated tyrosine (Scheme 2), would be of use for the synthesis of mannopeptimycin E. Herein, we report the successful implementation of our palladium-catalyzed glycosylation reaction^7,8 for the de novo installation of both a D,D- and an L,L-bis-mannodisaccharide fragment on a D-tyrosine. The flexibility of the approach is demonstrated by the syntheses of bis-2,3-dideoxy analogues in their D,D- and an L,L-forms.⁹

Scheme 2.

Retrosynthetic analysis of mannopeptimycins

Our retrosynthetic analysis of the disaccharide fragment 2a and its fully protected variant 3b is outlined in Scheme 2. We envisioned that the manno-stereochemistry in both 2a and 3b could be installed by a diastereoselective ketone reduction and a bis-dihydroxylation of a 1,4-linked pyran/pyranone 4. Similarly, we believed that the pyran/pyranone 4 could be assembled using a diastereoselective palladium-catalyzed glycosylation of tyrosine 5.⁷ Recently, we reported a diastereoselective palladium-catalyzed glycosylation reaction that used alcohols as nucleophiles and pyranones like 6 as glycosyl donors. Thus, sequential application of our Pd(0)-glycosylation/NaBH₄-reduction/Pd(0)-glycosylation sequence to tyrosine 5 and pyranone 6 was expected to allow for the rapid preparation of 4. Replacing the above-mentioned bis-dihydroxylation with a bis-diimide reduction might also allow for the preparation of the deoxy-analog 2b. Previously we have shown that pyranone 6 can be prepared in either enantiomeric form. Thus, this procedure was expected to allow the incorporation of either D- or L-sugars.¹⁰

Our synthesis studies began with the protected D-tyrosine 5 and pyranone 6 which, exposed to 1 mol% Pd₂(dba)₃•CHCl₃ and 2.5 mol% PPh₃, underwent a diastereoselective glycosylation with complete α-selectivity to afford the pyranone 8 in 92% yield. A diastereoselective 1,2-reduction of the enone 8 , when subjected to NaBH₄ in CH₂Cl₂/MeOH (1:1) at —24 °C, afforded allylic alcohol 9 as a single diastereomer (dr > 20:1). We next investigated the viability of the C-4 alcohol in the Pd-catalyzed glycosylation. Exposing allylic alcohol 9 to a second glycosylation using 1.2 equiv of pyranone 6 and 1 mol% Pd catalyst (1:2.5, Pd₂(dba)₃•CHCl₃/PPh₃) afforded the 1,4-linked-α-bis pyranone 4 in good yield (82%) and virtually complete stereocontrol.

The final post-glycosylation transformation of 4 is shown in Scheme 4. Treatment of 1,4-linked pyran/pyranone 4 under the same reduction conditions as before (8 to 9, Scheme 3) gave allylic alcohol 10 in excellent yield (91%) and diastereoselectivity (>20:1). The isovalerate group was installed by treating allylic alcohol 10 with isovaleric acid and DCC/DMAP in CH₂Cl₂, which provided the C-4 isovalerate disaccharide precursor 11 in excellent yield (96%). The manno-stereochemistry in 3a was diastereoselectively introduced¹¹ upon exposure of 11 to the Upjohn conditions (OsO₄/NMO, 85%).⁸ Removal of both TBS-ethers was accomplished with TBAF (0 °C in THF) affording the α-1,4-linked-bis-manno-disaccharide 2a in good yield (76%).

Scheme 4.

Synthesis of tyrosine-bis-manno-disaccharide

Scheme 3.

De novo synthesis of pyran/pyranone 4

Finally the bis-manno-sugar 3a could also be converted to the fully protected α-1,4-linked-bis-manno-disaccharide 3b without any ester migration (Scheme 5). This was easily accomplished by treating a CH₂Cl₂solution of tetraol 3a with 2,2-dimethoxypropane and 10 mol% CSA; conditions which provided the bis-acetonide 3b in good yield (80%).

Scheme 5.

Synthesis of fully protected bis-manno-disaccharide

Replacing pyranone 6 with its L-enantiomer (ent)-6, resulted in an equally efficient synthesis of the L,L-bis-manno-sugar diastereomer of 2a, 14 (Scheme 6). Thus in three analogous steps, D-tyrosine was converted into pyran/pyranone 12 (69% overall yield). The L,L-1,4-linked pyran/pyranone 12 was stereoselectively reduced and acylated to form 13 in good overall yield (86%). Once again, two diastereoselective dihydroxylations occurred upon exposure of 13 to the Upjohn conditions. This bis-dihydroxylation occurred with near perfect stereocontrol, as with the diastereomeric series (c.f. Scheme 4).¹¹ The tetraol product was converted to the unprotected bis-sugar 14 via a TBS group deprotection (TBAF, 78%) or to the fully protected diastereomer 15 by means of an acetonide protection (10 mol% CSA/2,2-DMP, 81%).

Scheme 6.

Synthesis of L,L-disaccharide analogs

Having synthesized the key disaccharide fragment of mannopeptimycin-E (2a and 3b) along with its L,L-diastereomers (14 and 15), we turned our attention to the preparation of deoxy analogues (Scheme 7). The simplest 2,3—deoxy analogue 16 was obtained by an exhaustive diimide reduction.¹² Both double bonds of 11 were reduced using excess of the diimide precursor in CH₂Cl₂ affording the 2,3-deoxy-bis-pyranoside 16 in nearly quantitative yield (95%).¹³ Under identical conditions the diasteromeric L,L-1,4-linked bis-pyran 13 reduced to give an excellent yield of the bis-dideoxy analog 17 (97%).¹³

Scheme 7.

Synthesis of bis-2,3-dideoxy-disaccharide analogues

In conclusion, an enantioselective synthesis of the manno-disaccharide fragments of mannopeptimycin-E has been achieved in 7 steps and 37% overall yield from D-tyrosine via an iterative palladium-glycosylation strategy. Key to the success of this approach was the ease with which the C-4 isovalerate group was introduced, and the high diastereoselectivity of the palladium-catalyzed glycosylation and bis-dihydroxylation reactions. The use of this methodology for the synthesis of mannopeptimycin-E as well as various analogues is ongoing.