Highly Efficient O-Glycosylations with p-Tolyl Thioribosides and p-TolSOTf

Abstract

A wide variety of p-tolyl thioriboside donors are examined for O-ribosylations of primary and secondary alcohols. p-Tolylsulfenyl trifluoromethanesulfonate (p-TolSOTf) is very effective in promoting O-ribosylations with p-tolyl thioriboside; all reactions are completed within 1~15 min. to provide the desired products in good yield with reliable α/β selectivity. A wide range of functional groups are tolerated under these conditions. The described O-ribosylation conditions are very useful for the generation of ribosamino-uridine library molecules in solution or on polymer-support.

The 5-deoxy-5-aminoribose moiety of ribosamino-uridine antibiotics (i.e. muraymycin, liposidomycin, caprazamycin, and FR-900493) has been considered as an important functional group to exhibit excellent antibacterial activities.¹ These antibiotics are known to interfere with the enzyme involved in the committed step of peptidoglycan biosynthesis, MraY which catalyzes the transformation of UDP-N-acylmuramyl-l-alanyl-γ-d-glutamyl-meso-diaminopimelyl-d-alanyl-d-alanine (Park’s nucleotide) to prenylpyrophosphoryl-N-acylmuramyl-l-Ala-γ-d-glu-meso-DAP²-d-Ala-d-Ala (lipid I), and are non-toxic or less toxic in vivo. Thus, MraY has been a target of interest for the discovery of novel antimycobacterial agents.³ However, medicinal chemistry programs of these natural products with an aim to improve the efficacy including pharmacokinetics are hampered by a limited number of modification reactions which can selectively modify the desired positions of such complex molecules.⁴ In addition, one of the drawbacks of performing medicinal chemistry with these complex antibiotics is the requirement for the fermentation of the natural product starting materials.⁵

In our ongoing effort to define the minimum structure requirement of ribosamino-uridine MraY inhibitors of natural product origin to exhibit equal biological activity,⁶ we envisioned synthesizing a library of ribosamino-uridine molecules in solution or on polymer-support.⁷ Because structural diversity of ribosamino-uridine antibiotics are observed not only in the lipid moiety (R group in Figure 1) but also at the 2-position of aminoribose moiety (i.e. methyl or sulfonyl group), it is desired to develop a versatile and robust ribosylation method which also amenable to glycosylations on polymer-support.⁸

FIGURE 1.

Representative Structure of Ribosamino-uridine Antibiotics.

Although numerous examples of N-glycosylations of purine or pyrimidine bases with furanose derivatives have been reported,⁹ O-ribosylation has rarely discussed in literatures. Recently, Merrer and co-workers reported O-ribosylations of primary alcohols using bromo- or fluoro-ribosides with the established promoters such as AgOTf or SnCl₂/AgClO₄.¹⁰ Reaction rates for O-ribosylations with 1-halo riboside are very slow (6~36h), and in our hands under these conditions O-ribosylations of the secondary alcohols resulted in unsatisfactory yields of 15~65%. In addition, because of the limited accessibility of 1-halo ribosides it is difficult to diversify ribosamino-uridine library molecules by O-ribosylations with these methods. In this note, we wish to report expeditious O-ribosylations of primary and secondary alcohols with a wide variety of p-tolyl thioriboside donors and p-TolSOTf that would be very useful to synthesize a library of ribosamino-uridine molecules.¹¹

Because thioglycosides 1) can conveniently be synthesized via acid catalyzed thioglycosylations of 1-O-acetylglycoside, and 2) exhibit excellent chemical stabilities against acids and bases, the anomeric thioethers have widely been utilized as a temporary protecting group of anomeric positions as well as glycosyl donors.¹² However, a choice of promoter requires careful consideration when use less electrophilic thioglycosides; in some cases no activation or very slow reaction rate was observed using conventional thiophilic reagents.¹³ In order to identify an effective thiophilic reagent for O-ribosylations, we first screened thiophilic reagents using an anomeric mixture (α/β = 1/6.6) of p-tolyl tribenzoyl thioglycosides 1a and the immobilized propargyl alcohol on the polymer-resin i¹⁴. As a result of extensive reaction screenings it was found that p-TolSOTf in-situ generated from p-TolSBr¹⁵ and AgOTf provided the β-O-ribosylated product ii in near quantitative yield within 15 min. at 0 °C;¹⁶ ii was characterized after cleavage from the polymer-resin with 20% TFA. Significantly, even in the presence of a large excess of the promoters (>10 equiv) all functional groups (acetal, alkyne, alkene, and BOM protecting group) were intact. It is worthy while mentioning that the other conditions tested (i.e. NIS, NIS/TfOH, NIS/AgOTf, and NIS/AgBF₄)¹⁷ provided ii in 0~15 % yields together with a significant amount of byproducts.¹⁸

To investigate how effectively p-TolSOTf can promote glycosylations of a wide range of p-tolyl thioribosides we synthesized the donor ribosides 1b~n and the conditions developed for an O-ribosylation on the polymer-resin (Scheme 1) were applied to primary and secondary alcohols in solution. All reactions were conducted with donor (2 equiv against acceptor), p-TolSOTf (1 equiv against donor), and MS4Å (2 times the weight of donor)¹⁹ in CH₂Cl₂ in the presence or absence of SrCO₃²⁰. The representative O-ribosylations using these conditions were summarized in Table 1. The reactions of the primary alcohols 2a~e with tri-O-acyl riboside 1a or 1b at 0 °C provided the corresponding β-glycosides 3a~f exclusively with greater than 85% yield within 1~5 min. (entries 1~6). It was realized that p-TolSOTf, which colours blue in CH₂Cl₂, could be generated even at -78 °C²¹ and could also activate 1a and 1b at the same temperature without noticeable decrease in the reaction rate. Because chemical modifications of the 5-position of 5-deoxy-5-aminoribose in ribosamino-uridine antibiotics is important to improve in vitro and in vivo efficacy, glycosylations of the acceptor 2e with the 5-TIPS-protected thioribose 1c and 5-azido-5-deoxy-thioriboside 1d, whose 5-positions can easily be diversified after glycosylations, were evaluated. Under the buffered conditions (condition B in Table 1) the azide and silyl groups in the ribosyl donors were intact (entries 7~8). The primary and secondary donor alcohols (i.e. alkanols, alkynols, and homoallylic alcohols)²² were ribosylated with the donors 1a~d to furnish the β-glycosides in greater than 80% yield; limited examples of the reactions of the secondary alcohols (i.e 2f) were shown in Table 1 (entries 19~21).²² However, primary and secondary allylic alcohols (i.e. 5-phenylpent-1-en-3-ol) were not applicable to these conditions due probably to its low nucleophilicity. Ribosylations of propargyl alcohol with the ether protected thioribosides 1e~g (entries 9~11) provided a mixture of α/β glycosides (5~6.6/1) in 75~90% yield. Thus, synthetically useful level of α-selectivity in O-ribosylations was observed by using the ether protecting groups. Glycosylations of propargyl alcohol with the acetonide protected thioribosides 1h~j (entries 12~14) provided a 1:1.1 (α/β) mixture of the corresponding products regardless of structure of the 5-position of 1h~j. As seen in muraymycins (Figure 1), an alkylation is observed at the 2-position of aminoribose unit. To study the effect of 3-O-acyl group in the 2-O-methylated thioglycosides, glycosylations of propargyl alcohol with 1k~n (entries 15~18) were examined. The 3-O-acetyl group in 1k~n were effective in reversing the selectivity; the reactions with 1k or 1l gave a 1:3 mixture of the α- and β-ribosides. The 3-O-benzoyl protected donors 1m or 1n improved α/β -selectivity (α/β = 1:5.5). These selectivities observed at 0 °C were not dramatically enhanced by lowering the reaction temperatures (i.e. -78 °C); the α/β selectivity was increased to 1:6.5 for 1m. The glycosylations of 2f, a versatile intermediate for the generation of ribosamino-uridine libraries, with 1m or 1n gave identical selectivities (α/β = 1:5.5) observed for the primary alcohol. Similarly, the reactions of 2f with the 3-O-acetate, 1k or 1l, provided an anomeric mixture (α/β = 1:2.5) of the glycosides (Scheme 2). Mechanistically, the ribosyl carbenium ion generated by p-TolSOTf would first be stabilized by the formation of an intimate ion pair²³ with triflate ion, and undergoes a pseudo 5-membered ring formation when 2-O-acyl ribosyl donor is applied. These processes must proceed prior to the glycosylation step. Although an anchimeric assistance by the 3-O-acyl group to form a pseudo 6-membered ring (iii in Scheme 2) has not been proven theoretically, the data summarized in Table 1 (e.g. Entry 11 vs. 17) clearly indicate that the 3-O-acyl carbonyl group is responsible for β-selective glycosylations. It is speculated that the formation of a pseudo 6-membered ring is a slower process than that of a 5-membered ring. Thus, non-selective glycosylation of an intermediate iv may compete with β-selective glycosylation (through iii) to provide an α/β anomeric mixture. The 3-O-benzoyl group seems to be stereoelectronically more favoured than the 3-O-acetyl group to participate in anchimeric assistance in ribosylations (Scheme 2).²⁴

SCHEME 1.

Screening of a Promoter for 1 Using the Immobilized Propargyl Alcohol on the Polymer Resin i.

TABLE 1.

Representative O-Ribosylations of Alcohols Using p-Tolyl thioglycosides and p-TolSOTf.^a

Entry	Donor	Acceptor	Temperature (°C)	Condition	Major Product	Yield(%)	α/β Selectivity (4/3)b
1			-78~0	A		98	0/1
2						98	0/1
3			-78~0	A		100	0/1
4			-78~0	B		95	0/1
5						98	0/1
6			0	B		98	0/1
7						95	0/1
8						85	0/1
9			0	B		90	6.6/1
10						75	6.0/1
11						78	5.0/1
12			0	B		95	1/1.1
13						85	1/1.1
14						80	1/1.1
15			0	B		85	1/3.0
16						90	1/3.0
17						85	1/5.5
18						90	1/5.5
19			0	B		90	0/1
20						80	0/1
21						90	0/1

^a2 equivalents of donors against acceptor were used in all reactions.

^bratio was established by ¹H-NMR analysis or based on isolated yield.

SCHEME 2.

Glycosylations of 2f with the thioribosides 1k~n.

In conclusion, thioribosides were, for the first time, generalized for O-ribosylations using p-TolSOTf.²⁵ p-TolSOTf can be generated by a mixing p-TolSBr and AgOTf in CH₂Cl₂ at -78~0 °C, and has a short half-life (ca. 15 min.) at 0 °C. However, the reaction rate of O-ribosylations under these conditions is very fast; all reactions summarized in Table 1 were completed within 1~15 min. The 2-O-acyl group is an important factor to obtain β-selective ribosylation product exclusively. The anchimeric assistance of 3-O-acyl group may attribute to furnish β-selective ribosylations with 2-O-alkyl thioribosides. The ether protected thioglycosides provided useful level of α-selectivities (i.e. α/β = 6.6/1 for benzyl ether). The 2,3-acetonide protected thioglycosides give rise to a 1:1.1 (α/β) anomeric mixture of ribosides. Most importantly, p-TolSOTf is applicable to substrates possessing a wide variety of functional groups such as alkyne, alkene, and commonly utilized protecting groups in multiple step syntheses (i.e. silyl, ketal, and ether protecting groups, and N₃ group). Moreover, the conditions reported here are applicable to O-ribosylations on the polymer-supported resin (Scheme 1). The 2- and 5-positions of aminoribose moiety of ribosamino-uridine library molecules can be diversified by using the thioriboside donors summarized in Table 1 and their related structure of molecules. The generation of diverse structures of ribosamino-uridine libraries and biological evaluations of these library molecules will be reported elsewhere.

Experimental Section

Typical Procedure for O-Ribosylation with 1a

A mixture of 1a (2.0 equiv), alcohols (1.0 equiv), MS4Å, AgOTf (2.0 equiv), and SrCO₃ (4.0 equiv; Condition B) in CH₂Cl₂ (0.1~0.2 M) was stirred at rt for 15 min. The reaction mixture was cooled 0 °C and p-TolSBr (2.0 M in ClCH₂CH₂Cl, 2.0 equiv) was added. After 1~15 min. the reaction mixture was quenched with Et₃N (5.0 equiv), and the resulting mixture was filtered through a SiO₂ plug. Purification by silica gel chromoatography (hexanes:EtOAc) provided the desired products. SrCO₃ was excluded for Condition A.

Propargyl 5-azido-5-deoxyl-2,3-O-isopropylidene-β-D-ribofuranoside (3n and 4n)

3n (β isomer):[α]²⁰_D = +78.4 (c 0.5 in CHCl₃); IR (film) 2102, 1077 cm^-1; ¹H NMR (300 MHz, CDCl₃) δ 5.30 (s, 1H), 4.65 (dd, J = 6.0 Hz, 18.9 Hz, 2H), 4.32 (t, J = 7.2 Hz, 1H), 4.26 (dd, J = 1.2, 2.4 Hz, 1H), 4.24 (dd, J = 1.2, 2.1 Hz, 1H), 3.46 (dd, J = 7.8, 12.6 Hz, 1H), 3.30 (dd, J = 6.9, 12.6 Hz, 1H), 2.46 (m, 1H), 1.50 (s, 3H), 1.33 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ 113.1, 107.1, 85.9, 85.4, 82.2, 75.2, 54.8, 53.8, 26.6, 25.1; HRMS (ESI) Calcd. for C₁₁H₁₅N₃NaO₄ (M+Na)⁺. 276.0960; found: 276.0960. 4n (α isomer): [α] ²⁰_D = −16.0 (c 0.1 in CHCl₃); ¹H NMR (300 MHz, CDCl₃) δ 5.23 (d, J = 4.8 Hz, 1H), 4.73 (dd, J = 4.8, 7.2 Hz, 1H), 4.55 (dd, J = 3.6 Hz, 7.2 Hz, 1H), 4.32 (dd, J = 2.4, 4.2 Hz, 2H), 4.27 (m, 1H), 3.55 (dd, J = 4.0, 13.2 Hz, 1H), 3.36 (dd, J = 4.4, 16.0 Hz, 1H), 2.39 (t, J = 2.4 Hz, 1H), 1.55 (s, 3H), 1.33 (s, 3H); ¹³C NMR (75 MHz, CDCl₃) δ116.2, 99.8, 81.2, 81.2, 80.7, 75.0, 55.1, 52.6, 26.2, 26.1; HRMS (ESI) Calcd. for C₁₁H₁₅N₃NaO₄ (M+Na)⁺. 276.0960; found: 276.0962.

Supplementary Material

1_si_001. Supporting Information Available.

Experimental procedures, full characterization data for new compounds, and copies of NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.