Synthetic oligosaccharides related to Group B streptococcal polysaccharides. The rhamnotriose moiety of the common antigen

Abstract

The methyl glycoside (1) of the α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → 2)-α-L-Rhap-(1 → portion of the group-specific, common antigen polysaccharide of Group B Streptococci has been synthesized. The key compound of the synthesis was methyl 3,4-di-O-benzyl-α-L-rhamnopyranoside (4). Controlled acetolysis of 4 gave 1,2-di-O-acetyl-3,4-di-O-benzyl-α-L-rhamnopyranose (10), which upon reaction with 4 under catalysis by trimethylsilyl trifluoromethanesulfonate (TMS-Tf) yielded the α-(1 → 2)-linked dirhamnoside. Subsequent rhamnosylation followed by deprotection gave 1. TMS-Tf was also used in rhamnosylation reactions with tetra-O-acetyl-α-L-rhamnopyranose. Acetylation of acceptor alcohols was found to be a major side reaction in the TMS-Tf assisted glycosylations. One-dimensional nuclear Overhauser enhancement differential spectroscopy indicated that, in solution, 1 takes up a bent instead of an extended conformation.