Strategies for the synthesis of branched oligosaccharides of the Shigella flexneri 5a, 5b, and variant X serogroups employing a multifunctional rhamnose precursor

Abstract

Allylation of methyl 2-O-benzoyl-4-O-benzyl-α-L-rhamnopyranoside (4) using allyl trichloroacetimidate under mildly acidic conditions provides the multifunctional rhamnose derivative (7). Glycosylation of the vicinal O-2 and O-3 positions of this derivative in either sequence permitted the synthesis, in good yield, of a branched trisaccharide (24) corresponding to a crucial portion of the Shigella flexneri variant X O-antigen. The flexibility of the derivative (7) was further demonstrated by the synthesis of a tetrasaccharide (36) containing the branch point of the S. flexneri serogroup 5a antigen. Attempts to synthesize the pentasaccharide (39) corresponding to the branched S. flexneri serogroup 5b antigen were hampered by the low yields and poor stereospecificity of the α-glucosylation reactions.