Total synthesis of a tetra- and two pentasaccharide fragments of the O-specific polysaccharide of Shigella flexneri serotype 2a

The synthesis of the methyl glycoside of the branched pentasaccharide biological repeating unit of the O-antigen of Shigella flexneri serotype 2a is described together with that of the methyl glycoside of the corresponding tetrasaccharide and frame-shifted linear pentasaccharide. All the strategies disclosed herein involve a key disaccharide corresponding to the branching point and otherwise appropriate monosaccharide building blocks activated as their trichloroacetimidate. Our data suggest partial lack of conformational flexibility at the branched residue.