Synthesis of specifically deoxygenated disaccharide derivatives of the Shigella dysenteriae type 1 O-antigen

The synthesis of methyl O-α-l-rhamnopyranosyl-(1 → 2)-α-d-galactopyranosides specifically deoxygenated at position 2 (31), or 4 (21) of the rhamnopyranosyl residue was accomplished using methyl 3,4,6-tri-O-benzoyl-α-d-galactopyranoside (18) as the glycosyl acceptor. Phenyl thionocarbonate activation of the penta-O-benzoylated disaccharide precursor followed by Barton reduction and Zemplén transesterification gave 31, while 21 was obtained via condensation of the deoxygenated monosaccharide donor with 18, and subsequent debenzoylation of the product.