Synthesis of an l-rhamnose tetrasaccharide, the common and major structure of the repeating unit of the O-antigenic polysaccharide of a strain of Klebsiella pneumoniae and Pseudomonas holci

A tetrasaccharide, α-l-Rhap-(1→3)-α-l-Rhap-(1→2)-α-l-Rhap-(1→2)-l-Rhap, the common and major structure of the repeating unit of the O-antigenic polysaccharide of a strain of Klebsiella pneumoniae and Pseudomonas holci was synthesized as its methyl and octyl glycosides. Selective 3-O-glycosylation of allyl α-l-rhamnopyranoside with 2,3,4-tri-O-acetyl-α-l-rhamnopyranosyl trichloroacetimidate gave allyl 2,3,4-tri-O-acetyl-α-l-rhamnopyranosyl-(1→3)-α-l-rhamnopyranoside (3). Benzoylation, deallylation, and trichloroacetimidation afforded 2,3,4-tri-O-acetyl-α-l-rhamnopyranosyl-(1→3)-2,4-di-O-benzoyl-α-l-rhamnopyranosyl trichloroacetimidate (6). Self condensation of 3,4-di-O-benzoyl-β-l-rhamnopyranose 1,2-methyl orthoester or 1,2-octyl orthoester gave methyl or octyl 2-O-acetyl-3,4-di-O-benzoyl-α-l-rhamnopyranosyl-(1→2)-3,4-di-O-benzoyl-α-l-rhamnopyranoside (16 or 17), and subsequent selective deacetylation gave the disaccharide acceptor (18 or 19). Coupling of 6 with 18 (or 19), followed by deacylation in ammonia-saturated methanol, produced the target tetrasacharide.