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

A tetrasaccharide, alpha -L-Rhap-(1 -->3)-alpha -L-Rhap-(1 -->2)-alpha -L-R hap-(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. Selec tive 3-O-glycosylation of allyl alpha -L-rhamnopyranoside with 2,3,4-tri-O- acetyl-alpha -L-rhamnopyranosyl trichloroacetimidate gave allyl 2,3,4-tri-O -acetyl-alpha -L-rhamnopyranosyl-(1 --> 3)-alpha -L-rhamnopyranoside (3). B enzoylation, deallylation, and trichloroacetimidation afforded 2,3,4-tri-O- acetyl-alpha -L-rhamnopyranosyl-(1 --> 3)-2,4-di-O-benzoyl-alpha -L-rhamnop yranosyl trichloroacetimidate (6). Self condensation of 3,4-di-O-benzoyl-be ta -L-rhamnopyranose 1,2-methyl orthoester or 1,2-octyl orthoester gave met hyl or octyl 2-O-acetyl-3,4-di-O-benzoyl-alpha -L-rhamnopyranosyl-(1 -->2)- 3,4-di-O-benzOyl-alpha -L-rhamnopyranoside (16 or 17), and subsequent selec tive deacetylation gave the disaccharide acceptor (18 or 19). Coupling of 6 with 18 (or 19), followed by deacylation in ammonia-saturated methanol, pr oduced the target tetrasacharide. (C) 2001 Elsevier Science Ltd. All rights reserved.