Convergent synthesis of the hexasaccharide repeating unit of the capsular polysaccharide of Klebsiella serotype K-34

Abstract

A convergent [4 + 2] block synthetic strategy has been developed for the synthesis of the hexasaccharide repeating unit of the capsular polysaccharide of Klebsiella serotype K-34 in very good yield. A tetrasaccharide acceptor was synthesized by sequential stereoselective glycosylations of four suitably protected d-glucose and l-rhamnose units and a disaccharide thioglycoside was prepared by orthogonal glycosylation of d-galactosyl thioglycoside acceptor and l-rhamnosyl trichloroacetimidate donor. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4–SiO2) has been used as thiophilic glycosylation activator in the stereoselective glycosylations. A late stage oxidation of primary hydroxyl group to a carboxylic group in the hexasaccharide was achieved in excellent yield using a combination of TEMPO and bis(acetoxy)iodobenzene (BAIB). All glycosylation reactions were highly stereoselective.