Benzylidene acetal fragmentation route to 6-deoxy sugars:: Direct reductive cleavage in the presence of ether protecting groups, permitting the efficient, highly stereocontrolled synthesis of β-D-rhamnosides from D-mannosyl glycosyl donors.: Total synthesis of α-D-Gal-(1→3)-α-D-Rha-(1→3)-β-D-Rha-(1→4)-β-D-Glu-OMe, the repeating unit of the antigenic lipopolysaccharide from Escherichia hermannii ATCC 33650 and 33652

An approach to the stereocontrolled synthesis Of beta-D-rhamnopyranosides is described in which 2,3-O-benzyl or related 4,6-O-[alpha-(2-(2-iodophenyl)ethylthiocarbonyl)benzylidene]-mannosyl thioglycosides are first used to introduce the beta-D-mannopyranoside linkage in high yield and stereoselectivity. Following glycosylation, treatment with tributyltin hydride in toluene at reflux brings about reductive radical fragmentation directly to the 6-deoxy sugar in high yield. A variation of these donors bearing a carboxylated donor on O(3) is a highly alpha-selective mannosyl and, after radical fragmentation, alpha-D-rhamnosyl donor. Using this stereoselective glycosylation/radical-fragmentation approach, a concise synthesis of the title tetrasaccharide is realized in which both the beta-D- and alpha-D-rhamnopyranosyl units are obtained in a single step by a double radical fragmentation of the modified benzylidene acetals.