Abstract
We achieved practical, highly stereoselective syntheses of three interglycosidic isomers of N-acetyl-β-D-mannosaminyl-L-rhamnoses, among which a β(1→4)-isomer corresponds to the repeating unit of the O-antigen of lipopolysaccharide (LPS) from the opportunistic pathogens Pseudomonas cepacia O5 and Pseudomonas aeruginosa X (Meitert). The other isomers are a β(1→2)-disaccharide, a constituent of LPS from Escherichia coli O1A, and an artificial β(1→3)-isomer. The disaccharides were obtained by simple three-step reaction sequences from 2-(benzoyloxyimino)-2-deoxyglycosyl halides (mannosamine progenitor). β-Selective glycosylations of appropriately protected L-rhamnosyl acceptors were performed. Subsequent reduction of the 2-acyloxyimino function to an amino group, N-acetylation, and removal of the protecting groups provided the target disaccharides. 13C-NMR and nuclear Overhauser effect spectra proved to be useful for stractural determination of the positional isomers of the disaccharides.
