A Siloxane-Bridged Glycosyl Donor Enables Highly Stereoselective β-Xylulofuranosylation

We report a siloxane-protected donor (7) for the highly stereoselective formation of beta-(2,3-cis)-xylulofuranosyl bonds. Glycosylation reactions with 7 gave >80% yields, and only beta-xylulofuranosides were isolated in all cases. The utility of 7 for the synthesis of complex glycans was shown by its successful application to the preparation of the repeating unit from the lipopolysaccharide O-antigen of Yersinia enterocolitica serovars O:5/O:5,27. This structure is a pentasaccharide with two beta-xylulofuranose residues; using 7, both were introduced simultaneously with excellent stereocontrol.