Efficient Strategy for α-Selective Glycosidation of d-Glucosamine and Its Application to the Synthesis of a Bacterial Capsular Polysaccharide Repeating Unit Containing Multiple α-Linked GlcNAc Residues

An efficient α-selective glycosylation method was developed for the synthesis of 2-deoxy-2-amino-d-glucosides based on synergetic α-directing effects of the TolSCl/AgOTf promotion system and the functional groups at the corresponding azido donor 6-O-position to exert steric β-shielding effect or remote participation in the glycosylation reaction. Its practicability was verified with a wide range of monosaccharide glycosyl acceptors and the first, one-pot synthesis of the challenging pentasaccharide repeating unit of an Acinetobacter baumannii K47 capsular polysaccharide.