A highly α-selective glycosylation for the convenient synthesis of repeating α-(1→4)-linked N-acetyl-galactosamine units

The repeating GalpNAc-alpha-(1 -> 4)-GalpNAc unit is part of a series of essential structures that can be found in many important biomolecules such as the glycoproteins and the O-antigenic polysaccharides of clinically important bacterial strains. In this paper, we describe an exclusive alpha-selective glycosylation reaction, using a 4,6-di-O-tert-butyldimethylsilyl-N-acetyloxazolidinone-protected thioglycoside as the glycosyl donor, under pre-activation conditions, with only half amount of the promoter, providing the product GalpNAc-alpha-(1 -> 4)-GalpNAc in high isolated yield. This reaction can be also applied to increasing the length of the repeating structure, which is of significant use in further synthesis of branched or linear oligosaccharides. (C) 2010 Elsevier Ltd. All rights reserved.