α:β Selectivity in the synthesis of 3-substituted, 4-methyl umbelliferone glycosides of N-acetyl glucosamine and chitobiose

The influence of phenolic acceptor nucleophilicity; for example, 3-substituted, 4-methylumbelliferones, and glycosyl donor electrophilicity; for example, 3- and 4-substituted N-acetylglucosamines, on glycosylation stereochemistry has been evaluated. In a systematic comparison, the stereochemical outcome as well as the reaction yield appeared to be influenced by the 3- and 4-substituents of the donor as well as the 3-substituent of the aryl acceptor. In the context of synthesizing a fluorogenic substrate for oligosaccharyltransferase, an alpha-glycoside was desired. Although most acceptor-donor pairs led to predominantly or exclusively the beta-glycoside, reaction of the most activated (3,4-di-O-benzyl) donor and the least nucleophilic acceptor (3-Br), resulted in a 1:1 ratio of alpha,beta arylglycosides. (C) 2004 Elsevier Ltd. All rights reserved.