Double diastereoselection explains limitations in synthesizing mannose-containing β-(1,3)-glucans

It is known that 3-O-glycosylation of glucosidic acceptors bearing acyl groups in the 4 and 6 positions instead of a 4,6-O-benzylidene ring mainly affords a-glycosides. Described here is an unexpected stereochemical outcome for elongation at glucose O-3 of a beta-D-Glcp-(1 -> 3)-alpha-D-Manp disaccharide using peracetylated ethyl thioglucoside as a donor. This unexpected reaction was correlated with match-mismatch effects, as shown by efficient coupling of the same acceptor by a donor of L-configuration. (C) 2010 Elsevier Ltd. All rights reserved.