Synthesis of the unique trisaccharide: 6-d-L-Talp alpha(1->2)-L-Rhaf beta(1->5)-DHA

Preparation of the title trisaccharide resulted from two remarkable facts: 1) during acetolysis of the anomeric t-butyl group in the fully protected disaccharide 4, the rhamnopyranoside ring underwent an unusual conversion into the furanose form to furnish 5a; 2) stereoselective coupling of 5-O-acetyl-3-O-benzyl-2-O-(2,3,4-tri-O-acetyl-6-deoxy-alpha-L-talopyranosyl)-alpha-L-rhamnofuranosyl bromide (5b) with the glycosyl acceptor 7b formed surprisingly the beta-linkage, to give the corresponding trisaccharide 8a. Copyright (C) 1996 Published by Elsevier Science Ltd