Stannylene‐Mediated Regioselective 6‐ O ‐Glycosylation of Unprotected Phenyl 1‐Thioglycopyranosides

A straightforward procedure is described for the synthesis of (1→6)‐linked saccharides by regioselective glycosylation of unprotected glycosyl acceptors. Phenyl 1‐thioglycopyranosides derived from D ‐glucose, D ‐galactose and D ‐mannose were treated with dibutyltin oxide to introduce a stannylene acetal, and then subjected to selective glycosylation at the 6‐position with the Koenigs–Knorr protocol. Peracylated glycosyl bromides of D ‐glucose, D ‐galactose, D ‐mannose and D ‐glucosamine were employed as the donors to give the corresponding (1→6)‐linked disaccharides in moderate to good yields. The best results were obtained with glycosyl donors and acceptors derived from D ‐glucose and D ‐galactose. Fully acylated disaccharide thioglycosides could also serve as glycosyl donors for the regioselective coupling. Brominolysis and subsequent Koenigs–Knorr coupling with the stannylene acetal of phenyl 1‐thio‐β‐ D ‐glucopyranoside gave rise to the corresponding (1→6)‐linked trisaccharides in moderate yields.