Reaction of 2-O-unprotected 1-O-silyl-protected D-glucose and D-galactose derivatives5a-dwith benzyl bromide in the presence of sodium hydride as the base afforded 1-O-benzyl 2-O-silyl derivatives6aα/β -6dα/β. Thus, prior to anomericO-benzylation,trans-1,2-silyl group migration takes place. Ensuing removal of the 2-O-silyl group furnishes 2-O-unprotected compounds8aα/β -8dα/β, which are useful building blocks. More prone to 1-O-silyl group migration is mannose as shown for derivatives of 4,6-O-benzylidene-D-mannose9.Cis-1,2- andcis-2,3-silyl group migrations affording compounds15and13were already observed on deacetylation of the thexyldimethylsilyl 2,3-di-O-acetyl derivative12β under Zemplén conditions.