Abstract Nucleophilic displacement by the cyanide anion of the 6- O -triflyl group in phenyl 6- O -triflyl-2,3-di- O -benzyl-4- O - p -methoxybenzyl-1-thio-β- d -galactopyranoside takes place via an intermediate 1,6-sulfonium salt resulting from the anchimeric assistance of the C-1 phenylthio group.