Catalytic and Stereoselective Glycosylation with Glucosyl Thioformimidates

A novel and efficient glycosyl donor having a p-trifluoromethylbenzylthio-N-p-trifluoromethylphenylformimidate group at an anomeric position is easily prepared by the addition of anomeric hydroxy group of 2,3,4,6-tetra-O-benzyl-(alpha),(beta)-D-glucopyranose to p-trifluoromethylphenyl isothiocyanate, followed by treatment with ptrifluoromethylbenzyl bromide. Catalytic and stereoselective glycosylation of various glycosyl acceptors with the above glycosyl donor smoothly proceeds by using various protic and Lewis acid catalysts which interact with its nitrogen atom. Further, catalytic and highly 1,2-cis or 1,2-trans stereoselective and chemoselective glycosylation between two different "armed" and "disarmed" glycosyl p-trifluoromethylbenzylthio-N-p-trifluoromethylphenylformimidates is also performed effectively in the presence of a catalytic amount of trifluoromethanesulfonic acid (TfOH) at -78 °C in tBuOMe or EtCN, respectively. These glycosylations are applied to successful one-pot sequential syntheses of trisaccharides.