Chemistry of 4,6-O-benzylidene-D-glycopyranosyl triflates: Contrasting behavior between the gluco and manno series

Activation of either anomer of S-phenyl 2,3-di-O-benzyl-4,6-O-benzylidene-1 -deoxy-1-thia-D-glucopyranoside with triflic anhydride in dichloromethane a t -78 degrees C in the presence of 2,6-di-tert-butyl-4-methylpyridine affor ds a highly active glycosylating species which, on addition of alcohols, pr ovides cl-glucosides with high selectivity. This selectivity stands in star k contrast to the analogous mannopyranoside series, which affords the beta- mannosides with excellent selectivity under the same conditions. Low-temper ature NMR experiments support the notion that a glucosyl triflate is formed in the initial activation step. Possible reasons for the diverging stereos electivity in the gluco and manno series are discussed.