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 at -78 degrees C in the presence of 2,6-di-tert-butyl-4-methylpyridine affords a highly active glycosylating species which, on addition of alcohols, provides alpha-glucosides with high selectivity. This selectivity stands in stark contrast to the analogous mannopyranoside series, which affords the beta-mannosides with excellent selectivity under the same conditions. Low-temperature NMR experiments support the notion that a glucosyl triflate is formed in the initial activation step. Possible reasons for the diverging stereoselectivity in the gluco and manno series are discussed.