Dehydrative glycosylation with activated diphenyl sulfonium reagents.: Scope, mode of C(1)-hemiacetal activation, and detection of reactive glycosyl intermediates

The development of a method for direct dehydrative glycosylations with 1-hydroxyglycosyl donors employing the reagent combination of triflic anhydride and diphenyl sulfoxide is described, The one-pot coupling method is a facile process which is applicable to a variety of carbohydrate coupling partners. Oxygen-18-labeling studies are consistent with the first step in carbohydrate activation being the formation of an anomeric oxosulfonium intermediate. This reactive glycosyl species (35) is observable in low-temperature NMR experiments when 2,3,4,6-tetra-O-methyl-D-mannopyranose is activated as the glycosyl donor. When the dehydrative glycosylation reaction is performed in the presence of the triflic acid scavenger 2-chloropyridine, NMR analysis reveals that the glycosyl oxosulfonium intermediate 35 is converted to the corresponding anomeric 2-chloropyridinium species 36.