ABSTRACT
Isopropenyl glycosides have been easily obtained in high yield by reacting the corresponding anomeric acetates with the Tebbe reagent. These compounds are efficient glycosyl donors in the presence of trimethylsilyl triflate or boron trifiuoride etherate, probably via a mixed acetal glycoside intermediate. On the basis of this mechanism, glycosylation of monosaccharide hemiacetal donors with acceptors bearing an isopropenyl ether function has been achieved. Glycosylating properties of isopropenyl glycosyl carbonate has also been investigated. Anomeric S-xanthates were conveniently prepared by a two step azidonitration-xanthation sequence from a series of galactals. They constitute a novel class of glycosyl donors for the stereoselective preparation of either α- or β- protected precursors of biologically important galactosamine-containing oligosaccharides. Substituted pyranoid glycals were in turn easily derived from glycosyl phenyl sulfones acetylated at C-2 by a reductive samariation-elimination sequence.
