ISOPROPENYL GLYCOSIDES AND CONGENERS AS NOVEL CLASSES OF GLYCOSYL DONORS - THEME AND VARIATIONS

Isopropenyl glycosides (i.e., 10 and 11) have been synthesized in high yields by reacting the corresponding anomeric acetates with the Tebbe reagent. These compounds undergo glycosylation with primary or secondary carbohydrate alcohols in the presence of trimethylsilyl triflate or boron trifluoride etherate, probably via a mixed acetal glycoside intermediate. On the basis of this principle, a quite efficient glycosylation of monosaccharide hemiacetal donors (i.e., 1, 7, and 9) with acceptors bearing an isopropenyl ether function at a primary or secondary position (i.e., 18 and 21) has been developed. Also investigated were the glycosylating properties of isopropenyl glucosyl and galactosyl carbonates (i.e., 12-15), easily prepared from the corresponding hemiacetals, toward sugar alcohols. In each case, the beta-selective synthesis of disaccharides from donors having nonparticipating groups at C-2 was ensured by the use of acetonitrile, at low temperature, as the solvent.