Stereoselective synthesis of 2-C-methylene glycosides and disaccharides via direct allylic substitution of hydroxy group in benzylated glycals

InCl3 efficiently catalyzes allylic substitution of the hydroxy group of 2-C-hydroxymethyl glycals to afford a diversity of 2-C-methylene alkyl and aryl glycosides as well as disaccharides in high yields. This protocol surpasses the existing methods for the synthesis of 2-C-methylene glycosides as it obviates the need for functionalizing the allylic hydroxy group of glycals. The interest of this methodology relies on the extremely mild conditions required even with a free hydroxyl group at the allylic position of the glycals and that too only with a catalytic amount of InCl3. The reaction is fast (30 min.), stereoselective and is compatible with a variety of oxygenated nucleophiles including those possessing acid-labile groups. A mechanistic investigation on the direct formation of an α,α-(1→1)linked disaccharide derivative from 2-C-hydroxymethyl galactal reveals that the reaction proceeds through a domino Ferrier rearrangement followed by a facile 1,3-alkoxy migration.