2,3-Unsaturated Allyl Glycosides as Glycosyl Donors for Selective α-Glycosylation

In the presence of NBS and a catalytic amount of a Lewis acid, 2,3-unsaturated allyl glycosides [6-(allyloxy)-3,6-dihydro-2-(hydroxymethyl)-2H-pyran-3-ol] have been successfully used as versatile glycosyl donors for the stereoselective alpha-glycosylation of a variety of alcohols comprising sensitive functions such as acetonide, keto, nitro, and ester in 50-90% yields. The methodology offers an equally facile alternative to 4-pentenyl replacement in unsaturated sugars.