Unsaturated carbohydrates. Part IX. Synthesis of 2,3-dideoxy-α-D-erythro-hex-2-enopyranosides from tri-O-acetyl-D-glucal

Tri-O-acetyl-D-glucal undergoes complete reaction with alcohols in benzene solution in the presence of boron trifluoride to give 4,6-di-O-acetyl-2,3-dideoxy-D-erythro-hex-2-enopyranosides. The α-anomers predominate (ca. 90%), and the method can be used to prepare the known crystalline ethyl α-glucoside easily and in greatly improved yield. Other alkyl glycosides have been prepared similarly, and the procedure has afforded means of obtaining the cholesteryl analogue and the disaccharide derivative 6-O-(4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranosyl)-1,2:3,4-di-O-isopropylidene-α-D-galactopyranose. Tri-O-acetyl-D-glucal again gave the 2,3-unsaturated glycosides on treatment with acetals in the presence of boron trifluoride; no evidence was obtained for the formation of branched-chain products produced by additions to the double bond.