Lithium Aluminum Hydride Reductive Rearrangement of Allylic Acetals to Vinyl Ethers: A Synthesis of 3-Deoxy Glycals

Carbohydrate allylic acetals (hex-2-enopyranosides) are reductively rearranged to vinyl ethers (3-deoxy glycals) by treatment with lithium aluminium hydride in refluxing ethereal solvents. Under similar conditions, some allylic alcohols are also reductively rearranged to olefins, although the reaction appears to be confined to carbohydrate substrates since the reaction fails with typical carbocyclic allylic alcohols.The results are rationalized by postulating the intermediacy of an oxygen–alane complex, this being formed more readily in the case of an hydroxyl (or ester) rather than an ethereal oxygen. An axial oxygen permits easier achievement of the reactive transition state than an equatorial oxygen. The complex normally leads to an SN2' reductive rearrangement in which the entering hydride and departing oxygen are syn-related. Alternatively, and particularly when the double bond is flanked by an hydroxyl group at one allylic position and a leaving group at the other, an abnormal SN2′ process may occur so that the hydride ion is delivered vicinal and cis to the hydroxyl group, with ejection of the leaving group. The process is at all times stereospecific but not always regiospecific.