Hydrogenolysis of carbohydrate acetals, ketals, and cyclic orthoesters with lithium aluminium hydride – aluminium trichloride

Abstract

The reduction of hexofuranose and hexopyranose acetals with LiAlH4–AlCl3 (1:1, probably AlH2Cl) to the corresponding ethers proceeds in similar fashion as that of simple acetals and ketals (Scheme I) (refs. 1–7). Certain O-methyl and O-benzyl derivatives, not readily obtainable otherwise, are conveniently synthesized from appropriate acetals in this way. Cyclic orthoesters of glucofuranose may be selectively reduced to acetals; the steric course of such reduction has been shown unequivocally in the case of orthoformates (Scheme II). Of several cleavable groups, the ease of hydrogenolysis is (a) cyclic orthoester > isopropylidene ketal, cyclohexylidene ketal > benzylidene acetal > ethylidene acetal > formal and (b) 5,6-O-linked or 3,5-O-linked acetal or ketal > 1,2-O-linked acetal or ketal.