Hydrogenolysis of the 4,6-O-Ketals of Glucopyranosides. Configuration-Dependent High Regio- and Stereo-Selectivity of the Diastereoisomeric Acetophenone Derivatives

Abstract

Hydrogenolysis (reductive cleavage) of the (R) isomers of the acetophenone 4,6-O-derivatives of glucopyranosides with LiAlH₄/AlCl₃ gives the 4-O-(1'-phenylethyl) ethers with (R) configuration; the corresponding (S) isomers produce the respective (R) 6-O-(1'-phenylethyl ) ethers are produced. The hydrogenolysis of other 4,6-O-ketals affords O 4 ether derivatives; the two diastereoisomeric 4,6-O-s-butylidene derivatives (cyclic ketals ) give O 4-ethers with the opposite absolute configuration. The stereoselectivity of these reactions is explained by the development of a four-centre transition state. The absolute configuration of the ethers has been determined by means of circular dichroism measurements.