The reduction of benzylidene derivatives of pentose diethyl dithioacetals with lithium aluminium hydride-aluminium trichloride

Abstract

Di- O-benzylidene derivatives of pentose diethyl dithioacetals were reduced to di- O-benzyl pentose diethyl dithioacetals in high yield. In three of the five cases studied, single products were obtained: 2,3:4,5-di- O-benzylidene- d-arabinose diethyl dithioacetal gave the 2,5-di- O-benzyl derivative; 2,4:3,5-di- O-benzylidene- d-xylose diethyl dithioacetal, the 2,3-di- O-benzyl derivative; and 2,4:3,5-di- O-benzylidene- d-lyxose diethyl dithioacetal, the 4,5-di- O-benzyl derivative. The others gave mixtures. It is suggested that the nature of the products obtained was determined by steric effects and by a preference for initial cleavage of secondary oxygen-acetal carbon bonds. The O-benzyl groups caused the conformations of the di- O-acetyl derivatives of these products to be significantly different than either the parent pentose diethyl dithioacetals or their peracetylated derivatives for four of the six compounds studied.