Lewis-acid-catalysed isomerisation of benzylidene acetals of methyl α- l-rhamnopyranoside and methyl β- l-arabinopyranoside derivatives

Abstract

The Lewis-acid-catalysed isomerisation of several derivatives of methyl 2,3- O-benzylidene-α- l-rhamnopyranoside and methyl 3,4- O-benzylidene-β- l-arabinopyranoside has been investigated. The presence of equatorial substituents vicinal to the benzylidene ring decreases the rate of isomerisation, and exo isomers isomerise more quickly than the corresponding endo isomers. The occurrence of isomerisation during the reductive cleavage reaction of acetal rings with LiAlH 4-AlCl 3 has been demonstrated.