Regioselective ring opening of exo- and endo-3,4-benzylidene acetals of arabinopyranoside derivatives with Lewis acids and reducing agents

Abstract

Dioxolane type 3,4-benzylidene acetals of benzyl β- l-arabinose either as a mixture or pure exo- and endo-isomers cleavaged with BF 3·OEt 2/Et 3SiH in dichloromethane or acetonitrile regioselectively, provided the 4- O-benzyl-3-hydroxy derivative. The reaction with TiCl 4/Et 3SiH or Cu(OTf) 2/Et 3SiH provided a mixture of 3- and 4- O-benzyl derivatives whereas with Cu(OTf) 2/BH 3·THF gave only hydrolyzed product. The regioselectivity of the reaction was proved to be directed by the acetyl substitution at C-2. Benzyl substitution provided a mixture of 3- and 4- O-benzyl derivatives in 1:1 ratio whereas non-substitution yielded the same mixture in 2:1 ratio.