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

Dioxolane type 3,4-benzylidene acetals of benzyl β-l-arabinose either as a mixture or pure exo- and endo-isomers cleavaged with BF3·OEt2/Et3SiH in dichloromethane or acetonitrile regioselectively, provided the 4-O-benzyl-3-hydroxy derivative. The reaction with TiCl4/Et3SiH or Cu(OTf)2/Et3SiH provided a mixture of 3- and 4-O-benzyl derivatives whereas with Cu(OTf)2/BH3·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.

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