Simple Synthesis of <i>trans</i>-<i>α</i>,<i>β</i>-Dibenzyl-<i>γ</i>-butyrolactone Lignans by Diastereoselective Reduction of <i>α</i>-Benzylidene-<i>β</i>-benzyl-<i>γ</i>-butyrolactones Using NaBH4–NiCl2

Abstract

Abstract trans-α,β-Dibenzyl-γ-butyrolactone lignans were synthesized by stereoselective reduction of α-benzylidene-β-benzyl-γ-butyrolactones using NaBH4–NiCl2. The reduction is found to proceed via conjugate addition of a hydride to an α-benzylidene-β-benzyl-γ-butyrolactone and the stereoselective protonation of the resulting metal enolate. The stereoselectivity would be brought about by the conformational rigidity of the phenyl moiety of the α-benzyl group induced by 1,3-allylic strain.