"Syn-effect" in the base-induced isomerization of vinylic sulfones to allylic sulfones and the related various reactions

Abstract

Syn-effect" has been recognized as a major cause of stabilizing the syn-conformation at the transition state of a number of reactions against the steric hindrance. This account deals with the origin of the "syn- effect" on the basis of the stereochemical outcome in the isomerization of vinylic sulfones leading to the formation of allylic sulfones and in the related various reactions. We proposed that a σ→π* interaction between the σ-orbital(s) of the allylic C-H σ-bond(s) (or of the C-H σ-bond(s) at the α-position for carbonyl compounds) and the antibonding orbital (π*) of the C=C double bond (or of the C=O double bond for carbonyl compounds) is the most important and essential factor for the "syn-effect", though the contribution of a 6π-electron homoaromaticity and/or of a hydrogen bonding cannot be entirely ruled out.