Theoretical approach to the conformational and configurational stability of α-sulphinyl carbanions derived from 1,4-oxathiane S-oxides

Abstract

cis- and trans-2-phenyl-1,4-oxathiane S-oxides were synthesized by cyclization of 1-phenyl-2-vinylsulphinylethanol diastereoisomers in basic medium; only the R*, S*-epimer gave a mixture of oxathianes. In order to explain the racemization observed in that case we carried out a theoretical study of the α-sulphinyl carbanions presumably involved, using an ab initio method (Gaussian 80) which satisfactorily predicted the energies associated with axial and equatorial conformations of 1,4-oxathiane S-oxide. To perform these calculations we used force-field (MM2) geometries. The results predicted a high energy barrier to pyramidal inversion at the sulphur atom, suggesting that the thermal stereomutation of stereogenic sulphur in these intermediates is improbable under the cyclization conditions. Nucleophilic attack of the alkoxide on the sulphinyl sulphur atom is therefore suggested to account for the observed epimerization.