Stereochemistry and structure in the tetrahydro-1-thio-4-pyrone and tetrahydro-4-pyrone series

Abstract

The preparation of the geometric isomers of 2,6-diphenyltetrahydro-1-thio-4-pyrone, and of its O-heterocyclic analogue is described, and configuration and conformation are assigned to each on the basis of n.m.r. spectra. In the formation of the thiopyrones from dibenzylidene acetone, the less stable isomer is shown to be kinetically favoured; a rationalisation is proposed. Evidence for a preferred quasi-boat conformation for the 5,6-dihydro-2H-thiopyran ring is presented. The structure and stereochemistry of some substituted tetrahydro-4-pyrones formed in the diethyl ketone–benzaldehyde condensation are described.