A SULFUR-STABILIZED ENOL

The preparation and X-ray structure of ,5,5,6,7,8a-hexahydro-4H-thioch romene-2-carboxylic acid methyl ester are described. The compound crys tallizes in the orthorhombic space group Pca2(1) with cell parameters a/angstrom 11.942(9), b/angstrom 5.399(1), c/angstrom 17.503(6), V/ang strom3 1129(2). The molecule is an enol with an intramolecular H-bond. NMR and IR spectra show that the main tautomer is also the enol form in solution, the enol content being 74% in CDCl3. The major solution c onformation is found to be similar to the solid-state conformation. Mo lecular mechanics calculations reproduce the conformational equilibriu m well but indicate that the enol form of the title compound is strain ed and would be less stable than the keto form. The role of the alpha- sulfur as a stabilizer of the enol is investigated with ab initio calc ulations.