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.