Synthesis and Diels-Alder reactions of 2-acetamido-3-phenylthio-buta-1,3-diene and its derivatives

3-Acetamido-4-sulfur-substituted 3-sulfolenes 2a-c which are convenient precursors to the corresponding dienes la-e have been synthesized, The Diels-Alder reactions of the dienes la-e can be efficiently carried out by heating the 3-sulfolenes 2a-c with dienophiles, The reactions with unsymmetrical dienophiles demonstrate that the 'para'-directing ability of the substituents on the 2,3-disubstituted I,3-dienes follows the order AcNH > PhSO2 > PhS > PhSO. However, Lewis acids can change significantly the regioselectivity of the reaction; for the diene 1a, the regiocontrol is even reversed, Semiempirical analyses of the diene FMO coefficients and the transition states involved in the cycloaddition provide some insight into the factors which affect the regioselectivity.