W. Tetrahedron Lett. 1983, 24,445,447. Troet, Vladuchick, and Bridges have previously studied in detail the internal competition of sulfur and oxygen substituents as regiocontrol elements in the diene component of the Diele-Alder reaction: Trost, BM; Vladuchhk, WC; Bridges, AJJ Am. Chem. SOC. 1980, 102, 3548, 3554. For a recent discussion of theoretical aspecta of the regiochemical outcome of Diels-Alder reactions, see: Alston, PV; Gordon, ...