Mechanism of cheletropic reactions of 1,3-dienes with sulfur dioxide

In a recent paper [N.S. Isaacs and A. Laila, J. Phys. Org. Chem. 7, 178 (1994)2] the volume profile for the cheletropic addition of sulphur dioxide to a 1,3-diene (2,3-dimethylbuta-1,3-diene) indicated that a two-step mechanism for this reaction, with a (4 + 2) cycloaddition at the S(DOUBLE BOND)O bond as the slow step followed by a rapid rearrangement of the resultant sulpheno-lactone, cannot be ruled out. However, ab initio calculations, at the MP2(FC)/6-31G*//RHF/6-31G* level, show that such a mechanism can be conclusively discarded.