1. All calculations were performed with GAUSSIAN 98.5Geometries were optimized at the B3LYP/6-31G(d) level,6which has been shown to be quite reliable for describing structures and energetics for pericyclic reactions.7All structures were characterized by frequency calculations at the B3LYP/6-31G(d) level, and zero-point energy corrections (scaled by 0.9806)8from these calculations are included in the reported energies. For selected structures, additional optimizations and frequency calculations at the B3LYP/6-311+G(d,p) level were performed. As described in the text, the geometries and activation barriers obtained at this level of theory did not differ significantly from those computed at the B3LYP/6-31G(d) level.