Given the various possible conformers of the two reactants, we consider the reaction path when the T1 state diene in its most stable conformer adds to an S0 state diene in its most stable conformer, and we discuss the paths with the least steric congestion between the two reactants which should lead to the lowest activation barriers. In the case of 5 and 6, the S0 state conformers considered were the most stable ones of the E-and E,E-isomers. Furthermore, two orthogonally twisted T1 state structures are possible for each of 2, 4, and 6 as these dienes can be twisted about either the C1-C2 or C3-C4 bond (Fig. ).