Mechanisms of both the photochemical and thermal Diels-Alder reactions of C-60 with Danishefsky's dienes are studied on the basis of product stereochemistry, kinetics, and the detection of radical intermediates. A stereochemically defined(1E,3Z)-1-methoxy-2-methyl-3-[(trimethylsilyl)oxy]-penta-1,3-diene (1a) is used as a stereochemical probe in the Diels-Alder reactions with C-60. The major Diels-Alder product is trans-adduct (3) rather than cis-adduct (4) in both the photochemical and thermal Diels-Alder reactions. Such stereochemistry indicates that the Diels-Alder reactions proceed by a stepwise mechanism rather than a concerted mechanism. The transient spectra of C-60(.-) formed in photoinduced electron transfer from 1a to the triplet excited state of C-60 (C-3(60)*) have been detected successfully in laser flash photolysis of the 1a-C-60 system. The observed rate constants determined from the dependence of the quantum yields on the concentrations of Danishefsky's dienes agree well with those for the photoinduced electron transfer from Danishefsky's dienes to C-3(60)*. Such an agreement together with the formation of trans-adduct (3) indicates that the photochemical Diels-Alder reaction proceeds via stepwise bond formation in the radical ion pair produced in the photoinduced electron transfer from Danishefsky's dienes to C-3(60)* On the other hand, the rate constants for the thermal Diels-Alder reactions of Danishefsky's dienes are 10(16) times larger than those expected for outer-sphere electron transfer from Danishefsky's dienes to C-60 Thus, a strongly unsymmetrical orbital interaction is required for the thermal Diels-Alder reaction to yield trans-adduct (3) which would not be produced by a concerted pathway.