Mechanisms of both the photochemical and thermal Diels-Alder reactions of C60 with Danishefsky's dienes are studied on the basis of product stereochemistry, kinetics, and the detection of radical intermediates. A stereochemically defined (1 E, 3 Z)-1-methoxy-2-methyl- 3-[(trimethylsilyl) oxy]-penta-1, 3-diene (1a) is used as a stereochemical probe in the Diels- Alder reactions with C60. The major Diels-Alder product is trans-adduct (3) rather than cis- ...