strong photosensitizing ability of the fullerene cage and to the valence isomerization of 1 to the norcaradiene form, which was facilitated by the bulky tert-butyl groups. Competitive oxygenation of 1 and a related cycloheptatriene lacking the fullerenyl group (2) resulted in the formation of endoperoxides of the two molecules in comparable yields. This result demonstrates that the cycloaddition is not