The [4+2]π cycloadduct (3aa) of phencyclone (1a) and benzoquinone (2a) readily transformed into the corresponding [2+2]π cycloadduct (4aa) during purification. The structure of the triketo cage compound was determined by single crystal X-ray analysis. The structure–reactivity relationships are discussed based on the PM6-calculated energy profiles for the whole photocycloaddition of the [4+2]π cycloadducts of some cyclopentadienones and benzoquinones.