CoII-tetraphenylporphyrin-catalyzed decomposition of bicyclic endoperoxides 4 and 5 with a strained double bond moiety has been studied. Compounds 4 and 5 have been synthesized by photooxygenation of 3 which itself was obtained by dichloroketene addition to cycloheptatriene, followed by removal of the chlorine atoms. An unusualdecompositionmode of 4 promoted by CoII-TPP resulted in the formation