changed to the [8+2] cycloadduct under refluxing in xylene by a retro–Diels–Alderreaction, a re-cycloaddition reaction followed by spontaneous decarboxylation. The structure of the [8+2] cycloadduct was clearly confirmed by chemical transformation to methyl 3-isopropyl-1H-cyclopent[a]azulene-9-carboxylate which was obtained by the hydride reduction of methyl 3-isopropylidene-3H-cyclopent[a]azulene-9-carboxylate