14π‐Electrocyclization across the two pentafulvenoid moieties of pentafulvene 1 occurs upon heating to provide dicyclopenta[a,d]cyclooctene 2 [Eq. (1)]. The nonalternant hydrocarbon 2 shows diatropic character and a distinctive absorption spectrum with the longest wavelength maximum at 767 nm.

14π‐Elektrocyclisierung über die beiden Pentafulvenoid‐Einheiten erfolgt beim Erhitzen von 1 zum Dicyclopenta[a,d]cycloocten 2 [Gl. (1)]. Dieser nichtalternierende Kohlenwasserstoff weist diatropen Ringstrom auf und hat ein charakteristisches Absorptionsspektrum mit einem längstwelligen Maximum bei 767 nm.

Dihydroazulenones 5 and 6 were prepared by photochemical isomerisation from their isomers 7 and 8, respectively. 5 and 6 were found to be substantially in equilibrium with the norcaradiene forms, although isomers 7 and 8 exist as the cycloheptatriene forms.