The 18-electron electrocyclisation of vinylogous fidecene. an unusual sequence of pericyclic processes

Upon thermal activation the vinylogous fidecene $\text{1}$ is isomerised into the pentacyclic indenoid hydrocarbon $\text{13}$ (benzene, t_{$\text{1}\text{2}$} (150°C) ca. 5 min), the formation of which is explained by an initial conrotatory (“symmetry-forbidden”) 18-electron electrocyclisation followed by two fast hydrogen migrations ([1.9], [1.5]) and 14π-electrocyclisation.