Singlet oxygenation of triquinacene, barrelene, and homobarrelene

The singlet oxygenation of three polycyclic hydrocarbons, triquinacene, barrelene and homobarrelene was studied. Triquinacene reacted by way of a perepoxide intermediate, transferring an oxygen atom to another triquinacene molecule to give exclusively the mono epoxide. Barrelene, on the other hand, underwent a rare homo-Diels-Alder reaction with ¹O₂ to give the decomposition product from the initial tetracyclic 1,2-dioxolane leading to benzofuran. The latter reacted with ¹O₂ in a [2+2] cycloaddition to give an unstable 1,2-dioxetane which collapsed to 2-formylphenyl formate. The latter was independently synthesize via singlet oxygenation of authentic benzofuran. Homobarrelene reacted in a similar fashion to give a homoDiels product, decomposition of which led to a keto aldehyde which was characterized spectroscopically. Computational work confirms the barrelene and homobarrelene reactions with ¹O₂ as concerted [_π2_s+_π2_s+_π2_s] cycloadditions.