Ab initia theoretical investigation of a formal alkene-enedione intramolecular [2 + 2] photocyclization

Irradiation of 1 with visible light results in intramolecular [2 + 2] photocyclization to afford the corresponding pentacyclic cage dike-tone, i.e., pentacyclo[5.4.0.02,6.03,10.0 5,9]undecane-8,11-dione (2). The mechanism of this reaction has been scrutinized by using ab initio theoretical methods. The results of these calculations provide new evidence which supports earlier suggestions that alkene-enedione photocyclizations may actually proceed via a diradical stepwise mechanism through the triplet excited state rather than as concerted [2 + 2] cycloadditions.