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

Irradiation of 1 with visible light results in intramolecular [2 + 2] photo cyclization to afford the corresponding pentacyclic cage diketone, i.e., pe ntacyclo [5.4.0.0(2.6).0(3,10).0(5,9)] undecane-8,11-dione (2). The mechani sm of this reaction has been scrutinized by using ab initio theoretical met hods. The results of these calculations provide new evidence which supports earlier suggestions that alkene-enedione photocyclizations may actually pr oceed via a diradical stepwise mechanism through the triplet excited state rather than as concerted [2 + 2] cycloadditions.