Photochemical cycloaddition between benzene derivatives and alkenes

The photochemical cycloaddition of benzene derivatives with alkenes yield polycyclic compounds, which are of great interest in organic synthesis. Three modes are observed: [2+2] (ortho), [3+2] (meta) and, less frequently, [4+2] (para) photocycloaddition. The [3+2] cycloaddition has preferentially been studied and its intramolecular version was frequently applied to the synthesis of natural products. The [2+2] photocycloaddition is observed when electron active subsituents are present on the reaction partners. This mode is often followed by photochemical or thermal rearrangements. Furthermore, the [2+2] mode occurs concomitantly with the [3+2] photocycloaddition and leads frequently to complex reaction mixtures. More recent investigations deal with this item and show how the reaction may be directed to the [2+2] mode.