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.
cycloadditions - arenes - photochemistry - stereoselectivity - natural products