[2+2]-Photocycloaddition Reactions of Cyclopentenones with Alkenes

The multistep mechanism of the reaction was established on the basis of spectroscopic and kinetic data and from the selective intermediate trapping. However, new experiments were needed not only to extend the value of the mechanistic conclusions to a larger variety of substituted cyclopentenones but also to give a better understanding of intramolecular photocycloaddition processes. These new investigations, using the selective trapping of intermediates, have allowed determination of the relative deactivation rates for 1,4-biradical intermediates. Furthermore, a verification of the validity of the “rule of five” in intramolecular processes has been possible. Simultaneously, attempts to develop asymmetric [2+2]-photocycloadditions have been described in connection with synthetic applications. The high efficiency and high selectivity usually observed in the [2+2]- photocycloaddition of cyclopentenone derivatives with alkenes have led synthetic chemists to use this reaction for the building of complex structures and polyfunctional molecules. Thus, the purpose of this chapter is to summarize the recent findings on the chemo-or stereoselectivity of the cycloaddition process of cyclopentenones and related systems (furanones, maleic anhydrides, and maleimides) with alkenes or alkynes. A review of some recent synthetic applications of the reaction is also presented.