Photocycloaddition of Enones Substrate Type Al

Cyclopentenones undergo a clean [2 + 2]-photocycloaddition to a variety of alkenes, resulting in products with a bicyclo[3.2.0]heptanone skeleton. The two annelated 

With regard to the simple diastereoselectivity, one can generalize that large substituents normally adopt opposite (trans) positions in the cydobutane product whenever feasible. 

In a pioneering study, Zandomeneghi and Cavazza showed that the [2 + 2]-photocycloaddition of racemic 4-acetyloxy-2-cydopentenone can lead to a small, but detectable, enantiomeric excess (ee) of the product if circularly polarized light (CPL) is used as an irradiation source. By preferential excitation of the (—(-antipode of the cyclopentenone with 1-CPL (351-363 nm laser) in the presence of acetylene, an 

In general, the stereoelectronic influence of substituents in [2 + 2]-photocycload-dition reactions is minor, and the preferred ground-state conformation often accounts for the formation of the major diastereoisomer. Inspection of molecular models and force field calculations provide a good picture of possible transition states leading via 1,4-biradicals to cyclobutane products. The total synthesis of (+)-guanacastepene represents another recent example for the use of stereoselective intramolecular cyclopentenone-olefin photocycloadditions in natural products synthesis [32]. 

Cyclohexenones require essentially identical irradiation conditions as cyclopente-nones (vide supra). The outcome of the intermolecular [2 + 2]-photocycloaddition to alkenes is somewhat more complex as compared to cyclopentenones, because the 

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