Photochemical -Cycloaddition and Dimerization Reactions

Open-chain alkenes undergo dimerizations in stereospecific [nl + tt ] paths in low yields on irradiation. For example, photodimerization of Z-2-butene 26 gives two products in which cw-geometry of methyl groups is retained. Similarly, the cycloadducts from irradiation of -2-butene 27 retains the trans-geovasXry of methyl groups [29]. Irradiation of Z-2-butene also gives cycloadduct with -stil-bene in a similar manner [29]. 

Cyclohexene 27a undergoes photo-induced dimerization in the presence of xylene sensitizer to give a mixture of stereoisomers in non-stereospecific manner [30]. 

1-Phenylcyclohexene 28 also undergoes dimerization on irradiation directly or in the presence of sensitizer to give tail-to-tail products. This indicates that both singlet and triplet excited states diradical intermediates are involved in the formation of the products [31]. 

Stereocontrolled photoaddition of allene to cyclopent-l-ene-l-carboxaldehyde 29 has been utilized in the introduction of exocyclic methylene group in steviol 30 [32]. 

Intramolecular -cycloadditions are also observed in cyclic 1,4-dienes 31 

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