Piperylenes, photosensitized dimerization

The photosensitized dimerization of cis- and /ra/w-piperylene has been found to lead to at least 15 products, six of which are cyclobutanes.(9>... 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

Very early in the study of photosensitization it was discovered that salicylaldehyde, 2,4-dihydroxybenzophenone, and 2-hydroxy-4-methoxy-benzophenone do not sensitize piperylene isomerization, indicating that enolization is much faster than quenching by the diene.37 Later it was shown that 2-hydroxybenzophenone would sensitize the dimerization of isoprene in concentrated solution, but that the reaction was much less efficient than when benzophenone was used as a sensitizer (Table I).36... 

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