Conjugated diene, 1,2-addition electrocyclic reactions

Photochemical electrocyclic ring-closure in a 4-electron system works well for many acvclic dienes (2.17) and related cvclic systems (2.18). The situation with conjugated trienes is more complex, and they can act as 6-e ectron systems (2.19) leading to cyclohexal, J-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond (which must be cis if a 6-electron electrocyclic nng-closure is to take place). 

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