Electrocyclization reaction of higher conjugated polyenals: Photochemical behaviors of retinal (vitamin A1 aldehyde) homologues.

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Abstract
During the studies on a photoreaction of retinal, involving various kinds of (Z)-(E) isomerization, a heretofore unknown photoproduct of retinal was isolated in a pure state and was characterized unambiguously. Direct irradiation of all-(E)-retinal and all-(E)-.beta.-ionylidenecrotonaldehyde in acetonitrile solution gave the corresponding 6e-electrocyclized photoproducts, both via the possible 7-(Z)-isomer intermediates of the parent conjugated polyenals. Unlike the lower members in the retinal series, it was also confirmed that sigmatropic rearrangement or photo-Diels-Alder reaction hardly proceeds in these higher members of the series mentioned above.