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The Photochemistry of Conjugated cis-Bicyclo[5.l.0]octenones, cis-and trans-Bicyclo[5.2.0]non-2-en-4-ones, and Their Methylene AnalogsThe photochemistry of 1,4,4-trimethyl-cis-bicyclo [5.1.0] oct-5-en-Z-one, cis-bicyclo [5.1 .O] oct-Z-en-4-one, and their methylene derivatives has been studied. In the first instance, 1,3 shift of the bond common to the two rings occurs, with rearrangement occurring from the singlet manifold. Photoisomerization of its methylene derivative likewise is a singlet state transformation. In sharp contrast, although the 2-en-4-one bicyclic and the derived conjugated diene also undergo excited state vinylcyclopropane-cyclopentene bond reorganization, these reactions proceed readily from the respective triplet states. These differences have been rationalized in terms of conformational factors particularly as they relate to bond overlap in the SI or the transoid (ix., as relates to Tbond overlap) TI states. The cis-and trans-bicyclo[5.2.0] non-2-en-4-ones, as well as their methylene congeners, do not exhibit an analogous propensity for rearrangement. Rather, polymerization was noted in aprotic solvents throughout this latter series.(31) R. S. Mulliken and C. C.

3-Cyclooctenone and its 6-oxa analog have been found to rearrange principally to 2-vinylcyclohexanone derivatives upon irradiation. Data are presented in support of the proposal that the formation of 2-vinylcyclohexanones as well as the minor products stems from an initial Norrish type I photochemical process followed by transannular rebonding or hydrogen abstraction.

Thermochemical non-2-en-4-ones, and

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