Photochemical Electrocyclic and Cycloaddition Reactions

Irradiation of cycloheptene in the presence of mercury gave a mixture of norcarane, vinylcyclopentane, hepta-1,6-diene, and other minor products. Deuterium labelling showed that the norcarane arose via a 1,2-hydrogen shift followed by ring-closure. The photochemical conrotatory ring-closure of all possible 1,3-dimethyl- and 1,4-dimethyl-cyclohepta-1,3-dienes has been carried out. ( )-Grandisol has been synthesized from eucarvone, the first step in the synthesis being the photoisomerization of eucarvone to l,4,4-trimethylbicyclo[3,2,0]hept-6-en-2-one. ° 

A key step in a reported synthesis of the ( )-longipinenes was the triplet sensitized, photochemical isomerization of the methylenedecadiene (202) to the tricyclic olefin (203) this is the first example of a cross-photocyclization in the cyclodeca-1,5-diene system.  

Kadohira, S. Takamuku, and H. Sakurai Tetrahedron Letters, 1974, 459. 

On irradiation, 2-(cycloheptylidene)cycloheptanone isomerizes to 2-(cyclohept-l-enyl)cycloheptanone, which on further irradiation cyclizes to a mixture of all four possible isomers of the oxetan (204). An analogous mixture of oxetans was obtained by irradiation of 2-(cyclo-oct-l-enyl)cyclo-octanone.  

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The standard observation that thermal and photochemical electrocyclic and cycloaddition reactions always take place by opposite stereochemical pathways [96] is explained directly by the unit of angular momentum carried by a photon. Photochemical activation disturbs the balance between angular-momentum vectors and dictates a different molecular conformation. 

Predict the structure, including all aspects of the stereochemistry, based on orbital symmetry principles for the following photochemical electrocyclic and cycloaddition reactions. 

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