Triplet photochemistry of medium ring trienes

The photochemistry of the triplets of 10- and 11-membered ring 1,3,5-trienes has been studied. At ?70° cis,trans,cis-cyclodeca-1,3,5-triene goes only to the cis,cis,cis-isomer. At 25°, this latter compound is converted into cis-bicyclo4.4.0]deca-2,4-diene via the thermally labile trans,cis,trans-cyclodeca-1,3,5-triene. At ?70° cis,trans,cis-cycloundeca-1,3,5-triene is converted to the cis,cis,cis-isomer. At 25°, this primary ptotochemical product undergoes a thermal 1,7-sigmatropic hydrogen migration to yield the trans,cis,cis, isomer. This latter triene upon sensitized irradiation yields cis-bicyclo5.4.0]undeca-8,10-diene and trans-bicyclo7.2.0]undeca-2,10-diene. The ratio of these latter two products changes with the temperature of the sensitized reaction. The possible mechanisims of these transformations are discussed.