Intramolecular photocycloaddition of 3-benzyloxyprop-1-enes

Abstract

The photochemistry of 3-benzyloxyprop-1-ene is markedly influenced by the presence of features which direct the intramolecular meta photocycloaddition to the 1,3-positions of the benzene ring and by constraining the benzylic position in the bichromophore. Thus, derivatives having 2′-methyl,2′-methoxy and/or 1,1 -dimethyl substitution yield meta photocycloadducts with the isomer having the linear triquinane unit greatly predominating. Adducts arising from 1,2-intramolecular attack are also observed from the substituted 3-benzyloxyprop-1 -enes but the major by-product from 1-benzyloxy-3-methylbut-2-ene is the novel spiro compound 28 derived from an ‘ene’ type reaction. The indanyl vinyl ethers undergo meta photocycloaddition to give both the bridged linear and angular triquinane skeleta with the former again predominating, while for the corresponding unsubstituted tetralin derivative the angular isomer is formed in the greater yield and with the dimethyl compound 36, the major photoisomer results from an initial intramolecular 1,2-attack in the bichromophore followed by thermal ring opening and 4π-photocyclisation.