Photochemical reactions. 143rd communication. Photochemistry of 5,6-epoxy-1,3-dienes in the ionone series. Influence of a hydroxy group in the 7-position

The synthesis and photolyses of the epoxydiene (E)- 5 are described. On triplet excitation (λ > 280 nm, acetone), (E)- 5 undergoes initial cleavage of the C(5) O bond leading to the intermediate c. Presumably an H-shift ( c → e ) followed by the fragmentation of the 1,4-diradical e leads (via the enol 37 ) to the diketones (E)- 6 and (Z)- 12 . Alternatively cleavage of the C(6) C(7) bond of c furnishes the diradical intermediate d which reacts by recombination leading to (E)- 13A + B, 16 , and 17A + B , or by an H-shift to the enol intermediate 38 . The latter undergoes an aldol-type reaction to (E/Z)- 14A + B and (E/Z)- 15A + B , as well as a photochemical 2 + 2]-cycloaddition to 18 . On singlet excitation (λ = 254 nm, MeCN), (E)- 5 undergoes photocleavage to the carbene intermediates f and g . The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 22 as the main product. From the carbene intermediate g , compounds 23, 24 , and 25 arise by carbene insertion into the neighboring C C or C H bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)- 20 , is formed via the ylide intermediate h .