Photochemical reactions. 144^th Communication. Photochemistry of 5,6‐epoxy‐1,3‐dienes in the ionone series: Influence of a methoxy group in position 7

Abstract

On triplet excitation (1 > 280 nm, acetone), the epoxydiene (E)-5 undergoes initial cleavage of the C(5)-0 bond of the oxirane and subsequent cleavage of the C(6)-C(7) bond leading to the diradical intermediate e which reacts by recombination furnishing the cyclic compounds (E/Z)-6, (E/Z)-7,8, and 9. Alternatively, a H-shift leads to the aliphatic methyl-en01 ether 10 which undergoes a photochemical [2 + 21-cycloaddition to compounds 12 and 13, the main products on triplet excitation of (E)-5. On singlet excitation (A = 254 nm, MeCN), (E)-5 undergoes cleavage to the carbene intermediates f and g. The vinyl carbene f reacts with the adjacent double bond furnishing the cyclopropene 14 as the main product. From the carbene intermediate g, the methyl-enol ether 15 arises by carbene insertion into the neighboring C-H bond. Furthermore, the diastereomer of the starting material, the epoxydiene (E)-16, and compounds 17A + Bare formed via the ylide intermediate h. Finally, the cyclobutene 18 is the product of an electrocyclic reaction of the diene side chain.