Conformation Dependent Photochemistry of Dienes. Substituent Effects on the Photochemical Reactions of Some 4-Acyloxy-2-azabuta-1,3-dienes.

The photochemical behaviour of 4-acyloxy-2-azabuta-1,3-dienes is dependent upon the nature of the 3- and 4-substituents of the diene and also, in some cases, on the type of the 4-acyloxy group. Thus when the 3-position is unsubstituted and the 4-substituent is a phenyl the dienes undergo trans-cis-isomerization on direct irradiation independent of the type of 4-acyloxy group. When the 3-substituent is a methyl and when there is a 4-methyl or 4-phenyl group then the benzoates undergo 1,3-benzoyl migration on direct or diene quenched irradiation. The acetates (7) and (9) in this series are unreactive on direct irradiation but compound (7) undergoes trans-cis-isomerization on sensitized irradiation. The difference in behaviour of the dienes is associated with the twist around the C(3)–N bond of the diene skeleton.