Cyclohexadienone Conjugated Derivatives

Derivatives of 6,6-disubstituted 2,4-cyclohexadienones are also photochemically reactive. Some examples are given in the following diagram. Electrocyclic ring opening to an unsaturated ketene is the favored transformation. Since nucleophilic compounds such as water, alcohols and amines add rapidly to ketenes, the resulting carboxylic acid or derivative is the final isolated product. If no nucleophilic reactants are present, the conjugated ketene diene will recyclize to the starting compound. Slower reactions leading to phenolic products may then occur. In the last (bottom) example, additional methyl substituents reduce the reactivity of the trans-ketene intermediate, so that only strongly nucleophilic amines are able to trap it. The ketene is removed by light induced isomerization and cyclization. As in the previous dienones, a triplet excited state undergoes decay to polar singlets that are thought to decompose in the manner depicted in the gray-shaded area.

Among the natural dienones displaying interesting photochemical behavior, the sesquiterpene santonin is exceptional, and has been the subject of exhaustive study.