Regiospecific photochemical transformations of 4, 5-diaryl-Δ⁴-imidazolin-2-ones: ketal formation and cyclodehydrogenation

Abstract

4,5-Diaryl-Δ⁴-imidazolin-2-ones undergo conversion to 2,2-dialkoxy-4,5-diaryl-Δ⁴-imidazolines by a characteristic solvent interaction of the carbonyl group on irradiation in alcoholic solvents. The photochemical oxidative cyclization of the stilbene system in these compounds to the corresponding phenanthrene derivatives does not occur under the usual conditions. Acidic conditions hinder the ketal formation of the carbonyl group and favour the oxidative dehydrocyclization reaction to the phenanthrene derivatives. These reactions have been found to be general to the 4,5-diaryl-Δ⁴-imidazolin-2-ones and plausible mechanistic pathways have been suggested for the two different selective reactions.