Photochemical cyclization of 2′-hydroxychalcones

Abstract

A series of 4-substituted 2′-hydroxychalcones underwent photocyclization to give the corresponding flavanones selectively, but in low quantum yields, on visible irradiation in polar aprotic solvents. Hydroxylic solvents, which disturbed the formation of a cyclic intramolecular hydrogen bond, strongly retarded the photoreaction, while neither triplet quenchers nor free radical inhibitors affected the reaction. The quantum yields were independent of the light intensity, implying a one-photon process, but strongly depended on the wavelength of irradiation. Mechanisms were proposed assuming π, π* excited states and Weller's postulate of a dynamic equilibrium in a proton-transfer double-minimum potential.