Photocyclization of 2′-Hydroxychalcones to 4-Flavanones

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Abstract
On irradiation with light of λ≥365 nm, parent and methoxy-substituted 2′-hydroxychalcones undergo facile photocyclization to give corresponding 4-flavanones in high efficiencies in ethyl acetate or dioxane, whereas in low or nil efficiencies in benzene, chloroform, carbon tetrachloride, ether, acetonitrile, ethanol, or t-butyl alcohol. The low reactivity in benzene is, however, significantly increased by the addition of liquid paraffin to increase the solvent viscosity. 3′,4′-Benzo-2′-hydroxychalcone showed low reactivities in any solvents. The formation of 4-flavanones is neither quenched by triplet quenchers nor depressed by the addition of free radical scavengers. The reaction mechanism is discussed in terms of enolization.