Photochemical synthesis of 2,2′-biflavanones from flavone

Photolysis of flavone (4) with the electron-donating amines including triethylamine or 2-(N,N-dimethyl-amino)ethanol in acetonitrile, benzene or methylene dichloride can easily afford two hydrodimers of 2,2'-biflavanone(racemate) (5a) and 2,2'-biflavanone(meso) (5b) and one reductive product of flavanone (6). Their yields were dependent on the molar ratios of substrate to amine, the kinds of amines, the solvents used and the irradiation sources. Higher yields were afforded 2,2'-biflavanone(racemate) (5a), 2,2'-biflavanone(meso) (5b) and flavanone (6) (30.0%, 20.9% and 15.8%, respectively) in the reaction condition of 1/2 molar ratio of flavone (4) to triethylamine in acetontrile with fourteen hours of irradiation.