Photohydrodimerization of 6-methoxyflavone to 6,6"-dimethoxy-2,2"-biflavanones

6-Methoxyflavone (7) easily afforded two hydrodimers of rac-6,6"-dimethoxy-2,2"-biflavanone (8a) and meso-6,6"-dimethoxy-2,2"-biflavanone (8b) and one reductive product of 6-methoxyflavanone (9) by using photolysis with the electron-donating amines including triethylamine, 2-(N,N-dimethylamino)ethanol Or N,N-dimethylaniline in solvents of acetonitrile, benzene or methylene dichloride. They were found to give higher yields of rac-6,6"-dimethoxy-2,2"-biflavanone (8a) and meso-6,6"-dimethoxy-2,2"-biflavanone (8b) (38.7% and 4.5%, 35.3% and 6.2%, respectively) in the reaction conditions of 1/10 molar ratio of 6-methoxyflavone (7) to triethylamine in a solvent of acetonitrile with irradiation of twenty-four hours by using 306 nm and 352 nm lamps.