Photoreaction of artonin E (1), 3-isoprenylated 2 ,4 ,5 -trioxygenated flavone, produced artobiloxanthone (2) and cycloartobiloxanthone (3). Furthermore, the treatment of artonin E (1) with a radical reagent (DPPH) resulted in the same products. These findings support that the flavone derivatives having the dihydrobenzoxanthone skeleton are biogenetically derived from the 3-isoprenylated 2 ,4 ,5 -trioxygenated flavones through the phenol oxidative cyclization.