Photodegradation of methoxy substituted curcuminoids.

Photodegradation of dimethoxy curcuminoids in acetonitrile solution was found to depend on the position of the methoxy group bonded to the phenyl ring. The rate of decomposition was expressed as the lifetime of the decomposing substrate, being the shortest in the case of the 3,5-dimethoxy and the longest for the 2,5-dimethoxy derivative. For the 3,5-dimethoxy curcuminoid, the major degradation products were 3,5-dimethoxy benzaldehyde, 3,5-dimethoxybenzoic acid and the Z and E isomers of dimethoxy cinnamic acid, together forming about 90% of the reaction mixture. Minor products found were 4,5-bis(3,5-dimethoxyphenyl)hex-2-endionic acid, and products with the molecular formula C23H24O6 and C23H22O6 attributed to the reaction of intramolecular [2+2] cycloaddition of the dimethoxy curcuminoid and the dioxygenated bicyclopentadione derivative (C23H24O8) derived from autoxidative transformation of the dimethoxy curcuminoid.