Photodegradation of natural substances: Photooxygenation and Ozonolysis of 4-methoxy-7-methyl-5 H-furo [3,2-g][1]benzopyran-5-one (Visnagin)

The photooxygenation of 4-methoxy-7-methyl-5 H-furo[3,2-g][1]benzopyran-5-one (Visnagin, 1 ) in methanol in absence and in presence of a sensitizer (methylene blue) has been studied, 6-Formyl-7-hydroxy-5-methoxy-2-methylchromone ( 4a ), methyl 7-hydroxy-5-methoxy-2-methylchromone-6-carboxylate ( 4b ) and 7-hydroxy-5-methoxy-2-methylchromone-6-carboxylic acid ( 4c ) could be isolated and identified in each case. The formation can be interpreted in terms of intermediate production of a 1.2-dioxetane like 2 . A comparative study on the ozonolysis of Visnagin ( 1 ) in ethyl acetate both in absence and in presence of dimethyl sulfide, was also undertaken. Ozone attacks 1 either at the furan ring (to give 4a , c ) or at both the furan and γ-pyrone site (to afford 10 ). Possible reaction mechanisms are considered and the structures of the new products are based upon compatible and spectroscopic evidences.