Photochemistry of phenazopyridine hydrochloride.

Phenazopyridine hydrochloride (1) is an azo dye with local analgesic and anaesthetic effects on the urinary tract. Its photochemistry was studied in different reaction media including the drug adsorbed on silica gel. This resulted in photochemical cyclodehydrogenation, reductive photodegradation and rearrangement of the drug molecule. Four major products were isolated and identified on the basis of IR, NMR and mass spectral studies. The products are: pyrido[3,4-c]cinnoline-2,4-diamine (2), N3-phenylpyridine-2,3,4,6-tetraamine (3), pyridine-2,3,6-triamine (4), 2,6-diamino-1-(4-aminophenyl)pyridin-4(1H)-one (5).