Photochemistry of ommochromes and related compounds

The photochemistry of some synthetic ommochromes, the 1‐methoxy‐11‐(β‐aspartoyl‐acetyl‐methyl ester)‐5 H ‐pyrido[3,2‐ a ]phenoxazin‐5‐one ( 2 ), the 1‐methyl‐1‐(1′‐[1′[11‐(β‐aspartoyl‐methyl ester‐imino)]ethenyl)‐ketal‐1 H ,5 H ‐pyrido[3,2‐ a ]phenoxazin‐5‐one ( 3 ) and the 1,5‐dimethoxy‐11‐(β‐aspartoyl‐ N ‐acetyl‐methyl ester)‐pyrido[3,2‐ a ]phenoxazine ( 4 ), obtained from the oxidation of the 3‐hydroxykynurenine ( 1 ), has been examined. In an acidic methanol solution of 2 , a reversible solvent photoaddition was observed, associated with a yellow‐red bathochromic shift. The mixture, stored in the dark, quantitatively records the starting absorption spectrum. In an acidic enviroment, 3 and 4 , affording 2 , show the same photochemical behaviour. A plausible mechanism, for explanation of the process is suggested.