Synthesis, photooxidation and Z ⇌ E photoisomerization of benzalpyrrolinones

In contrast to oxodipyrromethenes and bilirubin, benzalpyrrolinones (H, P‐OCH3, p‐Cl, p‐N(CH3)2 and o‐CH3) and α‐pyridalpyrrolinones appear not to undergo dye‐sensitized photo‐oxygenation. They do, however, undergo an unsensitized E ⇌ Z photoisomerization reminiscent of stilbene photoisomerization, and the photostationary state varies with substituent. Intramolecular H‐bonding is implicated in the α‐pyridalpyrrolinone isomerization. In each case, the Z isomers are the therrnodynamically more stable ones, but the corresponding E isomers have been isolated and characterized following photoirradialion.