Reactions of 1, 3-isoquinolinediones 5 and 4-alkylated 1, 3-isoquinolinediones 13 with singlet oxygen are entirely dominated by their enolization and proceed smoothly in benzene in the presence of pyridine as a base and a hydrogen bond acceptor. The products are triketones 6 and benzoisofuranones 7 for 5, and hydroperoxides 14, hydroxides 15 and benzoisofuranones 16 for 13. It was found that hydrolysis of 6 afforded the isoindolones 8 ...