On the reactions of 1,3-isoquinolinediones with singlet oxygen

Article abstract of DOI:10.1016/S0040-4020(99)00502-5

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 and not products 7, whereas alkaline cleavage of the hydroperoxide 14a yielded not only 16a, but also the isoindolone 19a, In view of these observations, an unusual [4+2] cycloaddition of the electron-rich enol 21 with singlet oxygen is proposed to be responsible for the formation of products 7 and 16, while products 6, 14 and 15 arise from both the [4+2] cycloaddition and the usual Schenck ene reaction pathways. This special diene reactivity of the isoquinolinone system towards singlet oxygen is further interpreted by frontier molecular orbital (FMO) interaction considerations.