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

Abstract 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.