view of these observations, an unusual [4+2] cycloaddition of the electron-rich enol 21 with singletoxygen 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 singletoxygen is further interpreted by frontier