Ketenimine and imine functions linked by an ethylene group. Intramolecular [4+2] cycloadditions leading to imidazo[1,2-b]isoquinolines

The intramolecular cyclization of imino-ketenimines where an ethylene or propylene chain is linking the nitrogen atoms of both functionalities is studied. The propylene tethered imino-ketenimines remain unchanged under thermal conditions, whereas their ethylene counterparts undergo a formal [4+2] cycloaddition, in which the ketenimine function acts as all-carbon diene and the imine as dienophile, to yield imidazo[1,2-b]isoquinolines. An X-ray crystal structure determination reveals that these cycloadducts incorporate an hydroxyl group at the benzylic carbon C10.