The Bischler–Napieralski synthesis

This synthesis was first suggested by Bischler and Napieralski and has been subjected to a number of improvements later on. This method involves the cyclodehydration of an acyl derivative of B-phenylethylamine to give 3,4-dihydro isoquinoline, in the presence of Lewis acids such as polyphosphoric acid, zinc chloride or phosphorous pentoxide. The 3,4-dihydro isoquinoline is then dehydrogenated by Pd(Palladium) at 160°C to Isoquinoline. It must be noted that the yield of this reaction is excellent if electron-donating groups are present on the benzene ring however if the electron-withdrawing groups are present on the benzene ring the yields are very poor. This is because of the electrophilic ring closure nature of the ring.