A convenient approach to the synthesis of benzo[c]phenanthridines via intramolecular cyclization of enamides

Article abstract of DOI:10.1016/S0040-4039(00)76675-5

A new synthetic route towards planar 8,9-disubstituted benzo[c]phenanthridinium salts via Bischler-Napieralski cyclization of enamides is reported. Treatment of ketoester 1b with benzylamine/TiCl₄ in dimethoxyethane, followed by acetylation with acetyl chloride afforded a mixture of naphthylamide 2 and E-enamide 3. Both enamides were efficiently cyclized by POCl₃ in acetonitrile under reflux. While the planar benzo[c]phenanthridinium salt 4 was directly produced from 2, the E-enamide 3 gave the 3-arylisoquinolinium salt 5, which was reduced and intramolecular cyclized to yield the tetracyclic nucleus of these alkaloids.