Abstract
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/TiCl 4 in dimethoxyethane, followed by acetylation with acetyl chloride afforded a mixture of naphthylamide 2 and E-enamide 3. Both enamides were efficiently cyclized by POCl 3 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.
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