A photochemical route to the protoberberine skeleton

Abstract

Irradiation of Mannich bases obtained from phthalimides, formaldehyde, and 1,2,3,4-tetrahydroiso-quinolines and carrying methoxy or methylenedioxy substituents gave, in moderate yield, pentacyclic photoproducts. These underwent acid-promoted reaction leading to 13-hydroxydibenzo-[a,g]quinolizin-8-ones, one of which was 13-hydroxy-8-oxoxylopinine. Reduction to remove the 8,13 oxygenated functions was not achieved. Unsymmetrical substrates derived from 3,4-dimethoxyph-thalimide gave a mixture of regio- and stereo-isomers in the photocyclisation, with no strong preference for one orientation of reaction.