Benzo phenanthridine alkaloids from protoberberines

A biomimetic synthesis of benzo[c]phenanthridine alkaloids from a protoberberine via the equivalent of a hypothetical aldehyde enamine intermediate has been developed (130,131). The enamide 230 derived from berberine (15) was subjected to hydroboration-oxidation to give alcohol 231, oxidation of which with pyridinium chlorochromate afforded directly oxyche-lerythrine (232) instead of the expected aldehyde enamide 233. However, the formation of oxychelerythrine can be rationalized in terms of the intermediacy of 233 as shown in Scheme 41. An alternative and more efficient... 

The protoberberine alkaloids (5-75) play important roles as precursors in the biosynthesis of a variety of related isoquinoline alkaloids such as protopine, phthalideisoquinoline, spirobenzylisoquinoline, rhoeadine, inde-nobenzazepine, secoberbine, and benzo[c]phenanthridine alkaloids. Chemical transformations of protoberberines to these alkaloids are particularly interesting and exciting from the biogenetic viewpoint and further from ready availability of starting protoberberines in nature or synthesis. 

The phenanthridine skeleton is synthesized by photocyclization of the enamides prepared from cyclohexanonimines and benzoyl chlorides (17,18). The benzo[c]phenanthridine skeletons are formed from the enamides prepared from 2-tetralonimines and benzoyl chlorides (19,20). More conveniently, the skeletons of protoberberine alkaloids are readily synthesized from the enamides prepared by simple acylation of 1 -methyl-3,4-dihy-droisoquinolines with benzoyl chlorides (21-24). This berbine synthesis is one of the most typical examples of the application of enamide photocyclization to alkaloid synthesis and can be further extended to the facile synthesis of the skeletons of the yohimbine group of indole alkaloids (25,26). 

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