Conversion of 21-deoxyajmaline into the 2-epi-compound

From a synthetic and biosynthetic point of view it is of interest to note that 21-deoxyajmalal-A in strongly acidic solution exists in the ring closed indolenium form (Chart 8.3 cf. demethoxy-carbonylakuammicine, Chart 6.7) which can be trapped either by acetylation or reduction (zinc). Although a trace of 21-deoxyaj-maline was formed in these experiments none of the more hindered 17-epi-compounds were detected. 

The ease with which advantage can be taken of the nucleophilic reactivity of the indolie j8-position toward a suitably placed group was further illustrated by the behaviour of 17-0-tosyl-21-deoxy-ajmalol-A which even upon heating gave 2-hydroxy-2-epi-17,21-dideoxyajmaline. 

Two congeners of ajmaline merit comment. Vomilenine a potential ojS-unsaturated aldehyde was readily convertible into perakine (Michael addition of an amine to an o -unsaturated aldehyde) by brief reflux in acetic acid. It is well possible that perakine is an artefact. 

Sarpagine. The structure of sarpagine was first put forward from a limited amount of data and biogenetic speculation. The correctness of this guess was established by its conversion by unexceptional methods into 1-methyl-lO-deoxydihydrosarpagine identical in all respects with 21-deoxyajmalol-B. So far 16-epi-sarpagine has not been recognized as a natural product. 

The action of cyanogen bromide on the alkaloid afforded the cyclic ether whose formation is sketched in Chart 8.5. Chromic acid oxidation gave not the expected aldehyde but instead a cyclic ether. Its formation may be mechanistically related to the course 

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