Regio- and stereospecific models for the biosynthesis of the indole alkaloids. Aspidosperma-Iboga relation


In 1968 we reported on the vivo and in vitro transformations of the Aspidosperma alkaloid tabersonine (1) to the Iboga alkaloid catharanthine (2) and proposed an intermediate 3 in this reaction which has since been found in a variety of stabilized versions. In 1969 Smith, et al., reported that, in their view, this reaction, which involves rupture of both the 7-21 and 17-20 bonds of 1, proceeds only as far as cleavage of the 7-21 bond. The resultant immonium species 4 then rearranges to 5 and cyclizes to allocatharanthine (6), an optically active isomer of catharanthine (2) and pseudocatharanthine (7), the latter two Iboga structures being interconvertible. We have emphasized for some time that both ionic and thermal requirements must be met in carrying out the full transformation which by passing through 3 affords the racemic products described earlier. In order to reconfirm and simplify the experiment, the ionic and thermal components have been separated as follows.



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