Biogenesis of Strychnos, Aspidosperma, and Iboga alkaloids. Structure and reactions of preakuammicine


In the course of biosynthetic and biogenetic investigation of the indole alkaloid family, models have been devised to simulate the Corynanthe -> Strychnos -> Aspidosperma -> Iboga transformations. However the information revealed by these and related feeding experiments still leaves unanswered both the nature of the intermediates between geissoschizine (I) (Corynanthe), akuammicine (II) (Strychnos), and the mechanisms connecting them. It has been suggested that a key member of this family situated at the main branching point of the pathway would be a prototype (IV) of the Strychnos alkaloids retaining all ten of the original geraniol carbon atoms which could suffer irreversible loss of a single carbon function to the "C" alkaloids, e.g. akuammicine (II) and strychnine, or by series of rearrangements generate the Aspidosperma and Iboga alkaloids. Such a model was demonstrated in the laboratory and detailed suggestions made implicating stemmadenine (III) as a key intermediate for this latter process. We now describe the isolation and properties of a new alkaloid, preakuammicine, for which the long sought structure IV is proposed and which, from its position in the Vinca sequence, meets at least one of the criteria as an intermediate. Furthermore the chemistry of IV provides in vitro analogy for its presumtive role as a true biointermediate.



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