It is well known that the special group of indole alkaloids derived from strictosidine is produced mainly by three tightly related plant families (Rubiaceae, Loganiaceae and Apocynaceae) from secologanin and tryptamine. The high number of alkaloids (more than 2500 isolated compounds) of common origin supposed a high degree of common chemical background, and suggested the possibility to detect at least the key steps of their formations by considering their structures, chemotaxonomic properties and the usual organic reactionmechnisms. This coherence could be demonstrated in the first part of our work (1) in alkaloids having the secologanin substructure in unrearranged form (type-I alkaloids). In the present work, the type II and type III alkaloids were investigated with the same method and according to the same aspects. The rearrangement of the type-I skeleton was interpreted by dissociation of bond C-15-C-16 (2) followed by association of C-17 to C-14 (in type II) or to C-20 (in type III) alkaloids in frame of an intramolecular Diels-Alder reaction. The position of the double bonds before and after the rearrangement and derivatives of secodine-type intermediates in plant species supported this mechanism. Further transformations involving cyclizations, fragmentations, rearrangements and functional group transformations afforded more than 500 ibogan (mainly type II), plumeran and eburnan (mainly type-III) alkaloids in great structural variety. Nearly all of them could be derived in the interplay of organic reactionmechanisms and chemotaxonomic properties (3) in a special stereochemical framework.

References:
[1] L. F. Szabó, J. Phys. Org. Chem. (accepted for publication).
[2] L. Károlyházy, Á. Patthy-Lukáts, L. F. Szabó, B. Podányi, Tetrahedron Lett. 2000, 41, 1575-1578.
[3] Dictionary of Natural Products, Version 14:2, 1982-2006, Chapman & Hall, CRC.

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