Fig. 6 in Alkaloid and iridoid glucosides from Palicourea luxurians (Rubiaceae: Palicoureeae) indicate tryptamine- and tryptophan-iridoid alkaloid formation apart the strictosidine pathway

Fig. 6. Possible pathways, leading to alstrostine-type as well as to isoalstrostine-type alkaloid structures. In the first step (a) an alline-like intermediate with two secondary amine functions is generated by oxidation and cyclization of tryptamine (Nakagawa and Hino, 1977). Subsequent in step (b), two secologanin molecules react with both secondary amine functions gaining two enamine moieties. In the following steps (c1/c2) and (d1/d2), protonation of one of the two enamine functions leads to the formation of a sixmembered ring with an iminium ion moiety. Depending on the location of the initial protonation in step (d1/d2), the six-membered ring carries its substituents either in position 3′ (–CH2–R) and 14 (-R), or in position 3 (–CH2–R) and 14′ (-R), respectively. The final deprotonation (e1/e2) leads in both cases to the formation of the enamine and thus results in the alstrostine-type or the isoalstrostine-type structure, respectively. Structure of residue "R" is shown in the box. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)