Fig. 5 in Alkaloid and iridoid glucosides from Palicourea luxurians (Rubiaceae: Palicoureeae) indicate tryptamine- and tryptophan-iridoid alkaloid formation apart the strictosidine pathway

Fig. 5. Two possible pathways (a and b), both leading to javaniside (4) by a cyclization which is related to the Pictet-Spengler reaction. In pathway (a) tryptamine is oxidized to 2-oxo- tryptamine prior to enamine formation of the amine function with secologanin. In the following Pictet-Spengler reaction comparable rearrangement the spiro-cyclic system as well as the five membered ring are formed according to the mechanism described by Bailey (1987). Alternatively in the similar pathway (b) the enamine is formed prior to the Pictet Spengler-like reaction (Bailey, 1987). Subsequent the imine moiety is oxidized to gain the five membered lactam ring. The intermediate resulting from both possible pathways (a and b) is finally trans- formed to javaniside (4) by saponification and formation of the lactam ring. As no strictosidine or follow-up products have been detected in Palicourea luxurians, the enamine possibly occurring in pathway (b) has barely or not been accepted in a classic Pictet-Spenglerase catalyzed reaction, as indicated in hampered pathway c. Electron pair shifts and formations of new bonds are indicated with red arrows. For structure of the residue "R" see the box in Fig. 6. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)