Ajmaline alkaloids ring system

Within the natural products field, the investigation of complete biosynthetic pathways at the enzyme level has been especially successful for plant alkaloids of the monoterpenoid indole alkaloid family generated from the monoterpene gluco-side secologanin and decarboxylation product of tryptophan, tryptamine [3-5]. The most comprehensive enzymatic data are now available for the alkaloids ajmalicine (raubasine) from Catharanthus roseus, and for ajmaline from Indian Rauvolfia serpentina [6] the latter alkaloid with a six-membered ring system bearing nine chiral carbon atoms. Entymatic data exsist also for vindoline, the vincaleucoblastin (VLB) precursor for which some studies on enzymatic coupling of vindoline and catharanthine exist in order to get the so-called dimeric Catharanthus indole-alkaloids VLB or vincristine [7-9] with pronounced anti-cancer activity [10, 11]. 

Thirty years ago van Tamelen and Oliver (162) presented a synthetic route, which they claimed to lead, via the sarpagan ring system ("deoxyajmalal system ), to the six-ring indole alkaloid ajmaline (17). The "crucial steps" in their scheme were the regioselective formation of the A -iminium ion 136 (realized by decarbonylation 135 136), and subsequent spontaneous bond formation between... 

Chart I. Variation in the ring systems among the ajmaline-sarpagine alkaloids. 

The quinuclidines (41) and, to a lesser extent, the iso-quinuclidines (42) have attracted considerable interest since they are themselves parts of alkaloids. The quinuclidine system is contained in the Cinchona alkaloids (43), in ajmaline (44) from Rauwolfia spp., and in macusine (45) obtained from Strychnos spp. The iso-quinuclidine ring structure occurs in the Iboga alkaloids (46) ... 

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