Ajmalicine alkaloids

R. serpentina Benth. Ajmalicine, ajmaline, ajmalinine, serpentine, serpentinine (Siddiqui and Siddiqui ). From a geographical variety of the species, Moajmaline, neoajmaline and unnamed alkaloids, m.p. 220° and m.p. 234 (S. Siddiqui ). From the same species van Itallie and Steenhauer isolated alkaloids B, C and A (rauwolfine) which may be identical with Siddiqui s serpentine, aimalinine and ajmaline respectively. 

Further elaboration of tetracycle 159c resulted in the syntheses of the racemate of indole alkaloids of the ajmalicine (61JA2594), tetrahydroalstunine (56JOC1315, 71JA5907), and akuammigine type (ajmalicinoid alkaloids). Similarly, 159d can be converted into yohimboid alkaloids (79JA5370). 

Microbial transformations of four heteroyohimbine stereoisomers [ajmalicine (81a) tetrahydroalstonine (81b), isoajmalicine (81c), and akumigine (81d)] yielded mixtures of 10- and 11-hydroxylation products (786) (Scheme 21). Microorganisms known for their abilities to metabolize indole alkaloids, steroids, and antibiotics were intitially screened, and seven cultures were further used for preparative-scale incubations with alkaloid substrate. The microorganisms used and yields (by HPLC) of metabolites obtained from 81a-81d are shown in Table HI. 

This overall biosynthetic scheme is summarized in Scheme 7, and it is well to remember that C. roseus is almost alone as a plant in which this whole pathway can be viewed in its entirety, for most indole alkaloid-containing plants produce only one or two of the major alkaloid classes, and not all four. In addition, C. roseus is without doubt the most economically important of the indole alkaloid-containing plants, and thus studies were, and continue to be, driven by the goal of increasing the availability of the commercially significant alkaloids ajmalicine (39), vinblastine (1), and vincristine (2). 

Leaf organ cultures of C. roseus have also been described 118). A typical 2.5-g fresh weight inoculum produced 29 g fresh weight of leaf material after 35 days dedifferentiated tissue was absent. The alkaloids found included ajmalicine (39), sitsirikine (48), tetrahydroalstonine (39), serpentine (40), and vindoline (3). 

Goodbody and co-workers (7/9) have examined the production of alkaloids in root and shoot cultures induced from seedlings of C. roseus. The pattern of alkaloids in the root cultures was similar to that of the roots from intact plants. Thus ajmalicine (39) and catharanthine (4) were produced, but no vindoline (3), a major leaf alkaloid, and no bisindole alkaloids. Similarly, the pattern of the alkaloid content of the shoot cultures was like that of the leaves of the intact plant, showing the presence of vindoline (3), catharanthine (4), and ajmalicine (39), with 3 predominating. A search for the bisindole alkaloids in the cultures indicated the presence of anhydrovinblastine (8) and leurosine (11) in the shoot cultures (2.6 and 0.3 xg/g fresh weight, respectively), but no vinblastine (1) or vincristine (2). 

The effect of light on alkaloid production in these cultures was also evaluated (7/9). More catharanthine was produced in the light than in the dark, and the same observation was made for ajmalicine (39). Knobloch et al. 120) examined the production of anthocyanins, ajmalicine (39), and serpentine (40) in cell suspension cultures and found that although serpentine levels increased 18-fold in the light, ajmalicine levels decreased 50%. In the work of Goodbody et al. 119) the biosynthesis of vindoline (3)... 

Scott and co-workers have also reported on the isolation of alkaloids from C. roseus cell suspension cultures 126). The cell line used, identified as CRW, afforded akuammicine (49), catharanthine (4), and strictosi-dine (33), and feeding experiments with labeled tryptophan led to incorporation into ajmalicine (39), akuammicine (49), catharanthine (4), and vindoline (3). The ability to produce alkaloids was carried through 8 successive generations. 

The 200GW line proved to be quite different, and of particular interest was the discovery that this line produced catharanthine (4) at levels about three times that of the intact plant (0.005%) (155,159,160). Curiously, the predominant alkaloid (60.48%) was strictosidine lactam (41), which is not normally seen in extracts of intact plants. Variation of the pH and added phytohormones did not significantly alter the pattern of alkaloids produced by this cell line (160). Further cell line studies (161) afforded one line (176G) which produced mainly ajmalicine (39) and lochnericine (73) and one (299Y) which apparently contained relatively inactive p-glucosi-dases, since the major alkaloids produced were strictosidine (33) (83%) and strictosidine lactam (41) (Table XIII). 

The work by Scott and Lee 165) on the isolation of a crude enzyme system from a callus tissue culture of C. roseus was followed by studies of Zenk et al. on an enzyme preparation from a cell suspension system which produced indole alkaloids 166). The cell-free preparation was incubated with tryptamine and secologanin (34) in the presence of NADPH to afford ajmalicine (39), 19-epiajmalicine (92), and tetrahydroalstonine (55) in the ratio 1 2 0.5. No geissoschizine (35) was detected. In the absence of NADPH, an intermediate accumulated which could be reduced with a crude homogenate of C. roseus cells in the presence of NADPH to ajmalicine (39). Thus, the reaction for the formation of ajmalicine is critically dependent on the availability of a reduced pyridine nucleotide. 

The incorporation of acetate into the monoterpene unit of the indole alkaloids has recently been reexamined (176). Using [l,2- C2]acetate it was established that no intact incorporation occurred, and a similar labeling pattern to that from [2- C2]acetate was observed, i.e., C-3, C-4, C-20, C-22, and C-23. Extensive scrambling of the acetate occurred via the Krebs cycle to label the 1 and 2 positions of acetate prior to incorporation. [2- C]Mevalonate was incorporated equally into C-17 and C-22 of ajmalicine (39), indicating that an equilibration occurs at some point in the pathway, as had been established previously with radiolabeled precursors 176). 

In vivo feeding experiments with singly and doubly labeled strictosidine (33) in C. roseus shoots afforded labeled ajmalicine (39), serpentine (40), vindoline (3), and catharanthine (4). Vincoside (85, page 37) was not incorporated into the alkaloids, suggesting that it was biologically inert 188). Brown and co-workers 190) conducted somewhat parallel studies examining the precursor relationship of strictosidine in C. roseus. Incorporation into tetrahydroalstonine (75), ajmalicine (39), catharanthine (4), and vindoline (3) was observed. 

Although ajmalicine (39) is not on the pathway to the bisindole alkaloids, it is a compound of substantial commercial interest, and several of the intermediates in its formation are probable intermediates in the extended biosynthetic pathway. This work is therefore reviewed for the purpose of completeness of studies on C. roseus. Considerable progress has been made on the biosynthesis of ajmalicine (39), and the studies on the formation of strictosidine (33) and cathenamine (76) have already been described. One of the preparations described by Scott and Lee was a supernatant from a suspension of young seedlings of C. roseus which af-... 

This structural group of indole alkaloids covers simple indole alkaloids (e.g., tryptamine, serotonin, psilocin and psilocybin), /3-carboline alkaloids (e.g., harmine), terpenoid indole (e.g., ajmalicine, catharanthine and tabersonine), quinoline alkaloids (e.g., quinine, quinidine and cinchonidine), pyrroloindole... 

Generally speaking, alkaloids are more toxic for vertebrates than for invertebrates. The coefficients of the selective toxicity show that alkaloids are very dominantly selective toxins to vertebrates (Table 26). Vertebrate very strong selectivity (<0.01) is observed in such alkaloids as ajmalicine, brucine, ephedrine, ergometrine, harmaline, lupanine, lupinine, scopolamine and... 

A similar vinylogous Mannich reaction has been used by Martin in the total syntheses of the heteroyohimboid alkaloids (—)-ajmalicine and (—)-tetrahydroalstonine <1995JOC3236>. An attempted synthesis of an opioid analgesic 2,4-dibenzyl-3,7-diazabicyclo[3.3.1]nonan-9-one-l,5-dicarboxylate (piperidone) by a double Mannich reaction of oxoglutarate, 2 equiv of phenylacetaldehyde, and methylamine did not give the expected product but instead gave rise to an unexpected [l,6]naphthyridine derivative (Scheme 57) <1998PHA442>. 

The above structure/activity tendency is seen again in heteroyohimbine alkaloids. Tetrahydroalstonine (83) exhibits considerable selectivity towards presynaptic ct2-adrenoceptors but raubasine (ajmalicine) (84)... 

Zenk, M. H., H. El-Shagi, H. Arens, J. StoV ckigt, E. W. Weiler, and B. Deus, "Formation of the Indole Alkaloids Serpentine and Ajmalicine in Cell Suspension Cultures of Catharanthus roseus," in Plant Tissue Culture and Its Biotechnological Application, Eds. W. Barz, E. Reinhard, M. H. Zenk, New York Springer-Verlag, 1977, pp. 27-43. 

Carbocyclic variants related to ajmalicine such as yohimbine are likely to arise from dehy-drogeissoschizine by the mechanism indicated in Figure 6.77. Yohimbine is found in Yohimbe bark (Pausinystalia yohimbe, Rubiaceae) and Aspidosperma bark (Aspidosperma species Apocy-naceae) and has been used in folk medicine as an aphrodisiac. It does have some pharmacological activity and is known to dilate blood vessels. More important examples containing the same carbocyclic ring system are the alkaloids found in species of Rauwolfla, especially R. serpentina (Apocynaceae). Reserpine and deserpi-dine (Figure 6.78) are trimethoxybenzoyl esters of yohimbine-like alkaloids, whilst rescinnamine is... 

The rauwolfia alkaloids are now hardly ever prescribed in the UK, either as antihypertensives or as tranquillizers. Over a period of a few years, they have been rapidly superseded by synthetic alternatives. Reserpine has also been suggested to play a role in the promotion of breast cancers. Both ajmalicine (= raubasine) (Figure 6.76) and ajmaline (Figure 6.82) are used clinically in Europe, though not in the UK. Ajmalicine is employed as an antihypertensive, whilst ajmaline is of value in the treatment of cardiac arrhythmias. Ajmalicine is also extracted commercially from Catharanthus roseus (see page 357). 

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