Of ajmalicine

On the other hand, oxymercuration of demethylcorynantheine (593), followed by sodium borohydride reduction, gave 1 -abeo-( 17- 16)-yohimbane derivative 596 as the major product besides small quantities of ajmalicine (597) and 19-epiajmalicine (598) (287). 

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). 

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-... 

Incubation of geissoschizine (35) with a cell-free extract from C. roseus 210) in the presence of NADPH caused the accumulation of an isomer of isositsrikine whose structure was established chemically to be the (167 ) isomer 58. None of the 16-epi isomer 95 was detected in the cell-free incubations or in feeding experiments with intact plants. Additionally, Stdck-igt has reviewed enzymatic studies on the formation of strictosidine (33) and cathenamine (76) (277), and Zenk has provided a very elegant summary of the enzymatic synthesis of ajmalicine (39) (272). 

DiCosmo, F., Quesnel, A., Misawa, M. and Tallevi, S. G. 1987. Increased synthesis of ajmalicine and catharanthine by cell suspension cultures of Catharanthus roseus in response to fungal culture-filtrates. Applied Biochemistry and Biotechnology, 14 101-106. 

Overman and co-workers have reported Mannich bis-cyclizations (carboxylate-terminated iV-acyliminium ion bis-cyclizations Scheme 56) and employed these in the total synthesis of (—)-ajmalicine <1995JA9139>. 

It was found that the polycarboxylic ester resin (XAD-7) was more selective in adsorbing indole alkaloids than XAD-4 despite a lower capacity [16]. When XAD-7 was added to the culture medium, the production of indole alkaloids was stimulated and increased in level compared to that of the undesired products. Selective recovery of ajmalicine, with the polycarboxyl ester resin XAD-7 has also been investigated. The accumulation of total indole alkaloids and the... 

An alternative mechanism (shown in Scheme 10) for the formation of (109) from demethylcorynantheine (102) postulates the prior formation of a hemiacetal (110) followed by an irreversible attack on the mercurinium ion by the hydroxy-group to give an intermediate of structure (111). The inherent plausibility of such a mechanism led Goutarel et al.75 to study the mercuration-demercuration of cory-nantheine which, in an aqueous medium, can in principle give rise to the same hemiacetal (110), and thence the acetal (111). In fact this reaction gave a mixture of the acetals (104), (109), and their C-16 epimers which, on treatment with polyphosphoric acid, gave a mixture of ajmalicine (85a) and 19-epiajmalicine (85b) in a ratio of ca. 45 55. 

In the transcript of a lecture, Zenk has reviewed58 the enzymic synthesis of ajmalicine and its biogenetic intermediates from secologanin and tryptamine in cell clones of Catharanthus roseus. 

The foregoing degradations, when considered in conjunction with the spectrographic properties of the molecule and the proved presence of one O-methyl group, resulted in the provisional formulation of mitraphylline as V, i.e., as the tetracyclic oxindole analog of ajmalicine (56). This expression was based on the molecular formula C21H26N2O4, and on the assumption that the molecule contains a carbomethoxy group. 

The mass spectra of ajmalicine (IXa) and its deuterated derivatives IXb-IXd exhibit a prominent ion at M-l (IXa) or M-2 (IXb-d), which is presumably due to formation of the corresponding 3-dehydro derivative XLI (R2, R j = H or D, as appropriate) by loss of the C-3 hydrogen atom. Other, but much less intense, ions at m/e 337 and 321 are formed by loss of a methyl or methoxyl group from ring E. 

Fifty per cent of the M-l peak is found at M-2 in the spectra of 3-deu-terioyohimbine (CCCVII, prepared by NaBD4 reduction of 3,4-dehydro-yohimbine perchlorate) and of 3,5,6-trideuterioajmalicine (CCCIX, prepared by NaBD4 reduction of serpentine hydrochloride), so that one-half of the hydrogen lost in the formation of this peak comes from C-3 or, in the case of ajmalicine, from C-3 and C-6. 

The spectra may be used to distinguish the two most common stereochemical forms, those of ajmalicine (CCCXVII) and tetrahydroalstonine (CCCXVIII), since in the former the peak x is more intense than v or w, while in the latter and in yohimbine it is weaker. Examples are seen in the spectra of CCCIII-CCCV and CCCXVII-CCCXIX (Table V, Refs. 165, 37). 

The stereochemistry of the heteroyohimbanes has largely been elucidated, and a total synthesis of ajmalicine has been achieved. 

Mass spectra of these alkaloids give useful structural information. The molecular ion, LXXVI, of ajmalicine is characteristic. The other fragments represent the indole portions following rupture and loss of ring E(l). 

The absolute configuration of ajmalicine is thus established through its conversion to dihydrocorynantheane, which itself has been related to cinchonamine (50-53). 

The UV- and IR-spectra indicate the presence of an a-oxindole chromo-phore superimposable without conjugation upon the Me02C=C0R chromophore characteristic of those of corynantheine and of ajmalicine. Hydrolysis followed by Wolff-Kishner reduction of dihydrocorynoxeine and of corynoxine yields two bases, dihydrocorynoxeinine (CXIX) and corynoxinine (CXX), respectively, which possess only the oxindole chromophore. The former base, dihydrocorynoxeine, has been shown to be identical with rhynchophylline (CXVIIb) (77), which possesses the ethyl side chain and hence corynoxeine (CXVIIa) has the vinyl chain (76). 

Figure 2.9 Enzymic formation of ajmalicine. TDC, tryptophan decarboxylase STS, strictosidine synthase STG, strictosidine glucosidase POD, peroxidase.

Strictosidine is the common precursor of ajmalicine (XLVni), on the one hand, and of vindoline (LIH) and catharanthine (LIV), on the other, these last two being the precursors of a-3 ,4 -anhydrovinblastine (XLVn), vinblastine (XLIX R = CH3) and vincristine (XLIX R = CHO), in that order [76],... 

The attempts by the same investigators to realize the synthesis of ajmalicine from corynantheine derivatives, previously reported" in brief, have now been published in detail " thus, while oxymercuration of demethylcorynantheine (115) followed by NaBHt reduction gave small quantities of ajmalicine (80) and 19-epiajmalicine, the major product was the IS-abeo (17 16) yohimbane derivative (116). 

In continuation of his biomimetic syntheses of heteroyohimbine alkaloids Brown" has succeeded in converting secologanin tetra-acetate (117) into elenolic acid (118), and in completing the synthesis of ajmalicine (80) essentially by the route published" earlier (Scheme 14). Contrary to the previous workers, who apparently isolated only ajmalicine. Brown eta/." obtained ajmalicine (80), 19-epiajmalicine (119) and tetrahydroalstonine (120). Since one of the lactams (121) afforded only 19-epiajmalicine (119) and in the general reaction sequence the proportion of (80), (119), and (120) in the final product depended on the length of time allowed for Schiff base formation rather than on the ratio of isomers present in the methyl elenolate [ester of (118)], it is apparent that interconversion of the imines (122) via the equilibria shown in Scheme 14 is responsible for the non-stereospecificity of the synthesis. 

The stereochemical assignments for the cyclization of 25.1 were based on conversion into synthetic intermediates for the synthesis of (— )-ajmalicine (25.6), (— )-tetrahydroalstonine (25.7), and ( — )-(10K)-hydroxydihydroquinine (25.8). No details of the stereochemical assignment of 25.5 were reported. These results can be rationalized by transition state 25.9, which allows for association of the donor and acceptor portions of the substrate. Attack occurs from the face of the enamine opposite to the phenyl group. As in the intermolecular reactions of similar imines, these reactions are probably under kinetic control. 

---