Cathenamine

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-... 

The next area for study was the pathway between strictosidine (33) and cathenamine (76), where the initial step is viewed as hydrolysis by p-glucosidase and opening of the hemiacetal to a dialdehyde 93. Attempts to trap this intermediate 200) have thus far failed, and it appears that recyclization and dehydration occur too rapidly, giving rise to 4,21-dehy-drocorynantheine aldehyde (94). Thus, an enzyme preparation was incubated at pH 7.0 in the presence of KBH4 to afford the C-16 epimers 69 and 95, thereby impuning the existence of 94. 

For the biosynthetic conversion of cathenamine (76) to the 19- and 20-epi derivatives, an equilibrium should exist between the enamine and imi-nium forms of cathenamine (i.e., 76 and 97) (767). This was examined (209) with deuterium labeling studies of incorporation into tetrahydro-alstonine (75), whereupon C-21 was labeled from both the enamine and iminium forms. When the enamine form was present, a second deuterium was incorporated (presumably at C-20) on reduction with NaBD DjO. Sulfate was effective in pushing the equilibrium toward the iminium species (209). 

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). 

Aspidosperma alkaloids These alkaloids have an aspidosperma-type nucleus. The a and are the same as in corynanthe type. Catharantine is the P4 from cathenamine (Figure 68). 

Cathenamine (100) has been identified as an early intermediate in terpenoid indole alkaloid biosynthesis (cf. Vol. 8, p. 27). It has also been isolated from Guettarda eximia. Another alkaloid, 4,21-dehydrogeissoschizine (99), has now been isolated from this plant it is readily converted into (100) in alkaline solution.29 On incubation with an enzyme preparation from Catharanthus roseus cell cultures in the presence of NADPH at pH 7, (99) was converted into ajmalicine (102), 19-ep/-ajmalicine (103), and tetrahydroalstonine (104), which are the normal products with this enzyme preparation. In the absence of NADPH, cathenamine (100) accumulated.30 The reaction to give (100) proceeded linearly with time, and was dependent on the concentration of protein and substrate. No conversion occurred in the absence of enzyme. 

The mechanism of formation of the (19i )-bases was studied by means of 2H and lsO experiments. In D20, at pH 6, strictosidine gives the monodeuterio-compounds (71) and (72), as expected, but whereas the proportion of (72) increases with time, no further deuterium is incorporated. Hence cathenamine does not revert in acid to the dienamine (67). Evidently cathenamine is protonated to... 

Crude SG expressing yeast extracts showed high enzyme activity that converted strictosidine into cathenamine. 

Reduction of cathenamine with NaBD4 in D20 resulted in the incorporation of two deuterium atoms in the reduced product, tetrahydroalstonine131 (Scheme 96). 

Ajmalicine, 19-epi-ajmalicine and tetrahydroalstonine are formed from 4,21-dehydrogeissoschizine via cathenamine (Fig. 2.9). The enzymatic synthesis of these corynanthe-type alkaloids has been investigated using C. roseus cell suspension cultures, and the enzymes involved have been reviewed by De Luca (1993) and Ziegler and Facchini (2008). Ajmalicine can be oxidized by POD to serpentine. This reaction may take in the vacuole. 

Ajmalicine (raubasine) affects smooth muscle function and is used to help prevent strokes (93), and tetrahydroalstonine exhibits antipsychotic properties (Fig. 2d) (94). These compounds are found in a variety of plants, including C. roseus and R. serpentina. A partially purified NADPH-dependent reductase isolated from a tetrahydroalstonine that produces a C. roseus cell line was shown to catalyze the conversion of cathenamine, a spontaneous reaction product that results after strictosidine deglycosylation, to tetrahydroalstonine in vitro (95). A second C. roseus cell line contains an additional reductase that produces ajmalicine. Labeling studies performed with crude C. 

El-Sayed M, Choi YH, Frederich M, Roytrakul S, Verpoorte R. Alkaloid accumulation in Catharanthus roseus cell suspension cultures fed with stemmadenine. Biotech. Lett. 2004 26 793-798. Heinstein P, Hofle G, Stockigt J. Involvement of cathenamine in the formation of N-analogues of indole akaloids. Planta Med. 1979 37 349-357. 

It has been proposed that the biogenesis of the less abundant 19R heteroyohimbine alkaloids involves 1,4-addition of an enol to a Z-alkene, e.g. (76). A similar intermediate is presumably involved in the conversion of cathenamine (77a) into 19-epicathenamine (77b) by means of alumina in chloroform. 

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