Mesembrine biosynthesis

Key intermediates in the biosynthesis of the crinine alkaloids are norbelladiene and 4 -0-methylnorbelladine (90). It became clear, however, as the result of experiments with doubly labelled precursors that neither these compounds nor 3 -0-methyl-, iV-methyl-, or 3 -OiV-dimethyl-norbelladine were directly involved in mesembrine biosynthesis the moderate levels of incorporation observed were found to be the result of prior fragmentation of the precursors. In an ultimate test... 

Further detail of biosynthesis is that 4 -0-demethylmesembrenone (107) and mesembrenone (108) are efficient precursors for mesembrenol (109) and mesembrine (110) (108) was also recorded as an efficient precursor for mesembranol (109 no 4,5-double bond) (108) was not incorporated into (107), indicating that O-demethylation of (108) does not occur to a significant extent.85 The compounds (107) and (108) lie on the pathway to (109) and (110) most reasonably after sceletenone (106), and the results may be fitted together to give the pathway shown in Scheme 10. 

Some preliminary results regarding the biosynthesis of mesembrine were obtained by Jeffs by feeding Sceletium plants with radioactive precursors (I6 ). It was possible to show that the Ce—C2—X unit of mesembrine is derived from tyrosine and that the six carbons of the aromatic ring are derived from phenylalanine. 

Mesembrine Alkaloids.— The way in which tyrosine, labelled in the side-chain, is incorporated into mesembrine alkaloids suggests that it is the source of all the carbon atoms of the octahydroindole moiety (phenylalanine provides the remaining C(, unit). The incorporation of L-[3, 5 - H2, f/- C]tyrosine [as (116)] into mesem-brenol (119) with retention of both labels (incomplete in the case of tritium see below) proves that this is so. Further, similar incorporations were recorded for [3, 5 - H2, l- C]tyramine [as (117)], [3, 5 - H2, l- C]-N-methyltyramine [as (118)], and [3, 5 - H2, N-merfty/- C]-N-methyltyramine thus defining the sequence tyrosine (116) — tyramine (117) A -methyltyramine (118)(119) as a major pathway in the biosynthesis of mesembrine alkaloids. 

Two new syntheses of racemic mesembrine have been described. In the first, outlined in Scheme 1, an improved synthesis of the penultimate 2-pyrroline (8) was achieved annelation of this afforded ( )-mesembrine (9). The second (Scheme 2) incorporates a regioselective borohydride imide reduction. Investigations on the biosynthesis of mesembrine alkaloids have revealed that a stereospecific protonation occurs at C-7 [see formula (9)] as one of the late stages. ... 

The research on mesembrine (76) has been much less straightforward but after several setbacks it has now reached a particularly interesting stage. The story of this problem so far shows how frustrating biosynthetic work can be when a persuasive structural relationship gives a false lead to the biosynthesis. 

Previous studies45 have demonstrated that tyrosine and phenylalanine follow separate pathways in providing the hydroaromatic C6—C2—N unit and the aromatic C6 unit of mesembrine. In this respect mesembrine follows the pattern, already established for the biosynthesis of the structurally related Amaryllidaceae alkaloids derived from O-methylnorbelladine, e.g. haemanthamine (70). The present investigation46 followed this lead and started with a reasonable working hypothesis (Scheme 12) in which the biosynthesis proceeds via O-methylnor-belladine (68) and intermediates of the crinine type such as (75). However, feeding... 

The mesembrine alkaloids, e.g. mesembrine (97), bear a strong structural resemblance to the Amaryllidaceae alkaloids of the crinine type, e.g. haemanthamine (91), but careful investigation has shown that the only aspect of biosynthesis... 

Full details have been published of part of the research on the biosynthesis of the mesembrine alkaloids of Sceletium strictum (Aizoaceae). Some of the work has appeared in a preliminary communicationand has been reviewed. 

The authors make two final comments. One is that in retrospect perhaps the differences in the biosynthesis of crinine and mesembrine alkaloids are not entirely unexpected in view of the widely different phylogenetic origins of the... 

Amaryllidaceae and Aizoceae families. Second, without use of doubly labelled precursors erroneous conclusions on mesembrine alkaloid biosynthesis would have been reached. 

Synthetic work on the mesembrane group will no doubt be further stimulated by reports on their central nervous system activity and somewhat surprising biosynthesis. An interesting asymmetric synthesis of unnatural (+ )-mesembrine (38) has been announced (Scheme 4). The key intermediate (35), prepared in nine steps from 1,2-dimethoxybenzene, was treated with L-proline pyrrolidide (36) under conditions typical for the preparation of enamines. The product was not isolated but subjected to reaction with methyl vinyl ketone followed by acid treatment to give the cyclohexenone (37) in 38 % overall yield. The last step in the synthesis [(37) — (38)] was based on previous synthetic work on mesembrine alkaloids. The synthetic (+ )-mesembrine (38) was shown to exhibit a positive Cotton effect and thus an antipodal relationship to natural (— )-mesembrine. The mesembrine analogues (40 = H or OMe, = H, Me, or CHjPh) have... 

The octahydroindole moiety of these alkaloids, e.g. mesembrenol (101), is known to arise from tyrosine whereas the aryl residue has a separate genesis in phenyl-alanine. ° Utilization of phenylalanine in alkaloid biosynthesis usually occurs via cinnamic acid and its derivatives. This has proved true for mesembrine bases too, efficient incorporations of cinnamic acid (95) and its 4 -hydroxy-derivative (96) being observed. A series of conventional feeding, as well as trapping and dilution, experiments established that alkaloid formation may occur via phloretic... 

A group of indole alkaloids from Mesembryanthe-mum species. (-)-Mesembrine, [C17H23NO3, Mr 289.37, oil, bp. 186-190°C (40 Pa), [a)g -55.4° (CH3OH)] has a cocaine-like activity. The plants are used in South West Africa to prepare the drug Channa. Joubertiamine (C,3H, N02, Mr 245.32) occurs together with mesembrine. The biosynthesis of M. a. is similar to that of Amaryllidaceae alkaloids of the cri-nine group and proceeds from phenylalanine via cinnamic acid, p-coumaric acid, and 3-(4-hydroxy-phenyOpropanoic acid. The latter is then coupled with yV-methyltyramine. 

Scheme 39. Utilization of phenylalanine, tyrosine, and methionine in the biosynthesis of mesembrine.

With the failure to demonstrate that norbelladine or its relatives plays a role in the biosynthesis of the mesembrine alkaloids, a reevaluation led to a modified approach in which attempts to identify the sequence of occurrence of the post-tyrosine and post-phenylalanine intermediates were made. There is now a substantial body of information available to suggest that phenylalanine and tyrosine have separate metabolic roles in plants belonging to the order Dictolyoden. Not only do they lack the enzyme phenylalanine hydroxylase (phenylalanine 4-monooxygenase) which is necessary for the conversion of phenylalanine to tyrosine, but the metabolic pathways of these two amino acids are generally quite different in secondary metabolism (70). Phenylalanine is involved in initial conversion to cinnamic acid and subsequent transformation to structural units of the so-called phenyl-propanoid pathway, which include Ar—C3, Ar—C2, and Ar—Cj structural entities. On the other hand, the role of tyrosine in the biosynthesis of secondary metabolities is most frequently seen as the precursor of Ar—Cj—N and Cg—C2—N units, and somewhat less frequently, as Ar—C2 and Q—C2 units. 

With regard to the utilization of tyrosine in the biosynthesis of mesembrine, various possibilities existed which pertained to the timing of the decarboxylation and N-methylation of the tyrosine-derived Q—C2—N unit during its incorporation into the octahydroindole portion of the mesembrine alkaloid skeleton. 

Several experiments provided interlocking evidence for the sequence tyrosine - tyramine -> iV-methyltyramine occurring during the biosynthesis of mesembrine-type alkaloids. These included the observation that both... 

While the evidence for the sequence tyrosine - tyramine - JV-methyl-tyramine as intermediates in the biosynthesis of the mesembrine alkaloids appeared to be secure, more compelling evidence was sought for the utilization of the aromatic ring and the attached C2—N side chain as an intact Cg—C2—N unit. The experiments, discussed below, which confirmed this proposal also brought to light several other important and unexpected findings (72). 

The utilization of phenylalanine for the biosynthesis of the mesembrine alkaloids was shown to involve its conversion to cinnamic acid (151) and 4-hydroxycinnamic acid (152), intermediates which are characteristic of the phenylpropanoid pathway (74). 

Since Ar—C2—N—C2—Cg units are involved in the biosynthesis of many alkaloid families (76), the amide 159 was examined as a possible precursor to the Sceletium bases in S. strictum. Again, no incorporation of radioactivity occurred on feeding a suspension of this compound in 20% Tween solution. Therefore, given the same reservation regarding the effect of high Tween concentrations on this result, it would appear that further studies will be required to determine the fate of iV-methyltyramine and 4 -hydroxy-dihydrocinnamic acid in their biosynthetic conversion to the octahydroindole skeleton of the mesembrine alkaloids. 

Scheme 47. Late stages in the biosynthesis of alkaloids of the mesembrine subgroi. ...

The group of alkaloids exemplified by mesembrine (6.202), shows a structural kinship with Amaryllidaceae alkaloids of the haemanthamine (6.187) type. However, the only aspect of biosynthesis common to these two groups of alkaloids is their origin in phenylalanine and tyrosine results, crucially with doubly labelled precursors, showed that various norbelladine (6.180) derivatives were only incorporated after fragmentation [148]. 

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