Cocaine biosynthesis

1 Cocaine Biosynthesis.- The first definitive work on the biosynthesis of the tropane moiety of this alkaloid has appeared .  

The results obtained are consistent with speculations that it is formed by the same general route which has long been established for the origin of hyoscyamine (16) in Datura species and Atropa belladonna, Extensive tracer experiments extending over 25 years have 

It was suggested that ornithine (1) is incorporated via -S-methylornithine (2), and this amino-acid was shown to be a direct precursor of hyoscyamine in Datura stramonium a.nd Atropa belladonna. This amlno-acid has been shown to be an authentic natural product since it was isolated in radioactive form after feeding [5- C]ornithine or [5- H]ornithine to A. belladorma. Decarboxylation then yields iP-methylputrescine (3), an established precursor of the tropane ring of hyoscyamine and scopolamine . Oxidation of AT-methylputrescine yields 4-methylaminobutanal (4), which was detected in Datura plants which had been fed [2- C]ornithine.  

The cyclized form of this amino-aldehyde is the y-methyl-A -pyrrol-inium salt (6). Condensation of this iminium salt with aceto-acetyl coenzyme A yields the coenzyme A ester of hygrine-1 -carboxylic acid (10). Hydrolysis of this ester and decarboxylation of the resultant 3-keto-acid (9) affords hygrine (8), which has been shown to be a precursor of tropine which is plausibly formed 

Having obtained a successful incorporation of ornithine into cocaine by the leaf-painting technique, the same method was used for administering [l- C]acetate and [4- H]phenylalanine . These two potential precursors were fed at the same time, the phenylalanine affording the benzoyl moiety in which almost all the trit- 

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A further intramolecular SB by condensation of 4-(methylamino)butanal leads to the A-methyl-A -pyrrolinium cation, which is the precursor of cocaine biosynthesis (Figure 1.28) [26, 27]. 

In E. coca (Erythroxylaceae), the stereospecific reduction of the 3-keto function of tropane alkaloids is catalyzed by methylecgonone reductase, which converts methylecgonone (2-carbomethoxy-3-tropinone) to methylecgonine (2-carbomethoxy-3p-tropme), the penultimate step in cocaine biosynthesis. The subsequent step is the benzoylation via benzoyl-CoA to yield cocaine [50]. 

Hoye TR, Bjorklund JA, Koltun DO, Renner MK (1999) N-Methylputrescine oxidation during cocaine biosynthesis study of prochiral methylene hydrogen discrimination using the remote isotope method. Org Lett 2(l) 3-5. doi 10.1021/ol990940s... 

In these cases the formal //-ammo acid relationship often is a result of late stage condensation or cyclization reactions (e.g. Mannich-type, Pictet-Spengler) within the biosynthesis Typical examples are cocaine and correlated tropane alkaloids, Catharanthus alkaloids or Iboga alkaloids like heyneanine. 

In the biosynthesis of cocaine, the carboxylic acid is retained as the methyl ester, and after stereospecific... 

Concomitant to the enhancement of the signal for C-3 in the C NMR spectrum, the signals for H2 and H4 decreased in the H-NMR spectrum of all the tropine moieties compared with those in authentic samples. This indicates the incorporation of deuterium at C2 and C4 derived from C Hj CXX)". As the diminution of the signals for H2 and H4 was not identical at both positions, we suppose a sequential incorporation of labeled acetate (example given in Fig. 15b). From these data we assume that the biosynthesis from N-methyl-pyrrolidinium ion to tropine is a two-step process which does not involve a four carbon unit (acetoacetyl coenzyme A) but two units of acetyl coenzyme A, which were added sequentially as has been suggested for the biosynthesis of cocaine [28, 37]. On the other hand it has been reported that l,2- C2-acetate was incorporated with an equal efficiency at C2 and C4 by non-transformed root cultures of H. albus [36]. Further investigations are required to clarify this matter. 

Putrescine A-methyltransferase (PMT, EC 2.1.1.53) catalyses the first specific step in the biosynthesis of tropane alkaloids, cocaine and nicotine [123]. Putrescine is methylated by PMT via SAM (S-adenosylmethionine) transferring the methyl group from SAM to an amino group of putrescine. Fig. (1). This enzyme has been isolated from roots of both Nicotiana tabacum and D. stramonium [124], and the activity of this enzyme is restricted to the roots of Solanaceous species corroborated by results describing a root pericycle-specific activity in A. belladonna [125]. Nevertheless, more recently, a low mRNA pmt transcript level in leaves of N. tabacum, with a rise in transcript level after mechanical wounding has been detected [126]. 

Amino acids act as precursor foe biosynthesis of alkaloids. Ornithine and lysine are common amino acids used as starting material for alkaloid biosynthesis. Cocaine and nicotine are classical examples form this series. 

The biosynthesis of tropane alkaloids has been extensively studied over the last few decades. This is mainly due to the pharmacological importance of compounds such as (-)-hyoscyamine, (-)- scopolamine and (-)-cocaine. An excellent review has been published on that subject by Leete [26]. 

The catecholamine hypothesis was then modified to include 5-HT in the etiology of depression (9,10). It should be noted, however, that not all Inhibitors of monoamine reuptake are antidepressants, because cocaine, a potent inhibitor of NE and dopamine reuptake, is not an antidepressant but, rather, an addictive stimulant. Subsequent studies with inhibitors of monoamine biosynthesis appear to confirm Kielholz s opinion and Schiidkraut s modified theory that clinical depression is the result of a deficiency in both 5-HT and NE and that the antidepressive mechanism of action most likely affects levels of both. 

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. 

It is considered that the biosynthesis of cocaine resembles that of (—)-hyoscyamine and (—)-scopolamine. It was shown, through the feeding experiments using E. coca [2—5], that the P-ketothioester was the biosynthetic intermediate of cocaine, as in the case of (—)-hyoscyamine and (—)-scopolamine. It was also demonstrated that a thioester of benzoic acid derived from phenylalanie was the precursor of the benzoyl moiety of cocaine [6]. 

The present review will focus on progress made in the elucidation of the biosynthesis of tropane and related alkaloids from a more chemical perspective and will attempt to outline what in our current understanding is deficient and to identify the remaining problems. For this purpose, the discussion will be divided into two parts, dealing with the assembly of the amino acid derived C4N portion, C-l,C-5,C-6,C-7,N-8, of the five-membered ring and then with the assembly of the acetate derived C3 fragment, C-2,C-3,C-4, and the formation of the azabicy-clooctane system. A liberal definition of the term tropane wiU be used, so that the biosynthesis of cocaine and some biogenetically related pelletierine-type alkaloids may be included in the discussion. In the view of this author, results... 

Scheme 6. Classical (path C) and revised proposals (path D) for the biosynthesis of methyl ecgonine 19 and thence of cocaine the illustrated incorporation experiment with [ N, C] 10 supported path D...

Feeding experiments have also been performed with more advanced putative precursors both for cocaine and tropane biosynthesis. In these experiments, the role of (21) and (22) in cocaine and tropane biosynthesis was examined. No incorporation of the ethyl ester derivative or of the N-acetylcysteamine thioester derivative of (21) into either of the alkaloids, cocaine or scopolamine, was observed [39]. However, isotope dilution experiments did provide evidence for the presence of (21) in Erythroxylon coca [46]. In the case of (22), fed as its ethyl ester, substantial specific incorporations into cocaine [47] and scopolamine [48] were recorded. These results were interpreted as corroborating evidence for the hypothesis that the acetate units in these alkaloids are introduced in a stepwise fashion via Path D2 in Scheme 8 with (22) as an intermediate. 

Scheme 12. Biosynthesis of cocaine via methyl ecgonine 19 only the (2J, 3S)-enantiomer is formed...

The discussion of the observed labeling patterns in the alkaloids so far has focused on the interpretation of the data according to either only one or a combination of several of the paths outlined in Scheme 8. Our investigation of the biosynthesis of a seemingly related alkaloid, lycopodine (27), uncovered yet another mechanism for the assembly of the C3 fragments of the tropane- and the pelletierine-type alkaloids. These results will be discussed in the following section and the implications of this discovery for the biosynthesis of cocaine and tropane will be discussed subsequently. 

Tropane alkaloid Biosynthesis Scopolamine, nicotine, hygrine, calystegines Bl, calystegines B2, spermidine, spermine, cocaine, tropinone, littorine, tropine... 

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