The Biosynthesis of Alkaloids

Unlike other classes of natural products such as the flavonoids, novel biosynthetic pathways continue to be discovered for the alkaloids, and some of these are discussed in more detail in Sect- 

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V. Aldol Reactions of Heterocyclic Enamines and Their Importance for the Biosynthesis of Alkaloids... 

Robins, R. J. 1998. The Biosynthesis of Alkaloids in Root Cultures. In Alkaloids. Biochemistry, Ecology, and Medicinal Applications (Roberts, M. F. and Wink, M., eds), pp.199-218. New York - London Plenum Press. 

The biosynthesis of alkaloids has been extensively studied, and although for a time it was thought that alkaloids arose primarily from amino acid precursors, strong evidence now is available that ethanoate also is involved. The mode of alkaloid biosynthesis is not yet as well understood as that of the terpenes and steroids. One experimental problem is the difficulty of feeding suitably labeled precursors to plants. 

During the last two decades, the use of enzyme-catalysed reactions in the total chemical synthesis and in the biological formation of alkaloids has been rapidly developed and a large number of enzymes and genes (cDNAs) involved in the biosynthesis of alkaloids have become available. 

The benzylisoquinoline alkaloids are widely distributed in nature and are intermediates in the biosynthesis of alkaloids of this family (2, 3). It is not surprising therefore that several groups (6, 7, 15, 23) have examined their spectra. Among the alkaloids that have been studied are reticuline (26) (7), norlaudanosine (27) (7), laudanosine (28) (6, 15), and the cis- and trans-N-oxides of laudanosine, 29 and 30, respectively (7). The chemical shifts of laudanosine are recorded in Table IV and the structures of the alkaloids may be found in Fig. 4. 

This observation was explained by the assumption that a portion of the protoberberine was formed via norreticuline (96) present in the same incubation mixture and derived from enzymic demethylation of reticuline. Reaction of 96 with an unlabeled one-carbon fragment and subsequent ring closure would then lead to C-8 unlabeled protoberberines. The authors suggest that this one-carbon fragment may be derived from S-adenosyl-methionine, and that the product of its combination with 96 may be converted directly to 91 or 94 without the intermediacy of free reticuline (99). If their assumption is correct, the conversion of norreticuline to the protoberberine alkaloids may not involve the formation of reticuline itself, a suggestion that is at variance with the known intermediacy of reticuline in the biosynthesis of alkaloids of this group. 

Tetracyclization. The Heathcock group1 has described a remarkably short and efficient synthesis of the skeleton of Daphniphyllum alkaloids (2) by reaction of the dialdehyde 1 with gaseous ammonia and then dissolution in acetic acid at 70°. The yield is 77%, based on the diol precursor to 1. The azadiene a and the imine b have both been isolated and identified. The conversion of a to b is an intramolecular Diels-Alder type reaction. The tetracyclization may well be involved in the biosynthesis of alkaloids such as Daphnilactone A (3). 

The established practice of including references to earlier Reports in this series for background information is continued. Two comprehensive reviews are also cited.1,2 An authoritative account of the biosynthesis of fungal metabolites has been published,3 as has an introductory text which includes a survey of the biosynthesis of alkaloids and nitrogenous microbial metabolites.4... 

The first intermediate beyond ornithine in the biosynthesis of tropane alkaloids has been deduced to be (S-A-methylornithine (2). Recently, (2) was identified as a natural constituent for the first time in a plant, namely Atropa belladonna, which produces tropane bases.5 The (2) was labelled by radioactive ornithine (1), but, unfortunately, the alkaloids were not, so correlation between the formation of (2) and the biosynthesis of alkaloids has not yet been achieved. 

In accord with a general body of evidence on the biosynthesis of alkaloids as against that of pipecolic acid (see above), L-lysine has been shown to be the preferred precursor for lupanine (27) and D-lysine the preferred precursor for l-pipecolic acid (24) in Lupinus angustifolia,36 A high retention of tritium, present at C-4 and C-5 in the lysine, on formation of (27) is to be noted. 

Further work on the biosynthesis of alkaloids by fractions of the latex of seed capsules of P. somniferum has been published.39... 

Steroidal Alkaloids.—In the biosynthesis of alkaloids such as solasodine (128), from cholesterol (129), it appears that the cholesterol side-chain is first functionalized at C-26 with the introduction of a hydroxy-group (cf. Vol. 8, p. 28 Vol. 7, p. 32). The 26-amino-compound, (25i )-26-aminocholesterol (130), has been found to act as a significant precursor for solasodine (128) in Solarium laciniatum, whereas (25i )-26-aminocholest-5-ene-3/ ,16/ -diol (131) was poorly utilized.105 This indicates that replacement of the hydroxy-group at C-26 by an amino-group may occur before further oxygenation elsewhere in the steroid nucleus (particularly at C-16). It may also be concluded from this and other evidence (cf. Vol. 9, p. 27) that oxidation at C-22 precedes hydroxylation at C-16. 

The association of cytochrome P450s with organelles other than the ER may have important implications for the biosynthesis of alkaloids. It may help to explain why biochemical localization studies have shown that certain reactions in a pathway occur within the chloroplast. In the case of Catharanthus alkaloids, the 3 rd to last step in vindoline biosynthesis involves a chloroplast thylakoid associated N-methyltransferase.22 The arguments for participation of chloroplasts in this reaction include the possibility that the previous step involving hydration of the 2,3 double bond might require a chloroplast based oxidation reaction (Fig 8.6). The conclusive identification of specific cytochrome P4S0 enzymes in chloroplasts suggests that this hypothesis should be tested. 

In the next chapter you will see how hydroxylation of benzene rings plays an important part in the biosynthesis of alkaloids and other aromatic natural products. 

Some of the most interesting applications of organic structural theory to the elucidation of biosynthetic pathways were stimulated by efforts to formulate mechanisms for the biosynthesis of alkaloids. Conversely, consideration of implied biogenetic relations have occasionally helped structural determination. An important aspect of theories concerning alkaloid biosynthesis has been the assumed role of the aromatic amino acids in their formation. Only limited experimental evidence is available in this area. The incorporation of tyrosine- 8-C into morphine has been shown to be in accordance with a theory for its formation from 3,4-dihydroxyphenyl-alanine plus 3,4-dihydroxyphenylacetaldehyde. A stimulating theory of the biosynthesis of indole alkaloids, based on a condensation between trypt-amine and a rearrangement product of prephenic acid, has recently been published. The unique stereochemistry of C15 of these alkaloids had an important part in the formulation of the theory. Experimental proof of this theory would be valuable for several areas of alkaloid chemistry and biosynthesis. 

Alkaloid metabolism in lupine was proved by Wink and Hartmann to be associated with chloroplasts (34). A series of enzymes involved in the biosynthesis of lupine alkaloids were localized in chloroplasts isolated from leaves of Lupinus polyphylls and seedlings of L. albus by differential centrifugation. They proposed a pathway for the biosynthesis of lupanine via conversion of exogenous 17-oxosparteine to lupanine with intact chloroplasts. The biosynthetic pathway of lupinine was also studied by Wink and Hartmann (35). Two enzymes involved in the biosynthesis of alkaloids, namely, lysine decarboxylase and 17-oxosparteine synthetase, were found in the chloroplast stoma. The activities of the two enzymes were as low as one-thousandth that of diaminopimelate decarboxylase, an enzyme involved in the biosynthetic pathway from lysine to diaminopimelate. It was suggested that these differences are not caused by substrate availability (e,g., lysine concentration) as a critical factor in the synthesis of alkaloids. Feedback inhibition would play a major role in the regulation of amino acid biosynthesis but not in the control of alkaloid formation. 

Cyclopentanes, Iridoids.—A very useful review of the biosynthesis of the known sweroside-, morroniside-, and oleuropein-type secoiridoid glucosides, together with the biosynthesis of alkaloidal glucosides which can be regarded as secoiridoid... 

Alkaloids are basically nitrogen bases. The amino acids act as building blocks for the biosynthesis of alkaloids. Majorities of the alkaloids contain a pyridine, quinoline, and isoquinoline or tropane nucleus and are responsible for physiological effects in man or in animal. The side chains in alkaloids are derived from terpene or acetate. Alkaloids have basic properties and are alkaline in reaction, turning red litmus paper blue. 

This review covers the biosynthesis of alkaloids other than those derived from tryptophan and a C9 or Cio terpenoid unit, which are surveyed in the succeeding chapter. 

Rehacek Z, Desai JD, Sajdl P, Pazoutova S. The cellular role of nitrogen in the biosynthesis of alkaloids by submerged culture of Claviceps purpurea (Fr.) Tul. Can J Microbiol 23(5) 596-600, 1977. 

The simple basis for the biosynthesis of alkaloids in the plant cell is that a few common amino acids can be converted simply into reactive intermediates which may then condense spontaneously in variants of the Mannich reaction to yield, virtually at a stroke, the fully elaborated nuclei of the alkaloids. More detailed accounts of the biogenetic concept... 

Takao (522) determined the structure and the relative configuration of cor3moline. This alkaloid can be derived from the alkaloids of the corydaline type (Table XV). These two groups of bases—corydaline and corynoline—have also been isolated from the plant genus Gorydalis. It has not yet been decided whether one of the initial compounds which arise during the biosynthesis of alkaloids of the corydaline type is a tropic acid derivative. 

If instead of protonation of the imine function in (14), in the lysine decarboxylase reaction, nucleophilic attack by the 6-amino-group of lysine [see (19)] occurs, (16) is obtained directly and independently of cadaverine, and without loss of the C-2 proton of lysine. This modified" decarboxylase would function, it is suggested, for the biosynthesis of alkaloids such Lysine decarboxylase... 

The biosynthesis of alkaloids containing a pyrrolidine ring such as hyoscyamine (18) and hygrine (19) is similar to the biosynthesis of those with a piperidine... 

The pinacol reaction is an example of radical dimerization (Scheme 4.29). Stabihzed free radicals have sufficiently long hfetimes to permit coupling outside solvent cage confinement Scheme 4.30 shows two such coupling reactions. The first is the photochemical reduction of benzophenone to benzopinacol (Scheme 4.30a). The second is an example of the oxidative coupling of phenols, a transformation that is an important step in the biosynthesis of alkaloids (Scheme 4.30b). 

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