Anthranilic Acid-Tryptophan Alkaloids

Department of Pharmaceutical Engineering, College of Chemistry and Chemical Engineering, Central South University, Changsha, China 

Anthranilic acid (or o-amino-benzoic acid) is an aromatic acid with the formula C H NO, which consists of a substituted benzene ring with two adjacent, or ortho- functional groups, a carboxylic acid, and an amine (Fig. 14.1). Anthranilic acid is biosynthesized from shikimic acid (for shikimic acid biosynthesis, see Chapter 10) following the chorismic acid-mediated pathway [1]. Based on its biosynthetic mechanism, shikimate is transformed to shikimate 3-phosphate with the consumption of one molecule of ATP, catalyzed by shikimate kinase. 5-Enolpyruvylshikimate-3-phosphate (EPSP) synthase is then catalyze the addition of phosphoenolpyruvate to 3-phospho-shikimate followed by the elimination of phosphate, which leads to EPSP. EPSP is further transformed into chorismate by chorismate synthase. Chorismate reacts with glutamine to afford the final product anthranilate and glutamate pyruvate catalyzed by anthranilate synthase (Fig. 14.1). 

Phosphorylation of 3-hydroxyl group of shikimate by shikimate kinase (EC 2.7.1.71) with ATP as a cosubstrate initiates the biosynthesis pathway of anthranilic acid [2], This step also presents the first step of the shikimate pathway, which is a metabolic route used by bacteria, fungi, and plants for the biosynthesis of many aromatic products such as lignins, alkaloids, flavonoids, benzoic acid, and plant hormones, in addition to the aromatic amino acids (phenylalaiune, tyrosine, and tryptophan). The sequential EPSP synthesis is catalyzed by EPSP synthase (EC 2.5.1.19) through the addition of phosphoenolpyruvate to 3-phospho-shikimate followed elimination of phosphate. EPSP synthase belongs to the family of transferases, specifically to those transferring aryl 

Chorismate synthase (EC 4.4.3.5) catalyzes the, A-trans elimination of the phosphate group from EPSP to form chorismate. It requires the presence of a reduced flavin mononucleotide (FMNH2 or FADH2) for its activity. Chorismate synthase from various sources shows a high degree of sequence conservation of about 360- 00 amino acid residues [5]. By now, nine structures have been solved for this class of enzymes. The X-ray crystal structure of chorismate synthase from Mycobacterium tuberculosis is a homotetramer with one FMN molecule noncovalently bound to each of the four monomers (Fig. 14.3). Each monomer is made up of 9 alpha helices and 18 beta strands, and the core is assembled in a uitique beta-alpha-beta sandwich fold. The active sites for FMN binding are made up of clusters 

From Biosynthesis to Total Synthesis Strategies and Tactics for Natural Products, First Edition. Edited by Alexandres L. Zografos. 2016 John Wiley Sons, Inc. Published 2016 by John Wiley Sons, Inc. 

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The majority of alkaloids have been found to be derived from amino acids, such as tyrosine, phenylalanine, anthranilic acid, tryptophan/tryptamine, ornithine/arginine, lysine, histidine and nicotinic acid (Fig. 2.1). However, alkaloids maybe derived from other precursors such as purines in case of caffeine, terpenoids, which become aminated after the main skeleton has been synthesized i.e. aconitine or the steroidal alkaloids, are found in the Solanaceae and Liliaceae. Alkaloids may also be formed from acetate-derived polyketides, where the amino nitrogen is introduced as in the hemlock alkaloid, coniine. 

Of all the alkaloid-producing families, one of the most prolific is the Rutaceae (Waterman, 1975). The alkaloids obtained included 1-benzyltetrahydroisoquinoline, simple tryptophan derivatives, imidazoles and, most commonly, quinoline alkaloids originating from anthranilic acid. The Rutaceae was the only family in which the direct use of anthranilic acid in alkaloid production occurs to any extent. 

The route of formation of the carbazole nucleus is still far from understood, and has been variously considered to arise from 3-prenylquinolone via a pathway involving shikimic acid (394) and mevalonic acid (MVA) (400) (Scheme 3.1) (1,112,362-366), anthranilic acid (397) and prephenic acid (404) via a pathway involving shikimic acid (394) (Scheme 3.2) (367), and also tryptophan (408) involving the mevalonate (400) pathway (Scheme 3.3) (133). All of these pathways lack experimental proof. However, based on the occurrence of the diverse carbazole alkaloids derived from anthranilic acid (397) in the family Rutaceae, the pathway... 

Alkaloids derived from L-tryptophan hold the indole nucleus in a ring system. The ring system originates in the shikimate secondary compounds building block and the anthranilic acid pathway. It is known that the shikimate block. 

Amination at the 2-position and loss of pyruvate yields anthranilic acid, the precursor of the quinoline alkaloids, distributed widely in the Rutaceae, and tryptophan, the precursor of the indole alkaloids... 

Highly significant results have been obtained by Monkovic and Spenser 160) from a study of the biosynthesis of dictamnine using tracer methods. Mature plants of Dictamnus albus incorporate carboxyl- C-labeled anthranilic acid to give radioactive dictamnine, specifically labeled at C-4, strongly supporting the derivation of the alkaloid directly from anthranilic acid failure to incorporate radioactivity from tryptophan-jS- C shows that the 4-hydroxyquinoline unit is not derived... 

Phenylalanine and tyrosine acts as precursor for opium alkaloid biosynthesis. Tryptophan is a significant source of Vinca alkaloids. Alkaloids are derived from anthranilic acid, which is an intermediate in biosynthesis of tryptophan. Some alkaloids are derived from acetate, terpene or shikimic acid. Shikimic acid is a significant metabolite as most of the aromatic constituents are derived from shikimic acid pathway. 

C]Tryptophan gave inactive alkaloids but tritiated 2,4-dihydroxy-quinoline (34) and its N-methyl derivative were incorporated into (47) (0.009 % and 0.020% respectively) an early route had suggested the derivation of what was essentially (34) from tryptophan. Radioisotope dilution showed the presence of both these quinoline precursors together with iV-acetyl- and N-methyl-anthranilic acid in A. baueri. A satisfactory incorporation of N-methylanthranilic acid into (47) was found in Evodia xanthoxyloides, and this, together with its natural occurrence, indicates that early methylation may be important in the biosynthesis of acridone alkaloids. 

The isolation of alkaloids bearing the 8-hydroxyquinoline moiety prompted postulation (17) of a biosynthetic pathway to 1 from the amino acid tryptophan, for which chemical analogies are known (22) (Scheme 2). It should be noted, however, that, in general, quinolines are biosynthesized from anthranilic acid (14). Biosynthesis of broussonetine (4) would involve condensation of two molecules of 1 with a molecule of acetyl-CoA followed by cyclization, as depicted in Scheme 3. 

Miscellaneous Alkaloids. Shikimic acid (57) is a precursor of anthranilic acid (28) and, in yeasts and Escherichia coli (a bacterium), anthranilic acid (o-aminobenzoic acid) is known to serve as a precursor of tryptophan (26). A similar but yet unknown path is presumed to operate in higher plants. Nonetheless, anthranilic acid itself is recognized as a precursor to a number of alkaloids. Thus damascenine [485-64-7] (134), C10H13NO3, from the seed coats of Nige/h damascena has been shown (95) to incorporate labeled anthranilic acid when unripe seeds of the plant are incubated with labeled precursor. 

Quinazoline Alkaloids.— The biosynthesis of vasicine (=peganine) (163) has been investigated in Adhatoda vasica (Acanthaceae) and Peganum harmala (Zygo-phyllaceae). In both plants the anthranoyl portion is known to derive from anthranilic acid [the reported incorporation of tryptophan (0.017 %) suggested as via anthranilic acid seems of questionable significance]. The remaining atoms of vasicine, however, appear to arise by different pathways in the two plants. 

Since tryptophan is recognized as a main constituent of plant proteins and as a common biogenetic precursor of the complex indole alkaloids, the wide occurrence of tryptamine derivatives in the plant kingdom is not unexpected. The presently known cases of these simple indole alkaloids have been ones in which a tryptamine unit formally appears as a slightly modified structure (e.g., by oxidation or methylation), as a cyclized form or a dimeric variation thereof, or as a modification which incorporates short carbon chains (e.g., C4, C2) or a simple aromatic structure (anthranilic acid) respectively. The great majority of the simple indole alkaloids are confined to the dicotyledon plants. 

Biosynthesis Like other aromatic amino acids, e.g., Phe and Tyr, Trp is formed on the shikimic acid pathway. There is a branching point at chorismic acid one branch leads to Phe and Tyr, the other to Trp choris-mic acid - anthranilic acid (anthranilic acid synthase, EC 4.1.3.27)- A-(5 -0-phosphoribosyl)-anthranilic acid (anthranilic acid phosphoribosyl transferase, EC 2.4.2.18)- 1 -o-carboxyphenylamino-1 -deoxyribu-lose 5-phosphate [A-(5 -phosphoribosyl)anthranilic acid isomerase]- indole-3-glycerol phosphate (in-dole-3-glycerol phosphate synthase, EC 4.1.1.48) - indole (tryptophan synthase, EC 4.2.1,20)+serine - Trp. Many biologically active indole compounds are derived from Trp, e. g., 5-hydroxytryptophan, 5-hydroxy-tryptamine ( serotonin), and melatonin as well as many indole alkaloids. 

Groger, D. Alkaloids Derived from Tryptophan and Anthranilic Acid. In Encyclopedia of Plant Physiology, New Series, Vol. 8 (Bell, E. A., B. V. Charlwood, eds.), p. 128. Berlin-Heidelberg Springer-Verlag. 1980. 

This approach though also has some complications, particularly when the alkaloid has more than one nitrogen and more than one biosynthetic amino acid precursor unit. For example, evodiamine and rutaecarpine, from the fruits of Euodia mtaecarpa (Rutaceae), can be classified as alkaloids derived from tryptophan. In addition, one of the nitrogen atoms of each alkaloid is derived from an anthranihc acid unit. Therefore, these alkaloids can be classified as alkaloids derived fiom tryptophan and as alkaloids derived from anthranilic acid. In this volume, these alkaloids are discussed in Chapter 2.19 as alkaloids derived from tryptophan. 

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