Prephenic acid formation

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... 

The biogenetic pathway proposed by Chakraborty for the formation of carbazole (1) and 3-methylcarbazole (2) proceeds through Af-phenylated anthranilic acid (406). This hypothesis is based on aromatic C-methylation of aniline with methionine, and originates from anthranilic acid (397) and prephenic acid (404). Until now, there are no N-phenylated anthranilic acid derivatives known naturally, therefore, this hypothesis is lacking substantial biogenetic evidence. However, the isolation of carbazole (1), 3-methylcarbazole (2), and several derivatives of 3-methylcarbazole... 

We have arrived at prephenic acid, which as its name suggests is the last compound before aromatic compounds are formed, and we may call this the end of the shikimic acid pathway. The final stages of the formation of phenylalanine and tyrosine start with aromatization. Prephenic acid is unstable and loses water and CO2 to form phenylpyruvic acid. This a-keto-acid can be converted into the amino acid by the usual transamination with pyridoxal. 

Pseudoakuammicine is the first racemic base to be discovered in the strychnine-yohimbine series of alkaloids, and the question of its origin naturally arises. The only stage in the extraction of Picralima seeds during which racemization of akuammicine might have occurred involved prolonged percolation with hot methanol however, as already discussed, akuammicine is not racemized under these conditions but suffers a more extensive decomposition. In any event, such a racemization would necessarily involve fission of the 3,7 and 15,16 bonds, followed by a nonspecific resynthesis, which is considered to be a very unlikely possibility. It was therefore suggested that, in the plant, pseudoakuammicine is produced by a nonspecific biosynthesis this would accord with its formation from a tryptophan-phenylalanine precursor, but not from an optically pure prephenic acid derivative (40). 

The next stage in the formation of the aromatic amino acids involves the conversion of shikimic acid to prephenic acid (XV) and anthranilic acid (XVI). The conversion of prephenic acid to phenylalanine and t3Tosine, and that of anthranilic acid to tryptophan are fairly well understood and... 

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. 

Otherwise no immediate precursors of tyrosine appear to have been reported. Transamination of p-hydroxyphenylpyruvic acid has been suggested to be the final stage in yeasts (474), and may occur in E. coli (809), and isotopic evidence, discussed later, suggests that even if tyrosine is not formed from phenylalanine, the method of introduction of the tyrosine side chain is very similar to that postulated above for phenylalanine. Formation of p-hydroxyphenylpyruvic acid from prephenic acid can be readily visualized. 

The three aromatic amino acids that are biosynthesized in the shikimic acid pathway have much in common. The many stereochemical events occurring between the condensation of compounds 288a and 289 derived from carbohydrates to the formation of prephenic acid 296 have been extensively reviewed including a recent review by ourselves (82), and so we have summarized the stereochemistry of the biosynthesis in Scheme 79. Prephenic acid 296 leads to phenylalanine 297 and tyrosine 298. The mem-substituted amino acids 299 are derived from chorismate 295, as is tryptophan 302, as shown. 

Fig. 7.9. Formation of prephenic acid from chorismic acid (modified from Dewick, 1984 used with permission of the copyright owner, the Royal Society of Chemistry, London).

An alternate reaction of prephenic acid was found in extracts of coli nxutants blocked in the conversion to phenylpyruvate. The product of the alternate reaction is p-hydroxyphenyllactic acid. Presumably oxidation to ketotyrosine and transamination follow the formation of p-hydroxyphenyllactate to yield tyrosine. 

Amination of chorismic acid 5.10) leads through anthranilic acid 5.13) to tryptophan 5.14). The formation of phenylalanine 5.17) and tyrosine 5.18), on the other hand, proceeds via prephenic acid 5.16), whose formation from chorismic acid 5.10 = 5.15) involves... 

Prephenic acid (86) is readily transformed with acid to give phenyl pyruvic acid (89) but is somewhat more stable towards alkali. However, on heating or prolonged standing in dilute alkaline solution it is converted to p-hydroxyphenyl lactic acid (87). Several proposals have been put forward to account for the formation of this product under these conditions and one of these is shown in Figure 2.13. 

Chemical properties appropriate to a compound found at a branch point of metabolism are displayed by chorismic acid. Simply warming the compound in acidic aqueous solution yields a mixture of prephen-ate and para-hydroxybenzoate (corresponding to reactions h and l of Fig. 25-1). Note that the latter reaction is a simple elimination of the enolate anion of pyruvate. As indicated in Fig. 25-1, these reactions correspond to only two of several metabolic reactions of the chorismate ion. In E. coli the formation of phe-nylpyruvate (steps h and i, Fig. 25-1) is catalyzed by a single protein molecule with two distinctly different enzymatic activities chorismate mutase and prephenate dehydratase.34-36 However, in some organisms the enzymes are separate.37 Both of the reactions catalyzed by these enzymes also occur spontaneously upon warming chorismic acid in acidic solution. The chorismate mutase reaction, which is unique in its mechanism,373 is discussed in Box 9-E. Stereochemical studies indicate that the formation of phenylpyruvate in Fig. 25-1, step z, occurs via a... 

Fig. 2 (1)]. A key intermediate in the pathway is chorismate from which branched pathways lead to tryptophan, phenylalanine, tyrosine, 4-amino benzoate, isoprenoid quinones, and metacarboxyphenylalanine. A secondary branch also occurs at prephenate leading to phenylalanine and tyrosine. A representative number of non amino acid compounds in relation to their shikimic acid pathway precursors are shown in Fig. 1. One side branch leads to quinate which participates in the formation of depsides. 

In higher plants, phenylalanine seems to be formed in an alternative manner by formation of prephenate (26) and conversion of this intermediate into arogenic acid (32). Chorismate mutase occurs as two isozymes which have been purified to homogeneity from mung bean and sorghum. All chorismate mutase isozymes show allosteric activation by chorismate (Poulsen and Verpoorte, 1991). One chorismate mutase isozyme is inhibited by phenylalanine or tyrosine and activated by tryprophan, whereas the second is not affected by any of the aromatic amino acids. 

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