Prephenic acid properties

These properties, combined with the fact that the second methoxyl group is not attached to the benzene ring and must, therefore, be derived from the dihydroxyphenylalanine (or prephenic acid) precursor, suggest the presence in rhynchophylline of a /3-methoxyacrylic ester residue. Convincing support for this conclusion is provided by a comparison of the UV- and IR-spectra of rhynchophylline with those of appropriate model compounds. Thus, the UV-spectrum of rhynchophylline is identical with the summation spectrum of 3-ethyloxindole and ethyl jS-ethoxy-a-methylacrylate (74), and it is also closely similar to that of formosanine. The IR-absorption of rhynchophylline in the carbonyl region also resembles that of formosanine, except that rhynchophylline exhibits an additional band, of medium intensity, at 1645 cm-1 this band, however, is also present in the spectrum of ethyl jS-ethoxy-a-methylacrylate. 

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... 

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