The precursor has the same carbon skeleton as the designated starting material. All that is necessaryis to hydrogenate the double bond of the alkynoic acid to the cis alkene. This can be done by usingthe Lindlar catalyst. Cyclization of the hydroxy acid to the lactone is spontaneous.19.31Hydration of the double bond can occur in two different directions:(a)The achiral isomer is citric acid.(b)The other isomer, isocitric acid, has two stereogenic centers (marked with an asterisk*). Iso-citric acid has the constitutionWith two stereogenic centers, there are 22, or four, stereoisomers represented by this constitu-tion. The one that is actually formed in this enzyme-catalyzed reaction is the 2R,3Sisomer.19.32Carboxylic acid protons give signals in the range d10–12 ppm. A signal in this region suggests thepresence of a carboxyl group but tells little about its environment. Thus, in assigning structures to
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