Synthesis of Labeled Citric Acid

2- metltyl-l,4-bulanedioic acid (a-methyl-a-hydroxysuccinic acid) and 3-methyl- 

There is a number of similar procedures starting fntm olefinic diols which by reaction with dinitrogen tetraoxide give citric acid. Sargsyan et al. [11] presented a short account of all known until 1989 synthetic preparations of citric acid. Their paper is based mainly on the patent literature and shows that with an exception of old classical methods, most of other ways to obtain citric acid is characterized by relatively low yield. Evidently, in the context of the Krebs tricarboxylic acid cycle, there is a large number of investigations dealing with enzymatic synthesis of citric acid by condensation of acetate and oxalacetate [12-20]. 

14C/12C and isotopes. These differences in isotope compositions depend on 

The chemical operations described in the literature to introduce or into citric acid molecule are based essentially on the Grimaux and Adam synthesis. Labeled citric acid was prepared by Wilcox et al. [35] in the reaction of Na CN with 3-chloro-2-carboxy-2-hydroxybutyric acid and the formed nitrile was hydrolyzed directly with hydrochloric acid. From this solution, citric acid was isolated in the form of calcium citrate and finally converted to the acid. An alternative procedme was proposed by Rothchild and Fields [36] to obtain trimethyl citrate from labeled sodium cyanide and di-chloromethyl glycolate. A more complex synthesis of C labeled citric acid is described by Winkel et al. [39]. They used labeled methyl acetate and acetyl chloride (in the presence of hthium 1,1,1,3,3,3,-hexamethyldisilazide, [(CH3)2Si]2NLi which was dissolved in tetrahydiofuran) to obtain methyl acetoac-etate. It reacts in the presence of lithium diisopropylamide, [(CH3)2CH]2NLi, also dissolved in tetrahydrofuran, with dimethyl carbonate to give dimethyl 1,3-ace-tonedicaiboxylate. It is dicarboxylated by the action of bisulfite and potassium cyanide is converted to 3-cyano-3-hydroxy-l,5 pentanedioate and finally hydrolyzed by hydrochloric acid to citric acid. 

The three step synthesis of labeled citric acid was proposed by Strouse [28]. The sequence of reactions includes the condensation of labeled ethyl bromoacetate 

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