Asymmetric Citric Acid 251 



isotope. It must be remembered that although this formulation 

 is purely arbitrary, the general proposition must hold true. 



Since it was evident from these considerations that the 

 determining factor is the asymmetry of the central carbon 



a KOOC\ ,.CH2C«00H 

 b KOOCCHo i>K OK c 



it 



HOOCv . 



^C— CHpCOOH 

 HOOCCK^ '^ 



Fig. 6. Enzymatic dehydration of a labelled antipode of citric 



acid. 



atom, it followed that a chemical preparation of the asym- 

 metrically labelled citric acid was entirely feasible. Therefore, 

 its preparation was carried out as illustrated in Fig. 7 (Wilcox, 

 Heidelberger, and Potter, 1950). By this means a compound 

 closely related to citric acid and having a true asymmetric 



Mg' HCN 

 ClCHpCOOEt -rrT-^ ClCHoCOCSoCOOEt T^ :* 



?^ H^ ?^ 



ClCH^CCH^COOEt ^ » ClCH«CCHoCOOH 



Fig. 7. Synthesis of citric acid precursor. 



carbon atom was synthesized. This substance was resolved 

 into optically active form with brucine, and was then con- 

 verted into labelled citric acid bv means of the reactions shown 

 in Fig. 8. Samples of this labelled citrate in various stages of 



pH * ^ OH 



ClCHgC-CHgCOOH ^*g^^^> HbOCCHgC-CHgCOOH 

 COOH ' ^ COOH 



*• •'D / 



Fig. 8. Conversion of optically active precursor into labelled 



citric acid. 



