Enol Oxalacetic Acid Exists in the Z Form in the Crystalline State and in Solution

Because of inconsistencies in the literature regarding the structure of the enol form of oxalacetic acid (OAA), this subject has been reinvestigated. It was approached initially by attempting to determine the crystal structure of enol OAA directly by X-ray crystallography. This approach failed due to the poor quality of enol OAA crystals. The subject was then approached indirectly as follows. First, the structure of enol di-tert-butyl OAA in the crystalline state was determined to be Z by X-ray crystallography. Second, the solution structure of enol di-t-butyl OAA was determined to also to be Z by and ¹³C-NMR studies combined with the results from X-ray crystallography of enol di-tert-butyl OAA. Third, the solution structure of enol OAA was determined to be Z by ¹³C-NMR studies using enol di-tert-butyl OAA as a reference. Fourth, the structure of enol OAA in the crystalline state was determined to be Z by ¹H-NMR studies using freshly dissolved enol OAA solutions at low temperature. Finally, the structure of enol OAA in aqueous solution was inferred to be Z on the basis of is activity as a substrate for fumar ase A. The previous reports in the literature of crystalline (E)-enol OAA are in error.