Studies on phenylpyruvic acid. I. Keto-enol tautomerism.

The tautomerism of phenylpyruvic acid (PPA) and its sodium salt was investigated. The 1H and 13C spectra of PPA in aprotic solvents and in methanol show almost complete prevalence of the enol form, whereas the keto form prevails only in water. The Z configuration was assigned to the sole enol tautomer present on the basis of the value of the vicinal coupling constant [3J1H, 13COOH]=3.7 Hz. A small amount of the hydrated form of PPA (1-phenyl-2, 2-dihydroxylpropanoic acid) was found in the aqueous solution of its sodium salt and in buffer solution (pD=6) of PPA. By means of infrared spectroscopy one can conclude that crystalline PPA is in the enol whereas its sodium salt is in the keto form. The keto-acid was not obtained in the solid state. The colorimetric method for testing PPA traces in urine depends on the formation of a enol-Fe3+ complex (2:1) which appears stable in dimethylsulfoxide (DMSO).