Crystallographic and NMR evidence for the enol tautomer of 3-methoxy-4-hydroxy-5-chloro-phenylpyruvic acid

Temperature-variable NMR Study of the keto-enol Tautomerism of Phenylpyruvic Acid

“…Many spectroscopic studies, ultra violet (UV), infrared (IR), Raman and nuclear magnetic resonance (NMR) studies, have been reported on the keto-enol tautomerism [3][4][5][6][7][8][9][10][11][12] and conformation 6,9,10,12,13 of PA and PPA. These many investigations have revealed that these compounds exist as equilibrium mixtures consisting of the keto and enol forms.…”

“…Carpy et al assigned the structures of the two tautomeric forms of PPA in DMSO solvent by 1 H and 13 C NMR spectroscopy, and found by X-ray crystallography of the single crystal that the crystalline form takes the keto isomer. 11,12 Moreover, the substituent effect on the tautomeric equilibrium was studied by using ab initio calculations in the gas phase, and the thermochemical parameters were obtained. They confirmed a large preference for the enol isomer relative to the keto isomer for PPA and o-chloro PPA.…”