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

The structures and conformation of 3-methoxy-4-hydroxy-5-chloro-phenylpyruvic acid3 and its acetate ester4 have been investigated by X-ray crystallography and by spectroscopic methods. Three independent molecules crystallized for the ester4 with the triclinic symmetry and the centrosymmetric space group (Z=6) and with the cell dimensionsa=11.764(6),b=13.549(5),c=13.978(15) A. α=100.08(7), β=94.62(8), γ=115.06(4)°. The three conformers of4 exist as the enolate tautomers and not the keto form.Trans extended side pyruvic acid side chains were observed for the three conformers. The crystalline cohesion was ensured by a strong network of hydrogen bonds. The NMR spectra of the phenylpyruvic acid3 and the acetate ester4 exhibited each a single proton on the benzylic carbon atom and chemical shift values supporting the existence of the enolate tautomer form for these phenylpyruvic acid compounds in solution.