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

Article abstract of DOI:10.1007/BF01668227

The structures and conformation of 3-methoxy-4-hydroxy-5-chloro-phenylpyruvic acid 3 and its acetate ester 4 have been investigated by X-ray crystallography and by spectroscopic methods.Three independent molecules crystallized for the ester 4 with the triclinic symmetry and the centrosymmetric space group (Z = 6) and with the cell dimensions a = 11.764(6), b = 13.549(5), c = 13.978(15) Angstroem, α = 100.08(7), β = 94.62(8), γ = 115.06(4) deg.The three conformers of 4 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 bu a strong network of hydrogen bonds.The NMR spectra of the phenylpyruvic acid 3 and the acetate ester 4 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.KEY WORDS: NMR, enol tautomer, phenylpyruvic acid.