A study of the tautomerism of β-dicarbonyl compounds with special emphasis on curcuminoids

Six β-diketones related to curcumin and curcumin itself have been studied by 13C NMR spectroscopy in chloroform in order to determine the equilibrium constant between the two keto/enol tautomers. In order to do this GIAO/B3LYP/6-31G∗∗ calculations of absolute shieldings (σ, ppm) were carried out. To establish relationships between σ and experimental chemical shifts (δ, ppm), three simple β-diketones (acetylacetone, dibenzoylmethane and benzoylacetone) have been studied. The preference for different groups to be conjugated with the Cdouble bond; length as m-dashO has been determined.