| Abstract
| - 1H and 13C NMR spectra of 2-acyl-substituted cyclohexane-1,3-diones (acyl = formyl, 1; 2-nitrobenzoyl,2; 2-nitro-4-trifluoromethylbenzoyl, 3) and lithium sodium and potassium salts of 1 have been measured.The compound 3, known as NTBC, is a life-saving medicine applied in tyrosinemia type I. The optimummolecular structures of the investigated objects in solutions have been found using the DFT method withB3LYP functional and 6-31G** and/or 6-311G(2d,p) basis set. The theoretical values of the NMRparameters of the investigated compounds have been calculated using GIAO DFT B3LYP/6-311G(2d,p)method. The theoretical data obtained for compounds 1−3 have been exploited to interpret theirexperimental NMR spectra in terms of the equilibrium between different tautomers. It has been foundthat for these triketones an endo-tautomer prevails. The differences in NMR spectra of the salts of 1 canbe rationalized taking into account the size of the cation and the degree of salt dissociation. It seems thatin DMSO solution the lithium salt exists mainly as an ion pair stabilized by the chelation of a lithiumcation with two oxygen atoms. The activation free energy the of formyl group rotation for this salt hasbeen estimated to be 51.5 kJ/mol. The obtained results suggest that in all the investigated objects, includingthe free enolate ions, all atoms directly bonded to the carbonyl carbons lie near the same plane. Someobservations concerning the chemical shift changes could indicate strong solvation of the anion of 1 bywater molecules. Implications of the results obtained in this work for the inhibition mechanism of (4-hydroxyphenyl) pyruvate dioxygenase by NTBC are commented upon.
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