Conformational analysis of 3-acetylamino-5,6-dihydrouracils by DFT computations

3-Acetylamino-6-aryl-5,6-dihydrouracils were found to exist in DMSO-d₆ solution as mixtures of four rotamers with an overwhelming predominance of two of them (96 mol%), caused by hindered rotation around the N-N bond (two major rotamers) and around the adjacent amide C-N bond (two minor rotamers). To explain these NMR spectroscopic data, thermodynamic and kinetic parameters for interconversion of all the rotamers of 3-acetylamino-5,6-dihydrouracil as a model compound were determined using the DFT B3LYP/6-311++G(d,p) method in the gas phase and DMSO solution. The calculations showed that the s-cis isomers relative to the amide C-N bond are significantly more stable than the corresponding s-trans isomers. Rotations around N-N bond in both the s-cis and s-trans isomers give two atropisomers in which the acetylamino group and the C(2)-N(3)-C(4) fragment are practically orthogonal. The interconversion between these isomers are characterized by fairly high energy barriers. The rather similar results were obtained from the DFT calculations performed for 3-acetylamino-6-(4-methylphenyl)-5,6-dihydrouracil in DMSO solution. Thus, the rotation around the N-N bond of the s-cis conformers of this compound gives two energetic minima [(6R*,S_a*)- and (6R*,R_a*)-diastereomers] and two transition states. The energy barriers between these diastereomers are in the range of ΔG = 17.65–19.17 kcal/mol (298 K, 1 atm) which are in good agreement with the value (ΔG^≠ = 19.6 kcal/mol) of hindered rotation around the N-N bond for the two major rotamers determined using ¹H NMR spectroscopic data at different temperatures.