Hydrogen's isotopic exchange reaction in the C-methyl sides in the medicinal agent xymedon: NMR spectroscopy and ab initio calculations

The kinetics of intramolecular and intermolecular exchange processes in xymedon (1-(2-hydroxyethyl)-4,6-dimethyl-1,2-dihydropyrimidin-2-one, a regeneratory, wound-healing drug) and its analogue were investigated in the solution. Hydrogen's mobility was detected in the C-methyl sides of these compounds. This mobility was monitored via NMR in the hydrogen/deuterium exchange reaction in water. Two models were proposed as explanations for this hydrogen-deuterium exchange. According to the main model, the key intermediates of these reactions are low-energy tautomers of xymedon in which the N-3 is protonated following which one proton leaves either 6-Me or 4-Me and thus its hybridization is changed. This hydrogen-to-deuterium exchange reaction is much faster under acidic conditions although it also occurs in alkaline conditions. Methylation via MeOTs or MeI leads to products with a quaternized ring N-3 atom in which a hydrogen-to-deuterium exchange reaction also takes place, although the rates of the 6-Me and 4-Me hydrogens exchange are reversed. According to density functional theory calculations, the presence of methyl groups at the C-4/C-6 positions and of the CO fragment is crucial to remarkably lower the energies of these rare tautomers. The exact position of the CO in heterocycle is also very important in the tautomers' relative stability.