Spectral and Quantum-Chemical Study of Nonequivalence of Methylene Protons in 1,4-Dihydropyridine Derivatives

Structural characteristics of 1,4-dihydropyridines influencing the nonequivalence of the diastereotopic methylene protons in substituents at positions 2 and 6 were studied. The combined effect of magnetically anisotropic groups and intramolecular hydrogen bonds leads to a considerable difference in the chemical shifts of the methylene protons of the AB system. The unusual reversible temperature evolution of these proton signals in monocyclic 1,4-dihydropyridines is associated with two simultaneous conformational processes. It was estimated that the energetically most favorable conformations are stabilized by an intramolecular СН⋯О hydrogen bond.