Intramolecular hydrogen bonds in 1,4-dihydropyridine derivatives

1,4-Dihydropyridine (1,4-DHP) derivatives have been synthesized and characterized by ¹H, ¹³C, ¹⁵N nuclear magnetic resonance (NMR) spectroscopy, secondary proton/deuterium ¹³C isotope shifts, variable temperature ¹H NMR experiments and quantum-chemical calculation. The intramolecular hydrogen bonds NH⋯O=C and CH⋯O=C in these compounds were established by NMR and quantum-chemical studies The downfield shift of the NH proton, accompanied by the upfield shift of the ¹⁵N nuclear magnetic resonance signals, the shift to the higher wavenumbers of the NH stretching vibration in the infrared spectra and the increase of the ¹J(¹⁵N,¹H) values may indicate the shortening of the N-H bond length upon intramolecular NH⋯O=C hydrogen bond formation.