Intramolecular hydrogen bonds in 1,4-dihydropyridine derivatives

1,4-Dihydropyridine (1,4-DHP) derivatives have been synthesized and characterized by H-1,C- 13, N-15 nuclear magnetic resonance (NMR) spectroscopy, secondary proton/deuterium C-13 isotope shifts, variable temperature H-1 NMR experiments and quantum-chemical calculation. The intramolecular hydrogen bonds NH center dot center dot center dot O=C and CH center dot center dot center dot O=C in these compounds were established by NMR and quantumchemical studies The downfield shift of the NH proton, accompanied by the upfield shift of the N-15 nuclear magnetic resonance signals, the shift to the higher wavenumbers of the NH stretching vibration in the infrared spectra and the increase of the (1)J(N-15,(1)<^>H) values may indicate the shortening of the N-H bond length upon intramolecular N center dot center dot center dot O0=C hydrogen bond formation.