DFT and structural studies of 2-oxo-1,2,3,4-tetrahydropyridines

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Quantum chemical calculations based on density functional theory (DFT) and X-ray crystal structure analysis of some ethyl 4-aryl-6-methyl-2-oxo-1,2,3,4-tetrahydropyridine-5-carboxylates (THPYs) were applied in order to elucidate the effect of variations of the additional substituent on the C4-aryl ring on the molecular geometry, conformation and packing of molecules in the crystalline lattice. Due to the presence of the C4 stereocenter, these compounds exist as a racemic mixture in the crystal structure. Intermolecular interactions between the potential hydrogen donor (NH) and hydrogen acceptor (2-CO) of the heterocycle in the enantiomeric pair forming a sheet like dimeric structure, which provides especial stability to the crystal packing. The existence of the short intramolecular interactions (CH⋯π) and (CH⋯O) are a key feature which imparts also additional stability to the molecular conformation in the solid state. Most computational data including conformation of the heterocycle, the values of the bond lengths and angles are well in agreement with the experimental data. Ring flip calculations for 4-phenyl substituent explain the extreme torsional strain caused by the eclipsing of the carbon-hydrogen bonds in 3-CH2 moiety with the neighboring C4Caryl and C4H bonds and also the changes of some characteristic bond lengths and angles. These calculations indicate the more favored pseudo-axial orientation of the phenyl group over the pseudo-equatorial orientation.