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

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 C-4-aryl ring on the molecular geometry, conformation and packing of molecules in the crystalline lattice. Due to the presence of the C-4 stereocenter, these compounds exist as a racemic mixture in the crystal structure. Intermolecular interactions between the potential hydrogen donor (N-H) 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 (C-H center dot center dot center dot pi) and (C-H center dot center dot center dot 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 C-4-C-aryl and C-4-H 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. (C) 2016 Elsevier B.V. All rights reserved.