Molecular structure and conformational analysis of two 2-oxo(thioxo)- 1,2,3,4-tetrahydropyrimidine-5-esters

X-ray crystal structure analysis and quantum chemical calculations based on density functional theory (DFT) were used for structural and electronic characterizations of two 1,2,3,4-tetrahydropyrimidine derivatives (THPMs), namely, ethyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5- carboxylate (1) and methyl 4-(4-bromophenyl)-1,6-dimethyl-2-oxo-1,2,3,4- tetrahydropyrimidine-5-carboxylate (2). The results of these studies revealed that the heterocyclic ring adopts a quasi-boat conformation, in which the 4-aryl group occupies the pseudo-axial position. The occurrence of the C ₄-stereocenter in the heterocyclic ring causes the formation of both R- and S-enantiomers. X-ray diffraction technique indicates that both compounds exist as a racemic mixture in the crystal structure and the enantiomers are orientated to each other via hydrogen bonding between N₃-H as donor and the C₂=S or C₂=O groups as acceptor species, in each layer under formation of an enantio-syndio packing. Most computational bond lengths and angles are well in agreement with experimental data, and support the pseudo-axial orientation of the C₄-aryl substitution.