O. V. Shishkin

Conformational flexibility of antiaromatic 1,4 heterocyclic analogues of 1,4-cyclohexadiene

Abstract The molecular structure and conformational flexibility of antiaromatic six-membered 1,4-diheterocycles have been studied using an ab initio quantum-chemical method with a 6-31G basis set. Molecules without the sulfur atom have a planar equilibrium conformation. The bending strain in the presence of S atoms causes a transition to boat conformation. Generally, all rings are more flexible than other 1,4-dihydrocycles because of the anti-aromatic character of cyclic the π-system. An increase in bending strain results in a change in the shape of the energy minimum from flat single to double.

MOLECULAR STRUCTURE AND CONFORMATION FLEXIBILITY OF 2-OXO- AND 2-THIOXO-1,2,3,4-TETRAHYDROPYRIMIDINES AND THEIR DERIVATIVES

The conformational flexibility and effects of the substituents in 2-oxo- and 2-thioxo-1,2,3,4-tetrahydropyrimidines were studied by the semiempirical quantum-chemical AM1 method. The substituents at the double bond have no appreciable effect on the conformational characteristics of the heterocycle. The introduction of substituents to the saturated carbon atom results in the conversion of the tetrahydrocycle to a half-chair conformation and in a substantial decrease in the ring flexibility. The results of calculations are confirmed by the X-ray study of both compounds.

Conformational flexibility of the 1,4-dihydropyridine ring in calcium channel agonists and antagonists molecules

Abstract Conformational flexibility of the 1,4-dihydropyridine ring in 4-aryl derivatives of the 2,6-dimethyl-3,5-dicarbmethoxy-1,4-dihydropyridine has been studied using the MM3 molecular mechanics method. Change of the CCCC endocyclic torsion angle of dihydrocycle in the range of 35° entails an increase of energy less than 1 kcal mol −1 . The energy levels below 99% of the molecular population for each molecule were estimated. The interval of the torsion angle change for these regions is about 60°.

Molecular, electronic structure and conformational flexibility of the 1,6-dihydropyrimidine, 4,5-dihydrofuro[2,3-d]pyrimidine and their oxo, imino and methylene derivatives

Abstract Molecular and electronic structures of 1,6-dihydropyrimidine, 4,5-dihydrofuro[2,3-d]pyrimidine and their oxo, imino and methylene derivatives have been studied using the semiempirical quantum-chemical method AM1. The equilibrium geometry of all molecules is planar. These results were confirmed by X-ray structural investigations of the 1-oxo- and 1-imino derivatives of 2,5-dimethyl-3-(2,2,2-trichloroethyl)-4,5-dihydrofuro[2,3-d]pyrimidine. However, the dihydropyrimidine ring in all compounds possesses a degree of conformational flexibility. The transition from a planar equilibrium conformation to a distorted sofa with the CNCC torsion angle ± 20° causes an increase of energy less than 1.5 kcal mol −1 . The factors that determine this property of the dihydroheterocycle (bending strain, non-aromaticity, conjugation, 1,2-allylic strain) have been considered.

5-Amino-3-trifluoromethyl-1H-1,2,4-triazole

The bond lengths in the five-membered ring of the title compound, C 3 H 3 F 3 N 4 , (1), are equal within three standard deviations to those in 5-amino-3-nitro-1H-1,2,4-triazole. The amino group in (1) has a trigonalpyramidal configuration. Molecules of (1) form layers in the ab plane via intermolecular hydrogen bonds of the type N-H...N.

REACTION OF 2-POLYFLUOROACYLCYCLOHEXANONES WITH SEMICARBAZIDES

Abstract2-Polyfluoroacylcyclohexanones react with semicarbazide and thiosemicarbazide to give 2-(thio)carbamoyl-3-hydroxy-3-polyfluoroalkyl-3,3a,4,5,6,7-hexahydro-2H-indazoles, one of which was studied by X-ray diffraction.

Synthesis and molecular structure of 5-(1-aminoisoindolenin-3-ylidene-amino)-1,3,4-thiadiazole-2(3H)-thione

The structure of 5-(1-aminoisoindolenin-3-ylideneamino)-1,3,4-thiadiazole-2(3H)-thione was studied by X-ray diffraction analysis and UV and IR spectroscopy. In the crystal, the molecule of the title compound is planar in spite of the substantially shortened intramolecular N…S contact. The factors that determine the structure of the heterocycle, namely, the nature of the N…S interaction, conjugation effects, and the tautomerism, were analyzed based on quantum-chemical calculations performed by the AM1 method.

The N-phenylamide of 2-cyano-5-phenylpent-2-en-4-ynoic acid

The central 2-cyanocarboxyaminoprop-2-enylic fragrent of the title compound, 2-cyano-5,N-diphenylpent 2-en-4-ynamide, C 18 H 12 N 2 O, is planar. The phenyl rings are rotated with respect to this plane. Molecules are linked by N-H…O hydrogen bonds and form stacks by translation along the crystallographic c axis.

Conformational flexibility of partially hydrogenated rings in the repeating units of rigid-chain heterocyclic polymers

The conformational flexibility of partially hydrogenated rings in naphthylimide, naphthalic anhydride, and some of their derivatives used as model compounds for the repeating unit in rigid-chain heterocyclic polymers, was studied by the semiempirical quantum-chemical AM1 method. The partially hydrogenated rings in all of these compounds possess conformational flexibility. Transition from a planar equilibrium form to a distorted sofa conformation with a torsion angle of ±20° results in an increase in the energy of less than 1 kcal mol−1. The high flexibility of partially hydrogenated rings, along with other factors, can cause finite length of the Kuhn segment in rigid-chain heterocyclic polymers.

Molecular structure and substituent electronic effects in 10-arylidenanthrones-9

X-ray structural studies of derivatives of 10-arylidenanthrone-9 containing π-electron donating and withdrawing substituents within arylidene fragment have been carried out. In the crystal the molecules are strongly disflattened with the central dihydrocycles adopting an asymmetric boat conformation. This geometry of the molecules is a result of unfavourable non-valence interactions between atoms at the C=C bond and the hydrogens at the peri-positions of the anthraquinone fragment. Calculations performed by the semiempirical quantum-chemical AM1 method demonstrate that in the gas phase equilibrium geometry of the molecules was similar to that in the crystal. It was shown as well that the central dihydrocycle possesses high conformational flexibility in spite of steric overcrowding in all of the compounds. Electronic effects of the substituents had significant influence only on the arylidene moiety, resulting in different rotation angles of the phenyl group around the C=C exocyclic bond and virtually no effect on the anthraquinone part of the molecules.