A. Yu. Kovalevsky

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility.

“Anhydrolycaconitine”, a new diterpene alkaloid fromAconitum septentrionale K

A new alkaloid, anhydrolycaconitine (C36H46N2O9), was isolated from roots ofAconitum septentrionale K. Based on the results of1H and13C NMR and IR spectroscopy and mass spectrometry of the alkaloid and the product of its alkaline hydrolysis and on the data of X-ray diffraction analysis of the hydrolysis product, the structure of 1α, 6β, 14α, 16β-tetramethoxy-7-oxo-18-succinylanthranoyloxy-17-(7→8)abeo-aconane was assigned to anhydrolycaconitine.

Molecular structure and conformational dynamics of 3,12-dichloro-5,14-diphenyl-7,9a,16,18a-tetrazadibenzo[a,l]pentacene-9, 18-dione

The X-ray study of 3,12-dichloro-5,14-diphenyl-7,9a,16,18a-tetrazadibenzo[a,l]pentacene-9,18-dione (1), a model compound for a novel class of thermostable polyheteroarylenes, polyquinoquinazolones, has been carried out. The nonsymmetric flattened structure of the molecule observed is a result of intermolecular interactions. It was established using quantum-chemical calculations by the semiempirical AM1 method that the annelation of the 1,6-dihydropyrimidinone ring by aromatic cycles results in increasing the conformational flexibility of the dihydrocycle due to weakening the conjugation between the carbonyl group and the remaining π-system of the molecule. It was shown by X-ray study and quantum-chemical calculations that protonation of1 results in a substantial change in the molecular structure due to the large contribution of the 1,4-dihydro tautomeric form to the structure of the 1,6-dihydropyrimidinone ring. A tendency for the conformational flexibility of the heterocycle to increase upon protonation was revealed.

6a,7-Dihydro-2,6-dimethyl-7-phenyl-6H-chromeno[4,3-d]-1,2,4-triazolo[1,5-a]pyrimidin-6a-ol

In the title compound, C 20 H 18 N 4 O 2 , the dihydropyrimidine and dihydropyran rings adopt a distorted sofa conformation. The phenyl substituent at C7 and the hydroxy group at C6a adopt axial positions, whereas the methyl group at C6 is equatorial.

Effect of benzannelation on the conformational flexibility of the rings in oxo, imino, and methylene derivatives of cyclohexa-1,4-diene

The effect of benzannelation on the equilibrium conformation and flexibility of the dihydrocycle in cyclohexa-1,4-dienone,para-quinone, and their imino and methylene analogs was studied by the semiempirical quantum-chemical AM1 method. The equilibrium conformations of the carbonyl derivatives are planar. In the imino- and methylene-substituted analogs, the dihydrocycle adopts a boat conformation due to repulsions between substituted analogs, the dihydrocycle adopts a boat conformation due to repulsions between the hydrogen atoms at the exocyclic double bond and in theperi positions of the benzene rings. Annelation of cyclohexa-2,5-dien-1-one andpara-quinone with benzene rings at the C=C double bonds causes an increase in the conformational flexibility of the partially hydrogenated ring owing to an increase in the bending strain in the first compound and a decrease in the conjugation between the carbonyl groups and the remaining part of the molecule in the second compound.

Redetermination of N-acetylanthranilic acid at 158K

A redetermination of the structure of N-acetylanthranylic acid has been carried out at 158 K. The molecule is not planar, with N1 and C8 deviating from the plane of benzene. Molecules form chains in which they are connected by intermolecular O—H···O hydrogen bonds of between carboxyl OH and acetamide C═O groups. These chains run parallel to [103] and [103].

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.

N,N'-Bis(3-methoxyphenyl)-1,8:4,5-naphthalenetetracarboximide–trifluoroacetic acid–phenol (1/2.5/0.5)

In the title compound, C 28 H 18 N 2 O 6 .2.5C 2 HF 3 O 2 .0.5C 6 H 6 O, the naphthalenetetracarboxylic diimide moiety lies about an inversion centre and is planar, but conjugation between the phenyl substituent and the tetracyclic fragment is disrupted due to rotation around C-N bonds.

MOLECULAR STRUCTURES AND REACTIVITIES OF 1,3-DIPHTHALIMIDOBENZENE DERIVATIVES

X-ray diffraction analysis of 4,6-di(morpholin-4-yl) and 4,6-di(2,2,2-trifluoroethoxy) derivatives of 1,3-diphthalimidobenzene, which are potent reagents in the synthesis of heterocyclic polymers, was carried out. The conjugation between the π-systems of the benzene ring and of the phthalimide and morpholine substituents is distorted due to the rotation of the substituents about the C(Ar)−N bonds. The AMI calculations demonstrated that the hydrogen atoms of the methylene groups are “acidic”, which is favorable for condensation reactions. Steric hindrances to intramolecular condensation were estimated.

Molecular structures of amino and nitro derivatives of 1,3-di(morpholin-4-yl)benzene

The 4,6-diamino-1,3-di(morpholin-4-yl)- and 5-amino-2,4-di(morpholin-4-yl)-1-nitro derivatives of benzene, which serve as model compounds in studies of spatial structures and reactivities of monomer units of many polymers of practical importance, have been studied by X-ray diffraction analysis. The morpholine rings are twisted so that they are virtually perpendicular to the benzene ring. According to the results of AM1 calculations, the hydrogen atoms of the methylene groups of these substituents carry noticeable positive charges. The introduction of different substituents into the benzene ring has no substantial effect on the charge distribution in the morpholine rings due to the absence of conjugation between the lone electron pairs of the nitrogen atoms of the ring and the π-system of the benzene ring.