Conformational studies on substituted purines

CNDO/2 calculations are performed on 9-methyl-8-phenyl-6-thiopurine in order to determine the molecular equilibrium conformation. The two methyl hydrogens are found to be staggered to the ortho hydrogen on the phenyl ring, and the dihedral angle between the phenyl group and the purine system was found to be equal to 30°, in good agreement with experimental data. The results are discussed in relation to other conformational analyses performed with the CNDO/2 method.     

Conformational studies of haloperidol

The high field¹H,¹⁹F, and¹³C NMR spectra of haloperidol (HP) in CDCl₃ solution were recorded and analyzed with the aid of both homonuclear (H, H-COSY, H, H-COSYLR) and heteronuclear (H,C-COSY) chemical shift correlation experiments. Through space interactions between (1) the piperidine ring and the chlorinated phenyl ring and (2) the fluorinated phenyl ring and a methylene group were identified using phase sensitive Nuclear Overhauser and Exchange Spectroscopy (NOESY). The NOESY results are consistent with a planar structure rather than a folded version of haloperidol.     

Conformational studies on chloramphenicol and related molecules

A conformational analysis has been performed on chloramphenicol and related molecules by potential energy functions, NMR and IR spectroscopy. Specific solvent effects have been considered via calculation in the case of chloramphenicol. The results establish that essentially only one conformer of chloramphenicol exists in solution with respect to the C_αC_β bond, but that this stabilization is not due to hydrogen bonding as proposed in a previous study. The importance of stabilization of polar solute conformations in polar media is also discussed.     

Conformational studies on (+)-anatoxin-a and derivatives

Summary Anatoxin-a (AnTX) is a highly potent agonist acting at the nicotinic acetylcholine receptor (nAChR) and represents a valuable tool in the study of this receptor. Molecular mechanics, semi-empirical and ab initio molecular orbital energy minimization procedures were conducted to investigate the conformation of AnTX. For each minimization procedure, the s-trans enone isomer of protonated AnTX was the energetically favoured conformer due to intramolecular electrostatic interactions. Our studies are discussed in the light of previous experimental observations and conformational studies, in addition to their importance in the development of future pharmacophore models for nAChR agonist binding.     

Conformational analysis of some ortho-hydroxyazoic dyes by the PCILO method

The quantum mechanical MO method PCILO is used to perform a detailed conformational analysis of ortho-hydroxyazobenzene and 1-phenyl azo 2-napthol. Several energy minima are obtained for each compound, after a simultaneous optimization of the main geometric parameters. Comparison is made with the corresponding para compounds. The calculated results are discussed in relation to the available experimental data.     

Conformational analysis of polymethine dyes derived from the 2-azaazulene

A systematic investigation of the conformational structure was performed for the series of symmetrical and unsymmetrical mono-, tri-, pentamethine cyanines, and styryl dyes bearing 2-azaazulenium terminal group. The rotation energy barriers of terminal groups were determined via the dynamic variable temperature NMR experiments. The conformational transformation energy was calculated by quantum chemical methods (B3LYP and M05-2X) both for the cases of considering the solvent influence and not tacking it into account. Based on the comparison of theoretical and experimental data, relative electron-donating abilities and geometrical features of the heterocyclic terminal groups in 2-azaazulenium dyes were estimated. The arrangement of certain heterocyclic nuclei in order of basicity by considering the results of the dynamic NMR investigations was proposed. Influence of the conjugated chain length and the solvent nature on the conformational lability of the investigated dye molecules was discussed.     

Conformational studies of tertiary oligo-m-benzanilides and oligo-p-benzanilides in solution

A series of oligo-m- and p-benzanilides were made and their conformations in solution were studied by NMR. In most cases, conformational mixtures were observed as soon as three or more monomers were incorporated into the oligomer. Some crystal structures were obtained, which indicated that helical conformations were adopted in the solid state.     

Aminoarsolanes—II. Conformational studies

¹H and ¹³C NMR spectra of six dialkylaminoarsolanes were obtained as a function of solvent, concentration, and temperature. CI mass spectral data were also obtained as a function of retention time for the same compounds. These results suggest that the AsN bond is not labile as compared to the AsCl bond in chloroarsolanes. No evidence for dimer formation was noted either in solution or the gas phase under the investigated experimental conditions. The averaged solution conformation of the 1,3-dioxarsolane ring is discussed in terms of rapid intramolecular conformational averaging; nature of substituents at the 2,4, and 5 positions; and effects of temperature, solute concentration and solvent type.     

Conformational studies utilizing asymmetric torsional frequencies

The utility of asymmetric torsional frequencies, obtained from low frequency infrared and/or Raman spectra of the gas phase, for conformational studies is presented. Vibrational spectroscopy may well be the most generally applicable method to be used in the study of the conformations of certain types of small molecules with few substituents because infrared and Raman spectra can be recorded in all phases, and variable temperature experiments can also be conducted. Additionally, gas phase band contours in both the infrared and Raman spectra, along with Raman depolarization data, provide considerable structural information and lend confidence to conclusions arrived at from vibrational studies. The theory and experimental difficulties involved in the determination of potential functions governing internal rotation is presented, and illustrated by a few examples.     

Conformational studies on heteroleptic trifluoromethyl‐substituted phenylboranes

Organoboranes carrying electron‐withdrawing substituents are commonly used as Lewis acidic catalysts or cocatalysts in a variety of organic processes. These Lewis acids also became popular through their application in `frustrated Lewis pairs', i.e. combinations of Lewis acids and bases that are unable to fully neutralize each other due to steric or electronic effects. We have determined the crystal and molecular structures of four heteroleptic arylboranes carrying 2‐(trifluoromethyl)phenyl, 2,6‐bis(trifluoromethyl)phenyl, 3,5‐bis(trifluoromethyl)phenyl or mesityl substituents. [3,5‐Bis(trifluoromethyl)phenyl]bis[2‐(trifluoromethyl)phenyl]borane, C₂₂H₁₁BF₁₂, (I), crystallizes with two molecules in the asymmetric unit which show very similar geometric parameters. In one of the two molecules, both trifluoromethyl groups of the 3,5‐bis(trifluoromethyl)phenyl substituent are disordered over two positions. In [3,5‐bis(trifluoromethyl)phenyl]bis[2,6‐bis(trifluoromethyl)phenyl]borane, C₂₄H₉BF₁₈, (II), only one of the two meta‐trifluoromethyl groups is disordered. In [2,6‐bis(trifluoromethyl)phenyl]bis[3,5‐bis(trifluoromethyl)phenyl]borane, C₂₄H₉BF₁₈, (III), both meta‐trifluoromethyl groups of only one 3,5‐bis(trifluoromethyl)phenyl ring are disordered. [3,5‐Bis(trifluoromethyl)phenyl]dimesitylborane, C₂₆H₂₅BF₆, (IV), carries only one meta‐trifluoromethyl‐substituted phenyl ring, with one of the two trifluoromethyl groups disordered over two positions. In addition to compounds (I)–(IV), the structure of bis[2,6‐bis(trifluoromethyl)phenyl]fluoroborane, C₁₆H₆BF₁₃, (V), is presented. None of the ortho‐trifluoromethyl groups is disordered in any of the five compounds. In all the structures, the boron centre is in a trigonal planar coordination. Nevertheless, the bond angles around this atom vary according to the bulkiness and mutual repulsion of the substituents of the phenyl rings. Also, the ortho‐trifluoromethyl‐substituted phenyl rings usually show longer B—C bonds and tend to be tilted out of the BC₃ plane by a higher degree than the phenyl rings carrying ortho H atoms. A comparison with related structures corroborates the conclusions regarding the geometric parameters of the boron centre drawn from the five structures in this paper. On the other hand, CF₃ groups in meta positions do not seem to have a marked effect on the geometry involving the boron centre. Furthermore, it has been observed for the structures reported here and those reported previously that for CF₃ groups in ortho positions of the aromatic ring, disorder of the F atoms is less probable than for CF₃ groups in meta or para positions of the ring.     

Spectroscopic studies of fluorescent perylene dyes

The lowest electronic transition of the fluorescent perylene dye bis-(3,5-di-tertbutylphenyl)-perylene-3, 4:9,10-biscarboximide has been investigated b     

Spectrophotometric studies on some 2-styryl benothiazole cyanine dyes

The ultraviolet and visible absorption-spectra of some 2-styryl benzothiazole ethiodides have been studied in ethanol and different organic solvents. The bands observed are assigned to different electronic transitions liable to exist. The effect of substituents on the band position and intensity is discussed. The pKa values of the hydroxy derivatives have been determined.     

Conformational studies of some carbocyclic nucleoside analogues

Proton NMR spectra of some carbocyclic nucleoside analogs of tuberculin have been analyzed at 100 MHz in DMSO-d₆. The spectral characteristics and nuclear Overhauser effects indicate a preferential syn conformation about the glycosidic bond when a hydroxyl group, oriented toward the base, is available for intramolecular hydrogen bond formation.     

Conformational studies on 2-guanidinylthiazole, famotidine and some analogues

Conformational analysis of famotidine (FAMO) and some analogues have been performed using AM1 calculations. In addition, Conformational analysis were done on the 2-guanidinylthiazole moiety in order to see the effect of the N-sulfamoyl fragment and the methylthioethyl chain of FAMO on the thiazole ring. The results revealed that the N6H form (the guanidinium cation) was the most stable and might therefore be the best candidate for interacting with the histamine H2-receptor. The calculations for the N6H forms of FAMO and analogues showed a strong hydrogen bond anchoring the guanidine chain in the same plane as the thiazole ring, in agreement with X-ray diffraction and ¹H NMR studies.     

Conformational studies of 2-anilino-substituted 1,3,2-oxazaphospholanes

A series of 2-anilino-2-thio-1,3,2-oxazaphospholanes derived from ephedrine has been synthesized and conformationally studied by proton NMR and X-ray crystallography. The NMR data can be interpreted in terms of twist-envelope conformations in which the anilino substituents on phosphorus adopt predominantly equatorial positions. X-ray crystal structures of (2R,4S,5S)-2-anilino-2-thio-3,4-dimethyl-5-phenyl-1,3-2-oxazaphospholane, (2R,4S,5S)-2-(4-fluoroanilino)-2-thio-3,4-dimethyl-5-phenyl-1,3,2-oxazaphospholane, and (2R,4S,5S)-2-(4-methoxyanilino)-2-thio-3,4-dimethyl-5-phenyl-1,3,2-oxazaphosholane have been carried out, and these compounds adopt envelop, twist-envelope, and twist-envelope conformations, respectively, with the anilino moieties equatorial.     

Chemistry of leucomycin-X: Conformational studies on leucomycin

The conformation of leucomycin, a macrolide antibiotic with a 16-membered ring lactone, was examined in solution by IR, NMR and CD spectral analysis. The IR reveals that the five hydroxyls are all involved in intramolecular H-bonding. The NMR of the 16-membered ring lactone forming the aglycone shows that the acetyl-carbonyl at C-3, the allylic proton at C-11 and the aldehyde proton are in close proximity on the lactone ring. CD studies suggest that the conformation of the 16-membered ring lactone, especially around the lactone, is mobile and solvent dependent.