Search

The structure and the conformation of the two isomeric aryl rings which exhibit a face to face disposition at the perhydrocyclopentaquinolizine moiety are reported. The studies provide insight into the corresponding rotation barriers as well as chair to boat and nitrogen inversion processes of the quinolizine rings.

The stereolabile atropisomerism arising from restricted rotation about the [sp.sup.2]-[sp.sup.3] bond in a number of hindered aryl carbinols and ethers analyzed by dynamic NMR spectroscopy is presented.

The methylbenzyl ethers, bearing substituents of different bulkiness is investigated. The dynamics of the enantiomerization processes occuring in the synclinical (sc-) and syn-periplanar (sp-) atropisomers is followed by low-temperature NMR and the corresponding barriers determined.

A detailed study of the statin and dynamic situation occurring in a compound having four o-tolyl substituents bonded to a benzene ring in the 1,2,4,5 or 1,2,4,5-tetra (o-tolyl)benzene is described. Variable temperature dynamic NMR experiments allowed the barrier for the interconversion of the atropisomers to be determined.

The stereomutations in nonsymmetrical salophen ligands were studied by means of dynamic NMR and HPCL methods. The values compare well with those determined by chromatography on an enantioselective HPLC column at low temperatures, thus confirming that DNMR and DHPLC can be conveniently employed as complementary techniques.

The structural, conformational and stereodynamic properties of a class of carbinols having substituents of different dimensions, so that either stereolabile or configurationally stable atropisomers can be obtained, are reported. It is indicated that the sc-atropisomer is the more stable form when R=Me, i-Pr, t-Bu but is less stable one when R=Et.

Research has been conducted on tetraethylmethane and analogous C(CH (sub)2 X) (sub)4 compounds. The variable-temperature NMR studies of these compounds have revealed their dynamic behavior.

Research has been conducted on the 1,3,5-tris(4-methylnaphth-1)benzene. The results of the low temperature (super)1 H NMR spectral analysis in solution indicate the presence of the rotational conformer with the naphthyl substituents in a syn relationship.

Research has been conducted on the O-alkyl benzyl aryl oximes with the halogen atoms in the ortho position. The hypothesis that the corresponding conformational antipodes may interconvert to allow their physical separation has been investigated via the dynamic NMR study and the results are presented.

Research has been conducted on amino alcohols, useful intermediates in the natural product synthesis. The conformational preferences and stereodynamics of dimethylamino-2,4,4-trimethyl-3-phenyl-3-pentanol and corresponding amide precursor have been investigated and the results of the study are presented.

Research has been conducted on the bis(1-naphthyl) derivative. The synthesis of this derivative and two cognates has been performed via the Suzuki coupling reaction and the results of the study are presented.

Research has been conducted on the unhindered aryl ketones. The synthesis of the aryl alkyl ketones where the tert-butyl substituent severely hinders the Ar-CO rotation process has been carried out and investigated and the results are discussed.

Research has been conducted on the N-aryl-tetrahydropyrimidines. The preparation of these tetrahydropyrimidines has been carried out and their stereochemical properties have been investigated.

Research has been conducted on the dimesityl derivatives of ethanol and of different ethers. The results of the NMR spectroscopy analysis of these derivatives demonstrate that these molecules exist as M and P propeller-like stereolabile enantiomers.

Research has been conducted on the electron-rich aromatic compounds. The results of the stereoselective and straightforward synthesis of aminoalkylnaphthols are discussed.

Research has been conducted on the dimesityl sulfoxide and sulfone. The results of the molecular mechanics calculations demonstrate that the process of enantiomerization of these compounds is due to the correlated rotation entailing a one-ring flip pathway.

Results reveal that dimesityl sulfine in solution adopts a chiral conformation with a C(sub)1 symmetry analogous to the crystal form. The isomeric Z and E mesitylphenyl sulfines, on the other hand, adopt an achiral C(sub)s symmetry in solution.

Results show that in the case of 1,1-dimesitylketone the two helical enantiomers interconvert through a one-ring flip pathway, while it occurs through a two-ring flip pathway in 1,1-dimesitylethylene as determined by dynamic nuclear magnetic resonance spectroscopy. The dimesitylthioketone exhibits an intermediary pathway.

Nuclear magnetic resonance spectroscopic data reveal that naphthalenes, exhibiting an acyl and a phenyl group, yield enantiomers, which show slow rotations of the acyl group but not the phenyl group as detected at different temperature settings.

The barriers for the stereomutation processes are much higher in crystals than those in solution of triisopropyl(aryl)silanes. The derivatives of triisopropyl(phenyl)silane and triisopropyl-(2-naphthyl)silane exhibit a single, symmetric conformation with aromatic rings rotating rapidly with rotation barriers measuring 5.5 kcal and 13.0 kcal per mole for solution and solid state, respectively.

Research has been conducted on the homochiral dicarvone compound and its existence in two helical forms in methylene chloride. The results of the NMR experiments are discussed.

Derivatives of 1,4-diacylnaphthalenes were examined to determine the relationship between the dimension of the carbonyl-bonded alkyl groups and the interconversion barriers. Structural assignment was made possible by observing the conformer ratio at equilibrium with respect to the solvent polarity. For the bulky tert-butyl derivative, both isomers were isolated at low temperature and, by using X-ray data, the solid-state assignment was transferred to solution nuclear magnetic resonance signals under nonequilibrium conditions.

Several methyl or ethyl ethers derived from 1-naphthyl carbinols were synthesized and investigated using variable temperature nuclear magnetic resonance spectroscopy. The sp and ap atropisomers of the carbinol ArC(But)2OH (Ar=1-naphthyl) yielded atropisomers corresponding to a particular ether when alkylated with methyl iodide. M and P conformational enantiomers were found to exist in the atropisomers of ethers of 1-naphthylcarbinols.

Dynamic NMR spectroscopy was used to investigate several 1,2-diacylbenzenes, and a 1,2-formyl derivative 1 was shown to exist in the nearly coplanar ZE and EE conformations. A more hindered 1,2-diisobutanoylbenzene (4) was seen to adopt a twisted conformation which could relate to a meso or a reacemic stereolabile structure. The interconversion barrier between the RR and SS conformational enantiomers was determined.

Low-temperature nuclear magnetic resonance (NMR) spectroscopy was utilized to characterize the molecular conformation of ArRR'COH (Ar = 1-naphthylyl-2-methyl) naphthyldialkylmethanols. Analysis of the low-temperature NMR spectra of ArRR'COH (Ar = 1-naphthylyl) atropoisomers indicated the presence of similar conformations and a configurationally stable chiral center. On the other hand, ArRR'COH (Ar = 1-napthylyl-2-methyl) atropoisomers exhibited different conformations due to restricted rotation along the Pri-COH stereogenic axis.

The carbon-carbon rotational barrier about the Ar-CHS bond and carbon-nitrogen rotational barrier near the Ar-NH2 bond were analyzed in furan and thiophene omicron-amino thioaldehydes. Nuclear magnetic resonance analysis of thioaldehydes in DMSO indicated the presence of rotational conformers. The minor rotamer exhibited a S,NH2-anti structure while the major rotamer exhibited a S,NH2-syn structure. Furthermore, the delocalization of the amino nitrogen lone pair occurred to a larger extent with the thiophene derivatives of omicron-amino thioaldehydes.

The barriers for interconverting the conformational enantiomers (stereolabile atropisomers) of pyridine substituted adamantane derivatives are determined by dynamic (super 13)C NMR spectroscopy. It is possible to achieve a physical separation of a pair of these stereolabile atropisomers at -60 degreeC by means of the enantioselective cryogenic HPLC technique.

Research elucidates the ring inversion barrier in ketones by nuclear magnetic resonance spectroscopic structural analysis of bicyclo[3.3.1]nonan-9-one. Data indicate that the compound exhibits three conformations similar to bicyclo[3.3.1]nonane.

The thiocarbonyl derivatives of ferrocenes yield stereolabile enantiomers as indicated by changes in the signal multiplicity of the nuclear magnetic resonance solution spectra.

A study was conducted to analyze the ring inversion of the silane analogue of dodecamethylcyclohexane, dodecamethylcylcohexasilane (Me2Si)6. The high field 13C spectra of (Me2Si)6 was monitored at low temperatures. The high resolution 13C and 29Si nuclear magnetic resonance spectra were determined at 75.5 and 59.6 MHz. Results indicated that (Me2Si6) may undergo a rotational motion which would make equivalent three Si atoms and Si-bonded pairs of carbons.

Coumarins substituted by an aryl group in position 4 have exhibited restricted rotation about the Ar-C4 bond, giving rise to conformational or configurational enantiomers when such a restricted motion has led to [C.sub.1] symmetry. The dynamics of the stereomutation processes of the axial enantiomers is monitored by dynamic NMR, which has depended on the activation energies involved.

Variable- temperature NMR spectroscopy, X-ray diffraction, and DFT calculations are employed to study the aromatic diimino derivatives, bearing two N=C[Ph.sub.2] moieties bonded at the 1,8 positions of the anthraquinone, anthracene, biphenylene, and naphthalene rings. The findings offer insight into imino substituents which are essentially orthogonal to the aromatic plane and the diastereotopic phenyl groups bonded to the C=N carbons and also suggested that the barriers for the rotation of the cis are larger compared to the trans phenyl groups.

Nuclear magnetic resonance (NMR), absorption, and emission spectroscopy as well as electrochemical techniques are employed to study the complex formation between dendritic guests and a molecular tweezer host. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of N[Bu.sub.4]P[F.sub.6] are also discussed.

The reactions between a superelectrophilic carbon reagent, 4,6-dinitrotetrazolopyridine, and 1,3,5-tris(N,N-dialkylamino)benzenes, a supernucleophilic carbon reagent series, has afforded C-C coupling products that are edouble [sigma]-complexesE (W-M), Wheland-like on the 1,3,5-tris (N,N-dialkylamino)benzene moiety and Meisenheimer-like on the 4,6-dinitrotetrazolopyridine moiety. The experiments at variable temperature have shown that the formation of these complexes by a nucleophile/electrophile attack is a reversible process.

The substitution of anthraquinone at 1,8 positions by 2-methyl-1-naphthyl groups to yield syn (meso) and anti (racemic) isomers that interconvert with a barrier of 35.4 kcal [mol.sup.-1] in solution is reported. The configurational stability of the atropisomers is attributed to the steric interaction between a single methylnaphthyl substituent and the carbonyl group of the anthraquinone moiety.

The dynamics of the rotation process about the C9-isopropyl bond in the title compound, which is selected as it is the key intermediate for the preparation of drugs acting as calcium antagonists in the case of cardiovascular activity, is studied. The low-temperature NMR spectra have shown that the title compound has existed as a pair of conformational enantiomers, generated by the restricted rotation about the C9-[Pr.sup.i] bond.

The synthesis of a new chiralmacrocyclic uranyl-salophen derivative and the analysis of its configuration stability carried out by dynamic chiral HPLC are described. The studies have shown that the new structural motif has imparted a higher configurational stability, thus raising the half-life for the enantiomerization to more than 2 months at room temperature.

Syn and anti conformers in similar proportions are seen at ambient temperature for the atropisomers of ditolyl anthrones and anthraquinone. The anti-to-syn interconversion barriers are obtained by line-shape simulation of the temperature-dependent NMR spectra.

A detailed multinuclear NMR study of the neutral and lithiated trans-N-propyl-2,3-diphenylaziridine performed in presence of both THF and toluene is described. The findings suggested the involvement of two differently configured lithiated aziridines.

Fluorene derivatives, having substituted benzyl groups bonded to position 9, are examined by variable temperature NMR spectroscopy. Stereodynamic processes involving restricted rotation about the fluorenyl-C[H.sub.2] and aryl-C[H.sub.2] bonds have led to conformers and enantiomeric forms.

A number of aryl moieties bearing, in the ortho positions, alkyl groups of various sizes were attached to the position 1,8 of biphenylene to obtain long-living isomers that could be physically isolated. The studies provide insight into structures, interconversion barriers of the conformers, stereodynamics, and absolute configuration of the two enantiomers of the isomer.

Several 1,8-di-arylbiphenylenes are studied to provide a new approach for the detection of various [pi]-barriers involved in the enantiomerization processes.

The measurement of rotation barriers of two 1,8-ditolylanthracene derivatives by accurate line-shape simulation of their variable-temperature NMR spectra was observed to be 2 times larger than the former result data.

A study uses low-temperature dynamic nuclear magnetic resonance (NMR) spectroscopy to determine the rotation barriers about the s[p.sup.3]-s[p.sup.2] bond in several hindered benzyl alcohols symmetrically substituted in the ortho positions. The free energies of activation covered the range 4.6-10.1 kcal [mol.sup.-1] and ab initio computations matched the trend of these values and predicted the conformation adopted by the compounds.

The hydrazinolysis reaction of 5-perfluoroalkyl-1,2,4-oxadiazoles with hydrazine or methylhydrazine as bidentate nucleophiles is examined. The reinvestigation of the reaction of 3-phenyl-5-perfluoroalkyl-1,2,4-oxadiazoles with methylhydrazinne has allowed an assignment of the correct structure to the formed products.

The ring inversion barrier for thianthrene tetraoxide is determined by using the variable temperature [.sup.13.C] NMR spectra of the 2,7-diisopropyl derivative. The ring inversion barrier is found to be lower than that measured for a trans thianthrene dioxide derivative and these results have agreed well with ab initio theoretical predictions.

Nuclear magneto resistance (NMR) spectroscopy was used to determine the barriers to rotation in a range up to 15.4 kcal [mol.sup.1] for a series of biphenyl compounds with a single ortho-substituent. The series of compounds was found to adopt a ground state conformation where the biphenyl dihedral angle varies between 40 degrees and 90 degrees, reflecting the size of the substituent.

Compounds comprising of two mesityl substituents bonded to trigonal(super 2,3) or tetrahedral(super 4-6) moieties adopt conformations resembling a two-bladed propeller that entails the existence of a pair of conformational enantiomers. A stereo dynamic consequence is studied with help of 1,1-diaryl ethylene o,o-diisopropyl-1,1-diphenylethylene, and the diaryl ketone o,o-diisopropylbenzophenone, where the aryl group is a 2-isopropylphenyl moiety, which is discussed.