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  • Inorganic Chemistry  (10,544)
  • Organic Chemistry  (10,398)
  • Chemical Engineering  (5,359)
  • 1990-1994  (11,125)
  • 1970-1974  (9,074)
  • 1955-1959  (6,102)
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  • 1
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Semi-empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene-1-azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4-iminobenzylfuran-2,3-diones are presented. A [4 + 2] type of reaction of the ketene C=C double bond leading to a six-membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon-nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two-step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa-1,3-dienes are discussed.
    Additional Material: 3 Ill.
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  • 2
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reaction of aryliminodimagnesium [ArN(MgBr)2, IDMg] with nitrobenzene (Ar1NO2) in tetrahydrofuran (THF) gives unsymmetrical (unsym) azoxybenzene, which is deoxygenated to give unsym-azobenzene. The reaction is utilized for the independent preparation of ONN and NNO isomers of unsym-azoxy compounds. The effects of the relative value of the difference between oxidation and reduction potentials of para-substituted reactants, special effects of ortho-substituents of the substrate and also effects of structure and concentration of mono- and bis-IDMg reagents were studied previously, and ‘cooperation of their aggregate excess needed for product formation’ was proposed. This unfamiliar concept for reactions of magnesium reagents was studied further. Supporting evidence was obtained from the retarding effect of trimethylene chains of nitro [3.3] metacyclophanes and the derived azoxy [3.3] metacyclophanes and also from the effect of the length of bis-IDMg's central α,ω-polymethylenedioxy chain appropriate for azoxy deoxygenation. By comparison with deoxygenation by X3P reagents (X = EtO, Me2N), the general needs of cooperation of excess magnesium reagents were confirmed and its role in their reactions is discussed.
    Additional Material: 3 Ill.
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  • 3
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The magnitude ρx(ρnuc) is shown to decrease with increase in the extent of bond making estimated by the kinetic isotope effect for the reactions of Y-benzoyl chlorides with anilines, XC6H4NH2, in acetonitrile at 25·0°C.
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  • 4
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The role of orbital control in product selectivity during electrophilic aromatic alkylation catalysed by zeolites was studied both theoretically and experimentally. In order to discuss this, the alkylation of toluene and m-xylene by methanol was carried out on a series of large-pore zeolites (HY). The changes in the para to ortho ratio observed on changing the framework Si/Al ratio of the zeolites were related to ab initio molecular orbital calculations of the LUMO energy of structurally alike model clusters but containing different tetrahedral cations around the active site. The observed correlation is discussed in terms of the HSAB principle by taking into account the influence of the catalyst composition on the reactivity of the electrophilic reagent.
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  • 5
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: N-Substituted carbonimidodithioic acid dimethyl esters, when reacted with an active methylene compound such as nitromethane, undergo carbon-carbon bond formation followed by carbon-sulphur bond cleavage in the presence of zeolite catalysts to give 1-substituted amino-1-methylthio-2-nitroethenes. This carbon-sulphur bond cleavage is facilitated by the presence of rare earth cations in the zeolite-Y framework. The shape selectivity of the zeolite also plays an important role in this cleavage reaction. Force field calculations adopted for the molecules involved in the reaction indicate the geometry and conformational flexibility of these molecules. The computer-simulated model for the zeolite-Y is correlated with the conformation and shape of the reactant and product molecules to explain the variation in yields obtained in the conversion of substituted dimethyl esters.
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  • 6
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effects of the concentrations of LiCl, NaCl, KCl, CsCl, Na2CO3 and BaCl2 on the rates of reactions of hydroxide ion with ionized N-hydroxyphthalimide (NHP-) at 30°C and in H2O-MeCN solvents containing 10, 50, 60, 66, 70, 76 and 80% (v/v) MeCN reveal the formation of ion-pair complexes between cations of the salts, which probably exist in solvent-separated loose ion-pair forms (Mn+⃛Xk-) and NHP-. An increase in MeCN content from 2 to 76% (v/v) causes an increase in the association constants (K) by factors of 40, 21 and 9 for LiCl, NaCl and KCl, respectively, while the respective increase in the rate constants (k′s1) for the collapse of the ion-pair complexes to product is ca two-fold. The values of k′s1 remain almost unchanged whereas the values of K increase 7·5-fold with an increase in MeCN content from 2 to 70% (v/v) for CsCl. Ion-pair complex formation was not detected in the presence of Me4NCl at 70% (v/v) MeCN. The observed pseudo-first-order rate constants are highly sensitive to the valence state of cations and almost insensitive to the valence state of the anions of the salts.
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  • 7
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates and products of solvolyses of 4-methylene-2ax- and -2eq-adamantyl p-toluenesulphonates (tosylates) (4a-OTs and 4e-OTs, respectively) were studied. Compound 4a-OTs solvolysed more slowly than 2-adamantyl tosylate (1) in methanol and 2,2,2-trifluoroethanol (TFE) by factors of 2·3 and 2·5, respectively, at 25°C. However, by taking the inductive decelerating effect of a β-methylene substituent into account, the rates were revealed to be enhanced by α-participation by a factor of 50. The products of solvolyses of 4a-OTs in methanol, 80% acetone and TFE at 100°C were 2ax- and 2eq-alkoxy(or hydroxy)-4-methyleneadamantanes (4a-OR and 4e-OR, respectively), exo-4-alkoxy(or hydroxy)-5-methyleneprotoadamantane (exo-5-OR) and 5-[alkoxy(or hydroxy)methyl]-4-protoadamantene (6-OR) with adamantyl to protoadamantyl product ratio of 39:61 (in methanol), 56:44 (in 80% acetone) and 71:29 (in TFE). Despite the nearly symmetric nature of the intermediate cation, the 4a-OR: 4e-OR product ratio was essentially constant with 83:17 (in methanol), 85:15 (in 80% acetone) and 82:18 (in TFE). The formation of considerable amounts of 4e-OR was interpreted as showing the intermediacy of a pair of rapidly equilibrating classical ions. The rates of 4e-OTs were 2300-4300 times faster than those expected from inductive electron-withdrawing effect of a β-methylene substituent. The major product (84·5% in methanolysis and 98·7% in trifluoroethanolysis) was 4e-OR accompanied by small amounts of 2-alkoxy-2,4-methanoadamantane (9-OR) and 2-(alkoxymethyl)-2,4-didehydroadamantane (10-OR), no formation of 4a-OR having been observed. These results suggested that 4e-OTs solvolyses via a π-bridged intermediate cation.
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  • 8
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 9
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Molecular mechanics (MM3) calculations were carried out on the title compounds. Comparison between the MM3 results and those of semi-empirical and ab initio calculations and experiment indicates that the MM3 results are at least as good as results of much more expensive calculations. The MM3 calculations predict that unlike corannulene and cyclopentacorannulene, the transition state of the bowl-to-bowl inversion of the related C30H10 is non-planar, and the activation barrier is too high for this motion to occur.
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  • 10
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reactions of benzo-, naphtho- and anthraquinone derivatives (unsubstituted and substituted) with aryliminodimagnesium [ArN(MgBr)2] and aryloxymagnesium of a weak electron-donating ability were studied. In addition to the reduction products (quinhydrones and hydroquinones), nuclear substitution and condensation products were formed. The efficiency of single electron transfer (SET) from ArN(MgBr)2, evaluated by the relative values of the difference between the oxidation and reduction potentials of the reactants (ΔE = Eox - Ered), varies with the electron-accepting power of quinones. ΔE governs the modes of semiquinone appearance (ESR signals) and the relative amounts of the heat of reactions, reflecting the types and yields of the products. It is concluded that condensation products of both components are produced in the reactions of quinones with the lower SET efficiency by the presence of a fused benzene ring or MeO substituent. The structure-reactivity relationship proposed previously for reactions of various magnesium reagents is extended in the reactions with a variety of quinones even in cases of higher SET efficiency.
    Additional Material: 6 Ill.
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  • 11
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The low temperature 1H NMR spectra of 2,2-bis(3,5-dibromomesityl)ethenol in CS2-CD2Cl2 (3:7) display new signals which indicate the presence of four enol species whose OH is hydrogen bonded. Oligomerization to intermolecularly hydrogen-bonded enol dimers or tetramers is suggested.
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  • 12
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 13
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Measurements of absorption and emission spectra, fluorescence quantum yields (Φf) and lifetimes (τf) of ω-(1-pyrenyl)alkyl para-substituted benzoates PnX (X = H, Cl, CF3, CN, NO2, with n = 1-4) were carried out in solvents of various polarity. Intramolecular interaction in the ground state is not observed in any of these compounds. PnCN (n = 2-4) shows an intramolecular exciplex emission in solvents of low to high polarity. The broad, structureless emission at longer wavelength observed in ethyl acetate solution of P1CN is ascribed to an ‘exciplex-type’ emission which does not require actual overlapping of the two chromophores. P2CF3 also shows an exciplex emission in solvents of medium to high polarity. The solvent dependence of both Φf and τf increases as the electron-withdrawing ability of the para-sustituents increases. The relationship between fluorescence quenching by electron transfer and para-substituent of PnX is discussed by means of the free energy for electron transfer, ΔGET, obtained from the oxidation and reduction potentials of pyrene and methyl para-substituted benzoates in acetonitrile, respectively. PnH, with positive ΔGET, does not show a solvent dependence of Φf, except for P1H, in which ca 40% of the fluorescence is quenched in acetonitrile. PnCl, with slightly negative ΔGET, shows more efficient quenching, but does not show exciplex emission. PnCN, PnCF3 and PnNO2 have ΔGET values between -0·36 and -0·65 eV, and their fluorescence is fairly efficiently quenched. The fluorescence of PnCN is concluded to be strongly quenched by intersystem crossing from the singlet exciplex to the locally excited pyrene, and by electron transfer from the pyrene part to the benzoate part. That the formation of a singlet exciplex is necessary for intersystem crossing in bichromophoric compounds containing pyrene is thus clearly illustrated.
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  • 14
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The evidence for the mechanisms proposed for aromatic nucleophilic substitution reactions by primary and secondary amines in aprotic solvents of low relative permittivity is reviewed.
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  • 15
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α-methylstyrenes (1-Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual-parameter equation and not by a single-parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
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  • 16
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Protonation of tetrafluoro-2,11-dithia [3·3]paracyclophane and tetrafluoro-2,11-dithia [3·3]metaparacyclophane in high-acidity super acid media, namely FSO3H.SbF5 (1:1) ‘magic acid’-SO2CIF, gave their corresponding acidic disulfonium ions. Additional ring protonation to give a disulfonium-monoarenium trication was not observed. With the non-fluorinated 2,11-dithia [3·3] cyclophanes, the disulfonium ions can be ring protonated in equilibrium to give a dynamic disulfonium-monoarenium trication. Tetrafluoro [2·2]-metaparacyclophane is monoprotonated at the meta ring and gives a complex mixture of conformational isomers. Multinuclear magnetic resonance data on the cyclophane precursors and their derived cations are compared and analysed. The energies, conformations and charge distributions of the isometric fluorinated and non-fluorinated [2·2] - and dithia [3·3] cyclophanes were calculated by the AM1 and PM3 methods, respectively. In all but one case the cyclophane arenium ions predicted by theory to be energetically most favoured are those observed in solution under stable ion conditions. In agreement with experiment, the instabilities of S,S,C-cyclophane trications are also theoretically predicted.
    Additional Material: 10 Ill.
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  • 17
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ionization constants for 214 dye molecules were calculated from molecular structures using the chemical reactivity models developed in SPARC (SPARC Performs Automated Reasoning in Chemistry). These models used fundamental chemical structure theory to predict chemical reactivities for a wide range of organic molecules from molecular structure. The energy differences between the protonated state and the unprotonated state for a molecule of interest are factored into mechanistic components including the electrostatic and resonance contributions and any additional contributions to these energy differences. The RMS deviation was found to be less than 0·62 pKa units, which is similar to the experimental error.
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  • 18
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    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 169-169 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 19
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvent effects on the absorption spectra of N-(p-nitrophenyl)dimethylenediamine (Ia), N-(p-nitrophenyl)trimethylenediamine (Ib), N-(p-nitrophenyl)tetramethylenediamine (Ic). N-methyl-N-(p-nitrophenyl)tetramethylenediamine (II), N-butyl-p-nitroaniline (III) and N-(o-nitrophenyl)trimethylenediamine (IV) were studied at different temperatures. Whereas II, III and IV do not show any variation in their spectra characteristics with changes in temperature, I shows a hypsochromic shift with a hypochromic effect when the temperature is increased. The Kamlet and Taft solvatochromic comparison method was applied. A strong effect of the β parameter on I and III was interpreted as being due to the hydrogen bond donor ability of the H atom in the aromatic amino groups. In I, the β influence increases with increase in temperature. These facts are explained by proposing the formation of intramolecular hydrogen bonds between amine groups in all compounds I, besides the intermolecular interactions between compounds I and the solvent. On the other hand, the values of vo, s and b for Ib are smaller than the corresponding values for Ia and Ic. Since in Ib a six-membered ring may be formed, a more stable bond is expected. Comparative 1H NMR of the aniline hydrogen for I and IV (in non-hydrogen bond acceptor solvents) shows a particular downfield chemical shift for I which suggests hydrogen bond formation. Since this effect is independent of concentration, the hydrogen bond is assumed to be intramolecular, in agreement with solvatochromic and thermochromic studies. These conclusions were corroborated by IR spectroscopy in the solid state and in chloroform solutions.
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  • 20
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 21
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The secondary α-deuterium isotpe effect on the equiliburium constant (KD/KH) for the hydration of p-nitrobenzaldehyde is 1·39 ± 0·05. The Kinetic secondary α-deuterium isotope effect (KD/KH) for the addition of water to p-nitrobenzaldehyde is 1·18 ± 0·07. This result demonstrates that there is extensive carbon-oxygen bond formation in the transition state for addition of water to the carbonyl group of p-nitrobezaldehyde.
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  • 22
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate of the reaction of 2-chloroquinoxaline with piperidine in dimethyl sulphoxide was measured over a wide range of amine concentrations and at several temperatures. It was found that the order with respect to the nucleophile is close to 1 between 300 and 320 K, but is definitely less at lower and higher temperature. It is suggested that below 300 K an unreactive charge-transfer complex is formed between the reactants which dissociates at higher temperatures, whereas at temperatures higher than 320 K an unproductive σ complex is formed, the concentration of which increases with increase in temperature.
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  • 23
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The enthalpic contribution to the anomeric effect in r-2-carboethoxy-trans-4,trans-6- and r-2-carboethoxy-cis-4,cis-6-dimethyl-1,3-dithianes was determined by reaction-solution calorimetry. The enthalpy of solution of both isomers in pure p-dioxane and the enthalpy of solution and isomerization in the same solvent and in the presence of trifluoroacetic acid were experimentally measured. From these results the corresponding enthalpies of isomerization in solution were calculated, and were found to be ΔHax→eq = -0·16 ± 0·04 kcal mol-1 (-0·67 ± 0·18 kJ mol-1) and ΔSax→eq = -2·68 ± 0·1 cal K-1 mol-1 (-11·2 ± 0·4 J K-1 mol-1). The slightly negative ΔH term nevertheless reflects a substantial anomeric effect owing to the countervailing steric effects in the axial isomer. The significant entropy loss in the equatorial isomer was explained in terms of intramolecular electrostatic effects. The results are in agreement with those obtained from NMR studies of the conformational behaviour of 2-carboethoxy-5-methyl-5-aza-1,3-dithiacyclohexane.
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  • 24
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    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 591-609 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
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  • 25
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title sulphenamides were pyrolysed in a stirred-flow reactor at temperatures of 310-410°C, pressures of 8-15 Torr and residence times of 0·4-2 s using toluene as the carrier gas. N-(tert-Butylthio)allylamine formed 73 ± 4% isobutene, 23 ± 3% propene and N-allylthiohydroxylamine. The first-order rate coefficients for the formation of isobutene and propene, respectively, followed the Arrhenius equations kC4(s-1) = 1012·52 ± 0·36 exp(-163 ± 5 kJ mol-1 RT) and kC3(s-1) = 1010·99 ± 0·29 exp(-151 ± 4 kJ mol-1 RT) N-(tert-Butylthio)diethylamine gave 97 ± 1% isobutene, 1·9 ± 0·4% isobutane and N,N-diethylthiohydroxylamine. The first-order rate coefficients for isobutene elimination followed the Arrhenius equation k(s-1) = 1013·45 ± 0·24 exp(-164 ± 3 kJ mol-1 RT). The formation of the products is interpreted in terms of an elimination reaction with a unimolecular, four-centered, cyclic transition state. The reactivity of these sulphenamides was found to be much higher than that of previously studied alkyl or aryl tert-butyl sulphides and disulphides.
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  • 26
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The aromatic odd-alternant phenalenyl anion and a number of its derivatives were prepared in order to study the perturbation of this conjugated anion by methyl and methoxy groups. The conjugated anions were studied by means of 1H and 13C NMR spectrometry, alkylation experiments and semi-empirical calculations. It was found that a substituent at a charged carbon atom perturbs the entire conjugated system, whereas substituents at inactive (uncharged) carbon atoms have a large effect on the positions ortho to the substituent.
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  • 27
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The spontaneous hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB) with a degree of substitution (DS) in the range 2-9 was studied in 10% aqueous ethanol at pH 10, and was followed spectrophotometrically by the appearance of p-nitro-α-toluenethiol, in a continuous-flow system where the reactor was shaken. CelXNB was characterized by solid-state 13C NMR spectra. The reaction occurs through two parallel processes due to two xanthate ester groups with different reactivities. The fast hydrolysis was ascribed to the reaction of the C-2 + C-3 isomers, whereas the slow hydrolysis was due to the C-6 isomer. The percentage of the latter is much higher than C-2 + C-3. The solvent isotope effect of the fast hydrolysis (k′H2O/)k′H2D 11 was 2·22 ± 0·16 and the proton inventory indicated that there is only one proton transfer involved in the transition state, where a second water (or a neighbouring OH group) acts as a general base. The entropy of activation of the fast hydrolysis was only 3·3 ± 0·8 e.u., suggesting that the water molecules involved are highly oriented with respect to the coordinates required to reach the transition state. It is proposed that they form part of the three-dimensional hydrogen-bonded ice-like structure that involves the cellulose matrix.
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  • 28
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 316-322 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rates of hydrolysis of the vinyl ether functional groups of (Z)- and (E)-β-methoxyacrylic acid and (Z)-and (E)-β-methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)-β-methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β-carboxy and β-carbomethoxy substituents produce strong rate retardations, ranging from 2000- to 25 000-fold, for both Z- and E-isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)-β-methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non-ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β-lactone formation as suggested in an earlier observation of the same phenomenon in a different system.
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  • 29
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The stereochemistry and kinetics of the methoxide ion substitution reactions (Z)- and (E)-methylbenzohydroximoyl cyanidc [PhC(CN)=NOCH3] were investigated. The reaction of the (Z)-hydroximoyl cyanide with sodium methoxide in DMSO-methanol (9:1) solution at 44·8°C gives a mixture of methyl (Z)-O-methylbenzohydroximate [PhC(OCH3)=NOCH3] and the O-methyloxime of α-ketophenylacetamide PhC(CONH2)=NOCH3. The (E)-hydroximoyl cyanide undergoes methoxide ion-catalyzed isomerization to the E-isomer faster than it undergoes nucleophilic substitution. These observations were interpreted in terms of an addition-elimination mechanism in which the rate-limiting step is elimination of the nucleofuge (AN + DN#).
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  • 30
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 394-394 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 31
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The reaction of nitrosobenzene with aniline, to give azobenzene, in basic conditions was studied. It was shown that the reaction exhibits general base catalysis by different buffers giving a Brønsted coefficient β = 0·318. As in previous studies, a two-step process with a first step of attack of aniline on nitrosobenzene to give an addition intermediate and a second step of dehydration of this intermediate is proposed to interpret the mechanism of the reaction. The analysis of the Brønsted relationship and of the intermediate of the reaction led to the suggestion that hydoxide ion catalyses the reaction by a mechanism of general base catalysis in the dehydration step.
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  • 32
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Imidazole (Imz)-catalysed hydrolysis of benzoate esters proceeds via the intermediate formation of N-benzoylimidazoles. This paper considers the second step of this reaction, viz., Imz-catalysed hydrolysis of N-(4-X-benzoyl)imidazoles, X = CH3, H, Cl, CN and NO2, and N-(disubstituted benzoyl)imidazoles, 2-chloro-4-nitro, 2, 4-dinitro and 3,5-dinitro, in water-acetonitrile mixtures (10% or 14%, v/v, in organic solvent). On the basis of catalytic rate constants and the kinetic solvent isotope effect, it is shown that catalysis by Imz is of the general-base type. Unexpectedly, the hydrolysis of N-(2,4-dinitrobenzoyl)imidazole was found to be slower than that of N-(4-nitrobenzoyl)imidazole. It is shown that this reactivity order is due to a combination of a steric effect and stabilization of the reactant state due to a donor-acceptor interaction between the Imz moiety and the 2,4-dinitrophenyl ring.
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  • 33
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 34
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates of the solvolysis of 3-R-4-homoadamantyl methanesulphonates (mesylates) (3) were determined in 80% aqueous ethanol. The relative first-order rate constants at 25 °C were 1·0 (R = H), 2·29 (R = Ph), 3·26 (R = p-anisyl), 73·6 (R = Me) and 209 (R = Et). The methanolysis of 3 gave rearranged methyl ethers and rearranged olefins as major products together with small amounts (0·9-3·4%) of unrearranged products. The order of the accelerating effect suggests that the transition states involve significant σ-participation, despite the fact that 3 (R = H) solvolyses via a classical ion intermediate. The logarithms of the solvolysis rate constants of 3 showed linear correlations with those of 1-R-2-adamantyl tosylates (1) and 1-R-exo-2-norbornyl tosylates (2), indicating that the linear free-energy relationship between the β-substituent effects on the solvolysis rate is not a definite measure to distinguish between classical and non-classical intermediates.
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  • 35
    Electronic Resource
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 7 (1994), S. 518-524 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics of the nucleophilic cleavage of phthalimide (PTH) in buffer solutions of ammonia and pyrrolidine were determined. The reaction rates for ammonolysis of PTH revealed a buffer-catalysed second-order term in the rate law, but the reaction rates for pyrrolidinolysis of PTH showed buffer-catalysed second- and third-order terms in the rate law. Both ammonia and pyrrolidine revealed nucleophilic reactivity towards ionized PTH (S-) only within the pH range of the present study. This is attributed to the occurrence of intramolecular general base-acid catalysis. General base catalysis is detected in the reactions of pyrrolidine with both non-ionized PTH (SH) and S-. The general base-catalysed third-order rate constant for pyrrolidinolysis of SH is nearly 28 times larger than that of S-.
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  • 36
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The bisporphyria N,N′-bis[4″-(meso-triphenylporphyrinyl)benzyl]-4,13-diaza-18-crown-6 and its mono- and dizinc derivatives were synthesized in 66%, 46% and 53% yields, respectively, from 5-(4′-bromomethylphenyl)-10,15,20-triphenylporphyrin or its zinc derivative and 4,13-diaza-18-crown-6. The zinc-containing bisporphyrins form dimers in solution at low temperature or at high concentration. The unsymmetrical bisporphyrin; monozinc N,N′-bis[4″-(meso-triphenylporphyrinyl)benzyl]-4,13-diaza-18-crown-6 shows singlet-singlet energy transfer from the zinc porphyrin moiety to the free base moiety in both the monomeric and the dimeric form. The energy transfer rates were determined using time-resolved fluorescence spectroscopy and were found to be 1·26 × 109 and 2·29 × 109 s-1 for the monomeric and dimeric form, respectively. The difference in energy transfer rates between the two forms can be rationalized by the difference in overlap between the donor fluorescence spectrum and acceptor absorption spectrum, donor-acceptor distance and donor-acceptor orientation.
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  • 37
    ISSN: 0899-0042
    Keywords: enantiomers ; propranolol ; enantioselective pharmacokinetics ; protein binding ; nicardipine ; drug-drug interaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The influence of a single oral dose of 30 mg nicardipine on the pharmacokinetics of (R)- and (S)-propranolol, given orally as rac-propranolol 80 mg, was studied in 12 healthy volunteers. The plasma concentrations were higher for the (S)-enantiomer than for the (R)-enantiomer. The Clo and the Cl′intr of (S)-propranolol were significantly lower than the Clo and Cl′intr of (R)-propranolol. The unbound fraction of (R)-propranolol was significantly higher than that of (S)-propranolol. Coadministration of nicardipine significantly increased the AUC and Cmax and significantly decreased the Clo and Cl′intr for unbound drug of (R)- and (S)-propranolol. These changes were more important for (R)- than for (S)-propranolol. The protein binding was not altered by nicardipine. The enantioselective effect of nicardipine on the metabolic clearance of propranolol appears to be due to an interaction at the level of the metabolizing enzymes. The effect on blood pressure of rac-propranolol was little affected when nicardipine was coadministered with rac-propranolol, and its bradycardic effect was reduced. © 1994 Wiley-Liss, Inc.
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  • 38
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 25-40 
    ISSN: 0899-0042
    Keywords: Review ; capillary electrophoresis ; enantiomeric resolution ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preconditions are outlined for enantioselective separations in capillary electrophoresis (CE) with chiral selectors as additives to the background electrolyte. Free solution capillary electrophoresis conditions are characterised by a single solution phase. Chiral separations are reviewed by selector type (chiral ligand exchange, cyclodextrins, crown ethers, glycoproteins) with the extensive studies on cyclodextrins grouped into sections on amino acids, pharmaceuticals, and speciality chemicals, optimisation, biological fluids, and quantitative aspects. In micellar electrokinetic capillary chromatography, enantioselective discrimination occurs by partition in a two-phase system, with a chiral micellar phase as selector. Optimum separation conditions can be readily predicted for a given selector-selectand combination, and absolute values of binding constants determined by CE. Advantages of CE in comparison with HPLC using a chiral stationary phase include robust, rapid assays and the use of small volumes of aqueous solutions; disadvantages include less favourable detection limits. © 1994 Wiley-Liss, Inc.
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  • 39
    ISSN: 0899-0042
    Keywords: chiral HPLC ; Chiralpak AD ; amylose carbamate stationary phase ; antiestrogen ; breast cancer ; dichlorotriarylcyclopropane ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (Z)-1,1-Dichloro-2-(4-benzyloxyphenyl)-2,3-bis(4-methoxyphenyl)cyclopropane (5), a potential antitumor agent designed to treat breast cancer, was prepared in three steps. A stereospecific palladium-catalyzed cross coupling reaction which provided the intermediate (Z)-triaryl alkene 4 was a crucial step in the synthesis. Makosza phase transfer reaction on 4 gave the enantiomeric (Z)-dichlorocyclopropane derivatives 5 which were resolved by semipreparative HPLC on a chiral stationary phase consisting of amylose tris-3,5-dimethylphenyl carbamate coated on silica gel. © 1994 Wiley-Liss, Inc.
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  • 40
    ISSN: 0899-0042
    Keywords: enantioselective ; chromatography ; validation ; column-switching ; robotic ; pharmacokinetic ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Lifibrol, a new drug for the treatment of hypercholesterolemia, contains a stereogenic center bearing a secondary alcohol group. A normal-phase achiral-chiral HPLC separation of the enantiomers of lifibrol and two of its metabolites was developed and validated for quantitation in dog plasma. A silica and a Chiralcel OD-H column were operated in series and all six enantiomeric components and internal standard were directly separated. An initial solid-phase extraction (phenyl) clean-up step and a column-switching step to eliminate late-eluting compounds were also utilized. The solid-phase extraction step was automated using a robotic system. Assay development, validation, and application of the method to a bioavailability study of the racemate and enantiomers of lifibrol in dogs are described. The lower limit of quantitation was 0.0125 μg/ml for each enantiomer of lifibrol using 200 μl of dog plasma with UV detection (255 nm). In dog plasma following oral or intravenous administration of the racemate, the (R)/(S) ratio of the enantiomers of lifibrol was greater than one and increased with time. Following administration of the individual enantiomers, chiral inversion of the (S)-enantiomer but not the (R)-enantiomer was observed. © 1994 Wiley-Liss, Inc.
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  • 41
    ISSN: 0899-0042
    Keywords: pargyline N-oxide ; chiral nitrogen centre ; flavin-containing monooxygenase ; chiral stationary phase ; high-performance liquid chromatography ; Chiralcel OD ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The monoamine oxidase inhibitor pargyline (N-benzyl-N-methyl-2-propynylamine) is known to undergo extensive in vitro microsomal N-oxidation, thought to be mediated predominantly by the flavin-containing monooxygenase (FMO) enzyme system. Formation of the pargyline N-oxide (PNO) metabolite creates a chiral nitrogen centre and thus asymmetric oxidation is possible. This study describes a reverse-phase high-performance liquid chromatographic (HPLC) method for the quantitation of PNO and a chiral-phase HPLC method for the determination of the enantiomeric ratio of PNO. In vitro microsomal N-oxidation of pargyline was found to be highly steroselective in a number of species, with the (+)-enantiomer being formed preferentially. This metabolic transformation was stereospecific when purified porcine hepatic FMO was used as the enzyme source. © 1994 Wiley-Liss, Inc.
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  • 42
    ISSN: 0899-0042
    Keywords: chirality ; fluoxetine ; norfluoxetine ; desipramine ; iprindole ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The antidepressant fluoxetine (FLU) and its N-demethylated metabolite, norfluoxetine (NFLU), each contains a chiral center. The combination of FLU and desipramine (DMI), another antidepressant, has been reported to be useful in treatment of depression, to dramatically increase plasma levels of DMI and also to produce more rapid β-adrenergic receptor down-regulation in brain than caused by DMI alone. We have now begun studies on the effects of this drug combination on the levels of FLU and NFLU enantiomers in the rat. In addition, the combination of FLU and iprindole (IPR) was also investigated. Male Sprague-Dawley rats were treated intraperitoneally with either normal saline vehicle, DMI (5 mg/kg/day), (R,S)-FLU (10 mg/kg/day) or DMI (5 mg/kg/day) + (R,S)-FLU (10 mg/kg/day) for 4 days. Following the last treatment, 24 h urine samples were collected. Rats were sacrificed and brains were removed. For the IPR study, rats were pretreated with either saline or IPR-HCl (11.2 mg/kg) and then treated 1 h later with (R,S)-FLU. After 5 h, the rats were sacrificed and brains were removed. Brain and urine samples were analyzed by gas chromatography with electron-capture detection for free (R)- and (S)-FLU and (R)- and (S)-NFLU after extraction and reaction with (-)-(S)-N-(trifluoroacetyl)prolyl chloride. The results from the brains of the rats treated with DMI/FLU indicate that levels of the enantiomers of both FLU and NFLU were significantly increased over those seen in the animals receiving (R,S)-FLU alone. In the IPR/FLU treated rats, an increase in the brain levels of both (R)- and (S)-FLU was noted when compared with rats receiving (R,S)-FLU alone; however, there appeared to be no increase in the brain levels of NFLU enantiomers. © 1994 Wiley-Liss, Inc.
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  • 43
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 148-155 
    ISSN: 0899-0042
    Keywords: chiral stationary phase ; dinitrobenzylphenylethylamine ; dinitrobenzoylphenylglycine ; enantiomers ; 2,2,2-trifluoro-1-(9-anthryl) ethanol ; competition ; nonlinear ; liquid chromatography ; system peaks ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Competition between the (+)- and (-) enantiomers of 2,2,2-trifluoro-1-(9-anthryl) ethanol as mobile phase additives was indicated by the chromatographic behavior of their system peaks. Two types of chiral stationary phases were used, one based on dinitrobenzoylphenylglycine and the other on dinitrobenzylphenylethylamine plus tartaric acid. The racemic mixture was used as the mobile phase additive and k′ of their system peaks was studied as a function of the mixture concentration in the mobile phase in both cases. A shift in k′ of the two system peaks was observed and considered as an indication that competition occurred. The areas of the two system peaks were also studied as a function of the concentration of the enantiomers in the samples, using two different compositions of the mobile phase. The dependency of system peaks' area on the sample composition indicated whether competition between the enantiomers occurred. One mobile phase contained 0.1 mM of the racemic mixture, where the area of the two retained system peaks behaved independently, i.e., only the peak corresponding to the enantiomer was affected by its presence in the sample. The other mobile phase contained 0.75 mM of the racemic mixture, and both peaks were affected by the injection of any one of the enantiomers. The interdependency of the system peaks' area on both the enantiomers indicated that their distribution in the chiral system was interrelated due to mutual interactions. A quantitative treatment of the interdependency and competition was excluded, due to the irreversible adsorption of the two enantiomers on the chiral stationary phase after using overloading concentrations. This irreversible adsorption was visualized by the appearance of two retained system peaks of the two residual enantiomers. These system peaks were detected only when the sample contained pure enantiomers due to competition between the enantiomer in the sample with the residual enantiomers in the stationary phase. © 1994 Wiley-Liss, Inc.
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  • 44
    Electronic Resource
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994) 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 45
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 165-168 
    ISSN: 0899-0042
    Keywords: chirality ; chemical evolution ; phase transitions ; optical activity ; spontaneous symmetry breakdown ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We discuss the origin of the chirality of protein amino acids from the point of view of a phase transition from a racemic mixture into an optically pure state. We assume that Bose-Einstein condensation may act as an amplification mechanism. The original theory is due to Salam. We suggest a new role for the phase transition. Following Quack we distinguish parity violation of two kinds (de facto and de lege symmetry breaking). While the Salam phase transition corresponds to parity violation of the second kind (de lege), the phase transition we discuss in this work corresponds to parity violation of what we may call a third kind. This is suggested by recent experimental phenomena which correlate chiral symmetry breaking and pattern formation (spontaneous symmetry breaking that separates an initial racemic mixture into right- and left-handed space domains by means of a substrate). Tentative comments are given on the eventual design of possible experiments that may test this new hypothesis. © 1994 Wiley-Liss, Inc.
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  • 46
    ISSN: 0899-0042
    Keywords: atenolol ; enantiomers ; pharmacokinetics ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The purpose of this study was to describe the pharmacokinetics of and heart rate and blood pressure responses to (S)-atenolol (SATN) and (R)-atenolol (RATN) after oral administration of (S)-atenolol and (R,S)-atenolol (Tenormin™) in man. Eight male subjects were given single oral doses of 50 mg of SATN as a single enantiomer formulation (SEF) and 100 mg of Tenormin™ (TMN) using a randomized, double-blind, 2-period, complete crossover study design. Subjects performed exercise tolerance tests (Bruce Protocol) before and 2, 4, 6, 8, 12, and 24 h after drug administration. Plasma samples were obtained 2 min before and 30 min, 1, 1.5, 2, 3, 4, 5, 6, 8, 10, 12, 16, and 24 h after dosing. Urine was collected for the first 48 h after dosing. Plasma and urine samples were analyzed for SATN and RATN by an enantioselective HPLC method. SEF and Tenormin™ attenuated exercise-induced increases in heart rate and systolic blood pressure. Mean changes in exercise heart rates 4 h after dosing were -38 ± 3 bpm and -37 ± 3 bpm for SEF and TMN, respectively, P = 0.792. Mean changes in exercise systolic blood pressure were -42 ± 12 mm Hg and -55 ± 14 mm Hg for SEF and TMN, respectively, P = 0.484. Mean area under the plasma level time curve (AUC0-24) and mean Cmax for SATN for SEF were significantly lower than for SATN after TMN. Mean SATN AUCs0-24 were 1867 ± 261 and 2543 ± 223 ng · h/ml (P = 0.005) and mean Cmaxs were 225 ± 29 and 333 ± 30 ng/ml, for SEF and TMN, respectively (P = 0.011). Mean Tmax for SATN occurred significantly earlier after SEF than after TMN (2.9 ± 0.3 and 3.3 ± 0.3 h, P = 0.028). The amount of SATN excreted in urine was significantly lower after SEF than after TMN (18.7 ± 2.7 and 24.2 ± 2.0 mg, P = 0.017). AUC, Cmax, and amount excreted in urine (Au) were significantly higher for RATN than SATN after TMN. Mean AUCs, Cmaxs, and Aus for RATN compared to SATN were 2806 ± 239 vs 2543 ± 223 ng ± h/ml (P 〈 0.0001), 360 ± 31 vs 333 ± 30 ng/ml (P 〈 0.0001), and 25 ± 2.1 vs 24 ± 2 mg (P = 0.004), respectively. SEF and TMN are equieffective in attenuating exercise-induced increases in heart rate and systolic blood pressure. The SEF has lower bioavailability compared to TMN and RATN plasma levels are higher than SATN after TMN administration. © 1994 Wiley-Liss, Inc.
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  • 47
    ISSN: 0899-0042
    Keywords: 77Se NMR spectroscopy ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Previously, a diazaphospholidine has been synthesized and evaluated as a chiral derivatizing reagent for the determination of the optical purity of chiral alcohols via 31P NMR spectroscopy (Alexakis et al., J. Org. Chem. 57:1224-1237, 1992). Our laboratory is interested in the advantageous and practical applications of 77Se NMR spectroscopic studies in many facets of chemistry and biochemistry. To this end we have used this diazaphospholidine as a starting point and have investigated chiral alcohols coupled to an optically pure diazaselenophospholidine. The diastereomers formed were then evaluated by 77Se NMR spectroscopy, and these results were compared to the 31P NMR results published by Alexakis and co-workers. It was found that addition of the Se atom produced diastereomers that were air stable and, in many cases, the individual diastereomers could be distinguished by 77Se NMR spectroscopy. Preliminary results indicate that the 77Se nucleus is somewhat more sensitive to remotely disposed chiral centers than is the 31P nucleus. Furthermore, because of their stability, these compounds do not readily decompose and can, therefore, be studied by a variety of chromatographic and spectroscopic techniques. © 1994 Wiley-Liss, Inc.
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  • 48
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    Chirality 6 (1994), S. 213-215 
    ISSN: 0899-0042
    Keywords: ICRF-187 ; ICRF-186 ; ICRF-159 ; dexrazoxane ; doxorubicin ; dihydropyrimidine amidohydrolase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enzymatic ring-opening hydrolyses of the doxorubicin cardioprotective agents (+)-(S)-ICRF-187 (dexrazoxane), (-)-(R)-ICRF-186, and rac-ICRF-159 by the enzyme dihydropyrimidine amidohydrolase (DHPase) have been studied. ICRF-187 underwent enzymatic ring-opening hydrolysis by DHPase 4.5 times faster than did ICRF-186. It was also shown that DHPase opens only one ring of ICRF-186 and does not act on this one-ring open hydrolysis product, as has been observed for ICRF-187. Differences in the rates at which the two optical isomers are acted upon by DHPase suggest that they could have differing protective effects. © 1994 Wiley-Liss, Inc.
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  • 49
    ISSN: 0899-0042
    Keywords: cytochrome P-450 ; microsomal epoxide hydrolase ; epoxidation rates ; epoxide hydrolysis rates ; enantiomeric excesses ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rabbit liver microsomal biotransformation of α-methylstyrene (1a), 2-methyl-1-hexene (1b), 2,4,4-trimethyl-1-pentene (1c), and 2,3,3-trimethyl-1-butene (1d) has been investigated with the aim at establishing the enantioface selection of the cytochrome P-450-promoted epoxidation of the double bond and the enantioselectivity of microsomal epoxide hydrolase (mEH)-catalyzed hydrolysis of the resulting epoxides. GLC on a Chiraldex G-TA (ASTEC) column was used to determine the enantiomeric composition of the products. The epoxides 2 first produced in incubations carried out in the presence of an NADPH regenerating system were not detected, being rapidly hydrolyzed by mEH to diols 3. The enantiomeric composition of the latter showed that no enantioface selection occurred in the epoxidation of 1c and 1d, and a very low (8%) ee of the (R)-epoxide was formed from 1b. Incubation of racemic epoxides 2b-d with the microsomal fraction showed that the mEH-catalyzed hydrolysis of 2c and 2d was practically nonenantioselective, while that of 2b exhibited a selectivity E = 4.9 favoring the hydrolysis of the (S)-enantiomer. A comparison of these results with those previously obtained for linear and branched chain alkyl monosubstituted oxiranes shows that the introduction of the second alkyl substituent suppresses the selectivity of the mEH reaction of the latter and reverses that of the former substrates. © 1994 Wiley-Liss, Inc.
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  • 50
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    Chirality 6 (1994), S. 216-219 
    ISSN: 0899-0042
    Keywords: supercritical fluid chromatography ; chiral separation ; NSAIDs ; ibuprofen ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Packed column SFC is used to resolve the enantiomers of two nonsteroidal antiinflammatories. A Chiralpak AD column is used with a binary mobile phase of carbon dioxide/methanol to achieve resolution. The effects of composition, pressure, temperature, and flow on ibuprofen enantiomer separation are examined. In this instance, temperature affords the greatest change in resolution followed by pressure and composition. Baseline resolution of ibuprofen is achieved in less than 7 min and fluibiprofen is baseline resolved in less than 4 min. © 1994 Wiley-Liss, Inc.
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  • 51
    ISSN: 0899-0042
    Keywords: thalidomide enantiomers ; HPLC ; hydroxylated metabolites ; mass spectrometry ; EM 12 ; in vitro metabolism ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereoselective metabolism of the former sedative thalidomide and the metabolism of its analogue EM 12 were studied in vitro with liver homogenates. In our study we focused on hydroxylated nonhydrolyzed metabolites of thalidomide. An analytical HPLC method was developed to determine these metabolites directly. The investigations showed a highly stereoselective biotransformation of thalidomide. 5-Hydroxy thalidomide was preferentially formed by (-)-(S)-thalidomide, whereas (+)-(R)-thalidomide was metabolized to two hitherto unknown compounds (Met A and B). Mass spectrometry of these metabolites Met A and B indicated that oxidation or hydroxylation took place in the glutarimide moiety. Biotransformation studies with the more stable thalidomide analogue EM 12 revealed four new metabolites (Met C—F) whose quantities differed in the selected liver homogenate. © 1994 Wiley-Liss, Inc.
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  • 52
    ISSN: 0899-0042
    Keywords: human serum albumin ; electrokinetic chromatography ; chiral drugs ; ligand-ligand interactions ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: When a protein such as human serum albumin is added to the separation buffer in capillary electrophoresis, the mobility of solutes which bind to that protein may be altered. The change in mobility of the solute is a function both of the strength of the binding interaction, and the difference in mobility between the free solute and protein additive. By adding other ligands which themselves bind to the protein, the strength of the solute-protein binding may be modified, leading to a measurable change in the mobility of the solute. These effects are particularly striking for chiral compounds, where enantioselectivity may be completely lost on addition of a competitive ligand. Capillary electrophoresis with human serum ablumin as a buffer additive was used to separate the enantiomers of benzoin and three phenothiazine derivatives. A comparison of the binding of (S)-benzoin to human serum albumin as determined by capillary electrophoresis and by ultrafiltration was made. A variety of other ligands were then added to the buffer along with the protein, and the effects on mobility and enantioselectivity were studied. The displacers included (R)- and (S)-oxazepam hemisuccinate, (R)- and (S)-warfarin, nitrazepam, phenylbutazone, and octanoic acid. From the results obtained, it seems that capillary electrophoresis may be a useful, rapid method to screen for drug-drug interactions. There are some advantages of using this technique to study protein-ligand interactions: Only very small amounts of ligand are needed (useful when dealing with metabolites); for chiral compounds, if protein binding is stereoselective, then the method is also stereoselective, so single enantiomers are not needed; finally, measurements are obtained in solution, without the need for immobilization of the protein. A disadvantage is that the ligand and protein must have significantly different electrophoretic mobilities. © 1994 Wiley-Liss, Inc.
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  • 53
    ISSN: 0899-0042
    Keywords: chiral HPLC ; experimental design ; quantitative substituent effects ; recognition mechanism ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The determination of lipophilicity of the title compounds allowed treatment of the data for chiral separation (capacity factors) on CTA and CTPB according to these parameters. A linear correlation between In k′(+) and log k′w was found on both CTA and CTPB, as far as the substituents are situated in the plane of the aryl ring or the heterocycle. This correlation with a nonchiral descriptor allows treatment of capacity factors for (-)-enantiomers as deviations from the lipophilicity line or derived parallels. It results in a clear description of the molecular area affecting enantioselectivity. Application to larger alkyl derivatives shows that the effect of the substituent should be treated on a basis of attractive effect in the case of CTA and on the basis of attractive and repulsive effects for CTPB. © 1994 Wiley-Liss, Inc.
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  • 54
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 270-276 
    ISSN: 0899-0042
    Keywords: honey ; D-amino acids ; cyclodextrin columns ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Highly sensitive and accurate HPLC methods were used for the determination of total amounts of proline, leucine and phenylalanine and their enantiomeric ratios in a variety of different honey samples. Significant amounts of D-leucine and D-phenylalanine and relatively low concentrations of D-proline have been found in honeys of different botanical and geographical origins. It is suggested that the enantiomeric ratios of amino acids could be used to test for storage effects, age, and the quality of the processing of the honey. © 1994 Wiley-Liss, Inc.
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  • 55
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 277-282 
    ISSN: 0899-0042
    Keywords: D-amino acids ; D-glutamate ; monosodium glutamate ; food analysis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monosodium glutamate (MSG) is added to many processed foods at significant levels for flavor enhancement. It is also naturally occurring at high levels in some foods. The enantiomeric composition of free glutamate in foods was examined and all foods analyzed were found to contain D-glutamate. The relative percent of D-glutamate in the food products studied depended on the origin of the glutamate. Foods to which MSG was added by the manufacturer had a high total level of MSG but a lower relative percentage of the D-enantiomer (usually less than 0.8%). In comparison, fermented foods tend to have high relative levels of D-glutamate but a lower total amount of the amino acid. The relative percent of D-glutamate in nonfermented foods containing no added MSG was also found to be low compared to fermented products. In some cases the percent D-glutamate could be related to the relative amounts of other food ingredients such as cheese. © 1994 Wiley-Liss, Inc.
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  • 56
    ISSN: 0899-0042
    Keywords: preparative chromatography ; chiral separation ; polarimetric detection ; dual detection technique ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Pure enantiomers of an agrochemical process intermediate, (RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)-pentan-3-one (1), have been prepared on the milligram scale under overload chromatographic conditions on an analytical chiral column (250 × 4.6 mm i.d.). The effects of variation of temperature and mobile phase composition on retention factor, separation factor, and peak resolution have been investigated. Effects of flow rate, enantiomer ratio, sample concentration, and column load on productivity are also studied. Seven milligrams of the less retained (+)-enantiomer and 5 mg of the (-)-enantiomer were obtained from a single injection of 21 mg of (RS)-1. © 1994 Wiley-Liss, Inc.
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  • 57
    ISSN: 0899-0042
    Keywords: SFC ; brush-type chiral stationary phase ; Whelk-O CSP ; pharmaceutical analysis ; chiral analysis ; chiral preparative chromatography ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Good results have been obtained with use of the new bonded chiral stationary phase Whelk-O 1 in analytical and preparative subcritical fluid chromatography. A wide variety of enantiomeric pairs of compounds with different functional groups that are of pharmaceutical and biological interest have been resolved. This Pirkle-concept CSP appears to be more rugged than cellulosic phases (e.g., Chiralcel) with regards to solvents and pressure. In comparing the usefulness of the column for SFC versus HPLC chiral analysis, we have observed a clear superiority of SFC in terms of higher speed and efficiency of analysis, and faster method development. This is consistent with our experience with Chiralcel CSPs. With the Whelk-O 1 we have shown that the effects of temperature and modifier on SFC separations are similar to what has been reported for most other CSPs. We also observed a unique selectivity advantage of SFC for verapamil. We had good success with using a 1-in. diameter column packed with Whelk-O 1 to perform preparative SFC separations of a number of enantiomeric mixtures. The advantages of preparative SFC over preparative HPLC will be discussed. The feasibility of preparative SFC is dependent on how well we meet the practical challenges such as sample introduction issues, special hardware requirements due to the high pressure, and fraction collection issues. © 1994 Wiley-Liss, Inc.
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  • 58
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    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 314-320 
    ISSN: 0899-0042
    Keywords: amphetamine ; distillation method ; clathrate ; optical activation of bases ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The present paper illustrates the development of an advanced technique in optical resolution. Both of the amphetamine enantiomers can be obtained by a two-step distillation in nearly quantitative yield without any loss of the resolving agent. It is proved that the second-order interactions (H-bond) are sufficient for separation of enantiomers by distillation. © 1994 Wiley-Liss, Inc.
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  • 59
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 6 (1994), S. 321-328 
    ISSN: 0899-0042
    Keywords: oxazepam ; 3-O-acyloxazepam ; racemization ; kinetics ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomers of 3-O-acyloxazepam (oxazepam 3-acetate; OXA) underwent base-catalyzed hydrolysis and racemization. Kinetics of reaction products formed from an OXA enantiomer in buffered and unbuffered alkaline solutions were analyzed by chiral stationary phase high-performance liquid chromatography. Racemization occurred with varying rates in aqueous solutions with pH ranging from 7.5 to 14. Racemization mechanism was studied by the dependence of rates of hydrolysis and racemization on temperature and pH. Mass spectral analysis of racemization products derived from an OXA enantiomer in a deuterated solvent indicated that racemization was accompanied by a proton exchange with the solvent. The results indicated that a base-catalyzed keto-enol tautomerism between the C2-carbonyl group and the C3 carbon was responsible for the observed racemization. © 1994 Wiley-Liss, Inc.This article is a US Goverment work and, as such, is in the public domain in the United States of America.
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  • 60
    ISSN: 0899-0042
    Keywords: antiarrhythmic agents ; mbe-lactole ; rsp-cyclodextrine ; preparation of enantiomers ; GE 68 ; pharmacodynamic activity ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An improved synthesis of the enantiomers of the new benzofurane-type antiarrhythmic compound 1 is described, which makes use of the enantiomerically pure mbe-lactol. Thus, acylation of the benzofurane 4 with acetic anhydride and subsequent bromination gave the bromoacetyl-derivative 6, which, after reduction with LiAlH4, was protected with mbe-lactol to give a mixture of the diastereomers 8A and 8b. After separation via column chromatography assignment of absolute configuration was carried out using a well-established NMR- method. Reaction with propylamine and cleavage of the protective group gave (R)-1 and (S)-1, respectively. Enantiomeric purity was confirmed using a direct HPLC method with rsp-cyclodextrine as stationary phase. Pharmacological investigations on isolated guinea pig heart muscle preparations showed that GE 68 and its two enantiomers did not significantly differ from each other with regard to their negative inotropic, negative chronotropic, and lack of β-adrenoceptor blocking action. In contrast, the reference drug propafenone was equally potent in its negative inotropic and chronotropic activity as GE 68, but additionally showed a weak β-adrenoceptor blocking activity. © 1994 Wiley-Liss, Inc.
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