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  • Organic Chemistry  (9,519)
  • 1995-1999  (2,690)
  • 1980-1984  (3,406)
  • 1975-1979  (3,423)
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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: The reaction of bis-silylated ynamines with different nitrosyl reagents affords products derived from an electrophilic attack at the β-carbon atom and not the expected alkynyldiazonium salts.
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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: The recently developed ‘ligated’ technique has proved to be very successful in promoting living anionic polymerization of (meth) acrylic ester monomers. The efficient ligands discovered to date include μ, σ and μ-σ dual types. The roles of these ligands in the control of livingness and stereoregularity of anionic polymerization of (meth) acrylates are discussed in detail. There is convincing evidences that both ligation thermodynamics (electron density redistribution and steric hindrance in the formed active complex) and kinetics (how fast and in which way ligation occurs) are very critical in promoting the livingness. Moreover, steric hindrance appears to be a determining factor maintaining the process ‘living’ under more demanding conditions.
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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 role of ester enolate anions in silicon-mediated polymerization (group transfer polymerization) of methyl methacrylate (MMA) was investigated using cesium, tetrabutylammonium and lithium salts of 9-methylfluorenide as the nucleophilic catalysts. The results of normal anionic polymerization of MMA using these carbanion salts were compared directly with those of analogous GTP procedures with these carbanions in the presence of the silyl ketene acetal initiator 1-methoxy-1-(trimethylsiloxy)-2-methylprop-1-ene (MTS). In the absence of MTS these reactions are characterized by low conversions (14-52%), no control of Mn and broad molecular weight distributions. In the presence of MTS, controlled MMA polymerization is observed with molecular weights calculated based on the moles of MTS and not fluorenide anion. The MMA stereochemistry was also determined for both anionic polymerization and GTP for the same counter ions (tetrabutylammonium, cesium, lithium); within experimental error, the triad tacticities are the same for anionic polymerization and GTP. It is concluded that these results are consistent with the ester enolate anions as propagating intermediates in nucleophile-catalyzed GTP. These results are consistent with an associative GTP mechanism only if it is postulated that the chemistry and stereochemistry of a pentacovalent siliconate anion are the same as those of an ester enolate anion intermediate.
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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: Polymerization of methacrylic esters such as benzyl methacrylate using as initiator an-equimolar mixture of methylaluminium 5,10,15,20-tetraphenylporphine [(TPP)AlMe] and 2,7,12,17-tetramethyl-3,8,13,18-tetramethylporphine [(EtioP)AlMe] proceeded from both initiators, affording a unimodal polymer of narrow molecular weight distribution, although the reactivities of (TPP)AlMe and (EtioP)AlMe are very different from each other. 1H NMR studies on a mixture of two different (porphinato)aluminium enolates, the growing species, and a mixture of a (porphinato)aluminium enolate and an alcoholate, in the absence and presence of methyl methacrylate (MMA or MMA-d8), indicated an acyclic transition state polymerization mechanism by the participation of two aluminium porphyrin molecules, where the growing species always exchange their enolate ligands whenever it grows.
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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: The autopolymerization of styrene in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical), with and without organic acids, such as benzoic acid and camphorsulfonic acid, was studied. Broad polydispersities are obtained in the absence of acid, whereas narrower polydispersities are obtained in their presence. The significance of these results for the synthesis of narrow polydispersity polystyrene by the TEMPO-mediated living polymerization process is discussed.
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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 Kinetics of the polymerization of isopropyl vinyl ether, isobutyl vinyl ether (IBVE) and chloroethyl vinyl ether initiated by HI in CH2Cl2, in the presence and in the absence of tetrabutylammonium iodide as common anion salt, were investigated. The polymerizations proceed directly through the carbon-iodide termini without any necessary electrophilic activator. As previously observed with cyclohexyl vinyl ether, it was found that the addition of small amounts of salt, (0·5-10% with respect to initial [HI], depending on the monomer) drastically reduces the polymerization rate and leads to living-type polymerizations. Higher amounts of salt have no influence on the polymerization rate which remains constant over a broad range of [NBu4I]/[HI] ratios. This general behaviour, observed for the all vinyl ethers, suggests a common ion salt effect and therefore an ionic polymerization mechanism involving ion pairs and free ions. In the absence of salt, both ion pairs and free ions of higher reactivity participate in the propagation, whereas only ion pairs contribute to the propagation in the presence of a common ion salt. According to this scheme, the living character of the polymerization is assumed to result from a propagation reaction governed by undissociated ionic species. The validity and the implications of this reaction scheme, in terms of active centre concentrations, are also discussed.
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  • 7
    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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  • 8
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The relative thermodynamic stabilities of the title compounds were determined by iodine catalyzed chemical equilibration in cyclohexane solution. The main point of interest was a determination of the thermodynamic stability of the —O—C=C—O— moiety found in the exocyclic isomers, i.e. the stabilizing effect of a MeO group on the olefinic linkage of 2-methylenetetrahydrofuran. All three isomeric compounds have essentially similar enthalpy values, which, in comparison with some previous thermodynamic data, shows that the double bond stabilization energy of the MeO group in the exo isomers is only ca 3 kJ mol-1. The entropy difference between the geometrical isomers is negligible, whereas the endo isomer is favoured by an entropy contribution of ca 9 J K-1 mol-1.
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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: On irradiation on a dry silica gel surface, 2-(N, N-dialkylamino)ethyl 3-benzoylacrylates undergo selective E-Z isomerization. Competing processes, such as remote-hydrogen abstraction via a charge-transfer state, with the photoisomerization in solution are completely suppressed on the surface. The yield of the Z - from the E- isomer increases with increase in the coverage of the E-isomer on the surface, reaching a limiting value. Thermal isomerization of the Z- to the E- isomer occurs easily on the silica gels surface when the alkyl group on the nitrogen atom is small. Benzoylacrylates without any amino group also undergo selective photoisomerization.
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  • 10
    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
    Notes: The associationof C6H5O with NO was studied with the cavity-ring-down method by directly monitoring the decay of C6H5O in the presence of varying, excess amounts of NO. The biomolecular rate constant determined in the temperature range 297-373 K can be effectively rate constant determined in the temperature range 297-373 K can be effectively represented by k1 = 10- 12 · 12 ± 0.24e (194±185)/r cm3 molecule-1 with a negative activation energy of 0.8 kcal mol-1 (1 kcal = 4.184 kJ). In order to understand better the mechanism of the reaction, ab initio molecular orbital calculations were also carried out at the MP4(SDQ)/6-31G* level of theory using the HF optimized geometries. The molecular structues and energetics of five C6H5N1O2 isomers were calculated. Among them, the most likely and stable association product, phenyl nitrite (C4H5ONO), was found to be 17 kal mol-1 below the reactants, C6H5O + NO. Combining the measured rate constant and the calculated equilibrium constant for the association reaction, C6H5O + NO = C6H5ONO the rate constant for the unimolecular decomposition of C6H5ONO was obtained as k-1 = 4.6 × 1015E-8580/T s-1. The relatively large frequency factor suggests that a loose transition state was involved in the reaction, akin to those of its alkyl analogs (RONO, R = CH3, C2H5, etc.).
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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 dependence of the amount of water bound to human serum albumin (HSA) suspended in water-dioxane mixtures vs the equilibrium water concentration in the liquid phase was determined by the Fisher method at 298 K. The Langmuir model was used in order to describe the isotherm of the sorption of water by HSA at low water concentrations in the solvent. The calculated equilibrium constant of water adsorption (3.8 ± 0.6 l mol-1) is in good agreement with the adsorption constant obtained earlier from calorimetric data. The comparison of the determined isotherm of water sorption by HSA with the reported enthalpies of suspension formation showed that at low water concentrations in the solvent, water sorption is the only process contributing to the heat effects of the formation of the ‘protein + liquid’ heterogeneous system. From this comparison, the enthalpy of water adsorption by HSA was evaluated as -11.9 ± 1.7 kJ mol-1. At higher water concentrations in the solvent the amount of water adsorbed by HSA increased considerably. This increase in the amount of water on HSA at water activities above 0.5 is assumed to be due to the enlargement of the protein surface area.
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  • 12
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The superaromaticity concept is examined and developed, taking into account what is known about aromaticity. Three new hardness indices indices are defined, and shown to be excellent parameters for characterizing superaromaticity. High superaromaticity indicates significant global annulenoid conjugation relative to local benzenoid conjugation in circularly annealed benzenoid molecules. Kekulene, coronene and corannulene tetraanion all are predicted to be superaromatic. The prediction for the corannulene tetraanion strongly supports the “annulene-within-an-annulene” structure derived from NMR data.
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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: Reactions of diazomethane with butanol, allyl alcohol and β- and γ-halo alcohols led to efficient methylation (giving the corresponding methyl ethers) with the use of a proton-exchanged X-type zeolite compared with H2SO4. The reactions with propylene and isobutylene glycols using the zeolite provided regioselective methylation of the primary OH rather than the secondary or tertiary OH, whereas regioselectivity was not observed in the reactions using H2SO4. The reactions with 2-aminoethanol and 2-mercaptoethanol showed high chemoselective S-methylation and N-monomethylation, respectively, in the presence of the zeolite instead of H2SO4. The mechanism for the reactions is proposed to involve acid-base bifunctional catalysis of the zeolite in which the acidic site reacts with diazomethane to form its conjugate acid, and the nucleophilicity of OH and SH groups is enhanced by the interaction of the basic site with the proton of the groups.
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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: UV spectra of 17 para-Y-substituted styrenes, 8 para-Y-substituted α-methylstyrenes and 20 para-Y-substituted α,β,β-trifluorostyrenes were measured and correlated against spin-delocalization substituent constants (σ.) with confidence levels above 99·9%.
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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: Crystalline host compounds consisting of a roof-shaped dicarboxinide framework and pendant diarylethanol analogous subunits were synthesized and shown to form inclusion complexes with small organic molecules such as alcohols, amines, ketones or polar and apolar organic solvents. Clathrate efficiency and selectivity depend on the particular host structure. The crystal and molecular structures of a free host compound (2a) and inclusion compounds [2a·3-methylcyclohexanone (1 : 1), 1a·3-methylcyclopentanone (1 : 1), 1a·2-methylcyclohexanone (1 : 1), 1b·butyronitrile (1 : 1), 1b·propan-1-ol (2 : 1) and 1b·(-)-fenchone (1 : 1)] were determined by x-ray diffraction analysis. In all the structures, the hydroxyl group is involved in intramolecular hydrogen bonds and the host and guest molecules are held by lattice forces only. The channels and cavities left in the host matrix are large enough to allow disorder or high thermal displacement parameters of the guest molecules. The local packing coefficients for all guests are 0·42 on average.
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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: The alkylation, silylation and other electrophilic reactions of 1-lithio-1,3-bis(2-pyridyl)butane (2a) in THF at -78°C leads to meso-like products that may be accounted for by intramolecular coordination of Li ion with the penultimate 2-pyridyl group. This is also demonstrated by proton-transfer equilibria between 1-lithio-2-ethylpyridine and 2a, allowing the determination of the ΔH and ΔS of intramolecular chelation in 2a. The ΔS of chelation is nearly zero, probably as a result of chelation-induced desolvation of Li ion. Surprisingly, however, the corresponding addition of 2-vinylpyridine to 2a and higher P2VPLi homologs in THF is not stereoselective. In toluene at -78°C the t-BuLi-initiated polymerization of 2-VP is actually somewhat syndiotactic (rr = 0·44). The presence in this polymerization of at least one equivalent of t-BuOLi or other tertiary ROLi surprisingly leads to isotactic P2VP. SEC viscometric and other studies indicate that both the P2VPLi- and P2VPLi-ROLi complexes are monomeric in toluene. The stereochemistry is apparently due to a 1 : 1 P2VPLi-ROLi complex that discriminates between the two pro-chiral faces of the anion by preferntial coordination of the (ROLi2) moiety to the meso-prochiral face. Isotactic-like addition now occurs by electrophilic attack of 2-VP that is syn with respect to the (ROLi2) ‘counter ion.’
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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: The isobutylene (IB)-p-methylstyrene (pMeSt) monomer pair readily produces by living copolymerization nearly random copolymers with up to M̄n ≈ 50,000 g mol-1 and M̄w/M̄n ≈ 1·4. The livingness of the copolymerization process and the compositional homogeneity of the copolymers have been demonstrated by the following four diagnostic plots: (A) M̄n (number-average molecular weight) and corresponding N (number of moles of copolymer formed) vs Wp (weight of copolymer formed) for the diagnosis of chaintransfer; (B) -ln (1 - C) (C = conversion of monomers) vs time for the diagnosis of termination; (C) copolymer composition vs molecular weight; and (D) cumulative copolymer composition vs conversion for the diagnosis of true homogeneous copolymers. The theory for the use of the -ln(1 - C) vs time plot for the quantitation of irreversible termination in copolymerization has been developed. The exact conditions (i.e. nature of reagents, reagent concentrations, temperature, etc.) which lead to desirable products in terms of conversion, molecular weight, molecular weight distribution (MWD) and copolymer composition, were developed systematically by analyzing the results of a large number of experiments. The following combination of chemicals was found to lead to living copolymerization, and uniform high molecular weight, narrow MWD copolymers: IB-pMeSt 97 : 3 mol/mol with 5-tert-butyl-1,3-dicumyl methyl ether (5-tBu-1,3-DiCumOMe) initiator, TiCl4 coinitiator, ethyl chloride (EtCl) solvent, triethylamine (TEA) electron pair donor (ED) and 2,6-di-tert-butylpyridine (DtBP) proton trap at -52, -65 and -85°C.
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  • 18
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Direct 1H, 13C and 19F NMR spectroscopic analysis was carried out on the carbocationic intermediate generated in the interaction of tin(IV) chloride (SnCl4) with the adduct [4, CH3CH(OiBu)OCOCF3] of isobutyl vinyl ether (IBVE) and trifluoroacetic acid (CF3CO2H), either in the presence or in the absence of tetrabutylammonium chloride. The reactions were to mimic the living cationic polymerization of IBVE by the 4-SnCl4 initiating system (with added nBu4NCl) that was also found in this study. In CD2Cl2 solvent at -78°C, the 1H and 13C NMR analysis revealed the formation of a carbocationic intermediate [CH3C+H(OiBu)] that is in a rapid exchange equilibrium with the covalent counterpart 4. For the first time, the cation formation was further supported by 19F NMR analysis on the counteranionic part (CF3COO-) in 4, where, with SnCl4, the CF3 group gave a broadened signal that appeared clearly downfield relative to that in the covalent form. These and other results, which are in close correlation with the corresponding polymerizations, demonstrated the following: (i) the SnCl4-assisted generation of carbocationic species from the covalent ester 4; (ii) a rapid exchange between the cation and its covalent precursor 4; (iii) the relatively high cationic concentration in the salt-free system, where no living polymerization occurs; and (iv) the effective suppression of the cationic species in the presence of the salt, which proved to be the prime key factor for living polymerization.
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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: Oscillations of electrical potential differences across the liquid-liquid interface in phase-transfer catalyst systems are reported. The occurrence of the oscillations is consistent with the Starks' shuttling mechanism.
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  • 20
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The photostimulated reaction of N,N-dibutyl-p-toluenesulfonamide with diphenylphosphide ions in liquid ammonia leads to the corresponding phosphinic amides after oxidation Inhibition experiments with p-dinitrobenzene and 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) and the fact that the reaction is retarded in the dark suggest that it proceeds by the SRN1 mechanism.
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  • 21
    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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  • 22
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 1H and 13C NMR data for 2-diethylaminomethyl-3,4,6-tri- and -tetrachlorophenols in CH2Cl2 solutions at a variety of temperatures are reported. The results are consistent with intramolecular hydrogen bonding and proton transfer as shown in the structure 3. Concentration dependence of the NMR data for the perchloro compound shows an additional hydrogen bonding process attributed to the solute self-association. The 1H, 13C and 15N NMR data support the proposed proton transfer structure.
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  • 23
    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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  • 24
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The molecular and electronic structures of the cyclic 1,2-dithiolium ylides (2) and their acyclic dithiocarbonyl valence isomers (4) were studied by ab initio methods. All molecular structures were fully optimized at the R(U)HF/6-31G* and R(U)MP2/6-31G* levels of theory. The results of the restricted and unrestricted Hartree-Fock (HF) calculations are less reliable since the RHF wavefunctions of the cyclic compounds proved to be singlet/triplet unstable. A marked ground-state energy depression occurs on passing from restricted to unrestricted HF methods. The title compounds are of diradicaloid nature and belong to the non-classical class of structures. The heterocycles are planar and display C2v symmetry, while the cisoid open-chain dithiocarbonyl isomers are slightly distorted (C2 symmetry). According to the calculated bond lengths and charge distributions, the title compounds are in fact more or less zwitterionic. The thiolate is the most polar compound of this series. According to RMP2 calculations, the thiolate (2, X = S) and olate (2, X = O) should exist in the single ground rather than in the triplet state. However, in these cases the isomeric acyclic compounds are predicted to be almost isoenergetic with the cyclic compounds. Since the cyclic ylides have higher dipole moments than the acyclic dithiocarbonyls, the ylides will gain stability in polar solvents. However, the continuum quantum chemical model used in estimating the solvent effect does not favour the ring compounds sufficiently to exclude valence isomerization between the ring-closed and ring-open compounds.
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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: As a rebuttal to recent criticisms aimed at the mechanistic significance of the determination of the mesitylene/durene relative reactivity, it is argued that this ratio does provide reliable information about the structure of the transition state of many SEAr reactions, supporting the operation of the ‘conventional’ ionic mechanism, as opposed to the ET mechanism. Not even in the case of competitive experiments does it appear that the criticisms are justified.
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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: Empirical solvent polarity parameters ET(30) were determined by UV-visible spectrophotometry using Dimroth- Reichardt's betaine dye, as a function of composition, for (aprotic + aprotic) and (aprotic + protic) binary solvent mixtures. For (aprotic + aprotic) solvent systems the cosolvent was toluene, and the other solvents used were selected with different structural characteristics and an extensive range of polarity: chloroform, 1,4-dioxane, ethyl acetate, tetrahydrofuran, acetone, nitromethane and N,N-dimethylformamide. For (aprotic + protic) solvent systems, the protic cosolvent used was methanol, and the aprotic solvents selected were toluene, chloroform, 1,1,1-trichloroethane, tetrahydrofuran, acetone, N,N-dimethylformamide, and dimethyl sulphoxide. Each system was analysed according to its deviations from additivity due to selective solvation of the betaine. A preliminary application of these empirical solvent polarity parameters was related to the solvent effects in a simple example of a nucleophilic aromatic substitution reaction.
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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: Octaethyltetrathiaporphyrin dication, OTP2+, is electrochemically reduced by two successive electron-transfer processes to OPT0. OPT2+ forms intermolecular complexes with a series of π-donors. The association constants of the resulting complexes are controlled by the oxidation potentials of the π-donors and solvent properties. OTP2+ was incorporated into a Nafion membrane cast on a Pt electrode. Formation of OTP2+-π-donor complexes in the polymer membrane was followed by electrochemical and spectroscopic means. The association constants of the OTP2+-π-donor complexes in the Nafion membrane exhibit lower values than a homogeneous phase owing to the high ionic strength in the polyelectrolyte.
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  • 28
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The original Hammett equation, Δ = ρσ, is transformed in a constrained tetralinear relationship where each straight line with variable intercept term correlates one of the following four groups or subsets of dipolar substituents: normal and special substituents (depending on the absence or the presence of a lone electron pair in their atom next to the aromatic ring) and, in each of these classes, separating meta and para derivatives. There are a total of four fitting parameters in the resulting plurilinear Hammettian transformation (PHT) from which the statistically corrected parameters λ and γ are derived; λ and γ are the asymptotic values in a hyperbolic model for the representation of Δ4 vs Δ4/Δ3. This meta-para interrelationship is assumed to hold in the absence of through-resonance effects which, in turn, are allowed for by the use of alternative sigma scales of substituent constants. By applying the PHT to a large number of selected literature data, parameters λ and γ are determined for the ionization equilibria of 3- and 4-monosubstituted benzoic acids, anilinium ions, phenols and pyridinium ions. In these reactions series, parameter λ, which measures the para/meta ratio of field/inductive effects, is lower than unity and shows a marked dependence on the basic molecular framework. It is best modelled in terms of a through-space field effect approach. The ratio γ/γ0, where γ0 is referred to the unified sigma-zero scale, is shown to correspond to the original Hammett's reaction constant ρ. It is concluded that the PHT constitutes an improved Hammett equation for the analysis of substituent effects in benzene derivatives taking into account statistical errors and making allowance for different transmission coefficients for the field/inductive effect from meta and para positions in different reaction series.
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  • 29
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 8 (1995), S. 696-698 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: peri-Bridged naphthalenes in which the C-1 and C-8 atoms of the naphthalene ring are bonded to a single atom in a bridging group Z (Z = CH2, CHBr, C=O, NH, O, S, S=O and SO2) were studied by the AM1 semi-empirical SCF MO method. Although in peri-naphthalenes bridged by carbon or sulfur the naphthalene structure is planar, the corresponding nitrogen- or oxygen-bridged molecules are distorted about 15 and 23°, respectively. There are significant variations (from 92 to 106°) of the C-1-C-9-C-8 angle that is calculated in the cycle formed by the bridge. The calculated strain-energy barrier for pyramidal atom inversion in amine 4 and sulfoxide 7 are 32·5 and 174 kJ mol-1, respectively.
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  • 30
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 1H, 13C and 15N NMR data for nitraminopyridines are discussed in terms of tautomeric equilibria in these compounds. The favoured tautomer is determined mainly from 15N NMR spectroscopy. The chemical shift of the nitrogen atom of the nitro group in nitraminopyridines and N-nitroanilines which cannot tautomerize vary from 28·0 to 35·4 ppm in DMSO solution. 3-Nitraminopyridine and 2-nitramino-3- and -5-nitropyridines behave similarly. In the 15N NMR spectra of nitrimino-1-methyldihydropyridines, used as models, an upfield shift of that atom, different from that observed for 2-nitraminopyridine, indicates the significance of the nitrimino tautomer. In contrast, a downfield shift of the ring nitrogen atom of some other compounds shows an increased weight of the nitramino tautomer. 3-Nitraminopyridine, when dissolved in DMSO, does not have the zwitterionic structure. Changes in the chemical shift of the amino nitrogen atom are not readily interpreted. Some proton and carbon chemical shifts and hydrogen-hydrogen and carbon-hydrogen spin-spin coupling constants can also be used to determine the predominant tautomer of 2- and 4-nitraminopyridines.
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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: Ab initio studies on the SN2 identity exchange reactions RCH2X + X- → X- + RCH2X for R = CH2CH with X- = H, NH2, OH, F, PH2, SH and Cl, and for R = CH3 and CH≡C with X- = Cl were carried out at the HF and MP2 levels using the 6-31 ++ G** basis sets. The activation barriers, ΔE≠, and major structural changes, Δd≠ (C-X), in the activation process are closely related to the electronegativity of the R and X groups. The effect of electronegativity is twofold: a stronger electronegativity of R and/or X leads to a lesser electronic as well as structural reorganization required in the activation and to a greater correlation energy in the transition state. The former effect lowers the energy barriers at both the HF and MP2 levels whereas the latter lowers only the correlated (MP2) activation energies. Results with R = CH2CH, as a model for R = C6H5, indicate that ‘benzylic effect’ arises mainly from the relatively stronger electron acceptor ability of the phenyl group.
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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: Rates of acid-catalysed hydrolysis of phenyldiazoacetic acid were measured in aqueous solution at 25°C across the entire acidity range pCH+ = 1-13. The reaction was found to occur through both the non-ionized and inonized forms of the substrate, with the ionized form reacting faster by a factor of 650. Appreciable solvent isotope effects in the normal direction (kH/kD 〉 1) and the occurrence of general acid analysis show that hydrolysis occurs by rate-determining proton transfer to the diazocarbon atom of the substrate. Analysis of the rate profile for the reaction provides an estimate of the acidity constant of phenyldiazoacetic acid, pKa = 3·70, which is apparently the first determination of the acid strength of an α-diazocarboxylic acid. Comparison of this result with the pKa of phenylacetic acid shows the diazo group to have a mild (4×) acid-strengthening effect, whose modest magnitude is attributed to opposing resonance and polar effects.
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  • 33
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    Journal of Physical Organic Chemistry 8 (1995), S. 742-746 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The equilibrium structures of cis- and trans-1,3-diphosphetane were evaluated at the RHF/6-31g** level of optimization, with MP2 electron correlation correction. Both structures are puckered, the trans isomer being slightly lower in energy than the cis isomer. The inversion barrier in 1,3-diphosphetane is lower in energy than in phosphetane.
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  • 34
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    Journal of Physical Organic Chemistry 8 (1995), S. 761-773 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Chiral polymethine dyes constitute a particular class of conjugated π systems. Artificial chiral polymethine dyes were first synthesized by König and Langbein in 1928 in order to clarify the electronic structure of cyanine dyes; some natural dyes such as musca-aurin I (from the toadstool fly agaric) and betanin (from the vegetable red beets) are known to be chiral pentamethinium cyanine dyes, the chirality of which stems from L-α-amino acids as end-groups. The reasons why chiral polymethine dyes are not only of historical interest but once again the subject of research are explained.
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  • 35
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The photolysis and thermolysis of 7,8-diaza-3-oxatricyclo[4.2.1.02,5]non-7-enes were studied. Photolysis showed a novel [4π + 2π + 2π] cycloreversion in a formal way, the first example in azoalkane chemistry. This cycloreversion was found to be sensitized by the benzophenone produced. In contrast, thermolysis showed the typical decomposition pattern of azoalkanes, extrusion of nitrogen and subsequent σ-bond formation of the resulting biradical to afford the tricyclic oxetanes
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  • 36
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    Chirality 8 (1996), S. 351-351 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 37
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    Chirality 8 (1996), S. 353-353 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 38
    ISSN: 0899-0042
    Keywords: NSAIDs ; esterification ; resolution ; Candida antarctica lipase ; Mucor miehei lipase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The resolution of rac-suprofen (1) catalysed by lipase in organic solvents was investigated. Direct esterification of rac-1 with methanol in dichorometane catalysed by Novozym® 435 furnished the pharmacologically active (+)-(S)-suprofen as unreacted product with excellent enantiomeric excess. The same procedure in toluene using Mucor miehei lipase adsorbed in Celite as catalyst afforded (-)-(R)-suprofen with good optical purity. © 1996 Wiley-Liss, Inc.
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  • 39
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    Chirality 8 (1996), S. 357-363 
    ISSN: 0899-0042
    Keywords: excited state ; CPL spectroscopy ; CD spectroscopy ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 40
    ISSN: 0899-0042
    Keywords: 13C NMR ; chiral recognition difference ; elution order ; cellulose tris(4-methylbenzoate) ; 1-phenylethanol ; 1-indanol ; 1,2,3,4-tetrahydro-1-naphthol ; glutethimide ; Tröger's base ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To elucidate chiral recognition difference between pairs of enantiomers which interacted with a cellulosic stationary phase of chiral discrimination high performance liquid chromatography (HPLC), 13C NMR spectroscopy was applied for enantiomers of 1-phenylethanol, 1-indanol, 1,2,3,4,-tetrahydro-1-naphthol, glutethimide, and Tröger's base. Their 13C nuclear spin-spin relaxation times (T2Cs) and spectra were obtained in deuterated HPLC solvents in the presence of column packing material made of cellulose tris(4-methylbenzoate) coated on silica gel. The first-eluted enantiomers on the HPLC showed longer T2Cs and stronger signal intensities of 13C NMR spectra than the second-eluted enantiomers. These results indicated that the chiral recognition difference of enantiomers was observed by their T2Cs and intensities of 13C NMR spectra. The T2C difference was found to reflect the retention order of enantiomers on the chiral HPLC column. © 1996 Wiley-Liss, Inc.
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  • 41
    ISSN: 0899-0042
    Keywords: aminotetralin ; catechol group ; homochiral derivatization ; (+)-(R)-1-phenylethyl isocyanate ; enantiomers ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new chiral derivatization procedure for the HPLC resolution of chiral catecholamines and structurally related compounds is described. The homochiral reagent, (+)-(R)-1-phenylethyl isocyanate (RPEIC), was added to separate and quantitate the enantiomers of rac-5,6-dihydroxy-2-methyl-aminotetralin, the main metabolite of rac-5,6-diisobutyryl-2-methyl-aminotetralin, a potent dopamine agonist, by reversed-phase HLPC analysis. To avoid catecholamine degradation in the basic reaction medium and to obtain the selective and quantitative derivatization of the amino group of the compound, the reversible complex formation between diphenylborinic acid (DPBA) and the catechol group, in alkaline medium, was performed before homochiral isocyanate addition. The RPEIC derivatization was completed in 30 min and then the DPBA complex was dissociated by adding dilute acid. The structure of intermediates and urea derivatives was confirmed by mass spectrometry. The use of an electrochemical detector, operating in redox mode, allowed HPLC quantitation of enantiomers at the nanogram level in plasma and urine. The derivatization procedure is also suitable for other catecholamine-related compounds. © 1996 Wiley-Liss, Inc.
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  • 42
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    Chirality 8 (1996), S. 408-408 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 43
    ISSN: 0899-0042
    Keywords: N-ethyl-N-methylaniline N-oxide ; chiral nitrogen centre ; flavin-containing monooxygenase ; stereoselective metabolic N-oxidation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The prochiral tertiary amine N-ethyl-N-methylaniline (EMA) is known to be stereoselectively N-oxygenated in the presence of hepatic microsomal preparations. This biotransformation is thought to be mediated predominantly by the flavin-containing monooxygenase (FMO) enzyme system. In order to characterise this reaction further, the in vitro metabolism of EMA in the presence of hepatic microsomal preparations derived from a number of laboratory species has been examined. EMA N-oxide formation was stereoselective with respect to the (-)-S-enantiomer in the presence of microsomal preparations from all species examined, with the degree of selectivity decreasing in the order of rabbit 〉 rat ∼ LACA mouse ∼ DBA/2Ha mouse 〉 guinea-pig 〉 dog. The enantiomeric composition of the metabolically derived EMA N-oxide appeared to be determined solely by the differential rate of formation of the two enantiomers as opposed to any differences in affinities for the substrate in its pro-R and pro-S conformations. The use of enzyme inhibitors, activators and inducers indicated that EMA N-oxide formation was predominantly mediated by FMO in the presence of rabbit hepatic microsomes and that these agents did not generally affect the stereochemical outcome of the biotransformation. © 1996 Wiley-Liss, Inc.
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  • 44
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    Chirality 8 (1996) 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 45
    ISSN: 0899-0042
    Keywords: chirality of agonist-efficacy ; β-adrenoceptor ; stereoselective change of receptor ; epinephrine enantiomers ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The presence of β-adrenoceptor in a rat lung membrane preparation was confirmed by stereoselective competition of enantiomers of epinephrine with labeled iodocyanopindolol. The receptor-rich protein fraction, when combined with the pharmacologically active (-)-epinephrine, exhibited specific changes in the 205-220 nm region of circular dichroism spectra, indicating that the receptor helices may be perturbed. The (+)-epinephrine combined with the lung protein produced little or no change of the spectra. Bovine serum albumin, when combined with either enantiomer of epinephrine, produced nonstereoselective alterations of the spectra. Thus, the data provide important evidence for the higher intrinsic pharmacologic activity of the natural (-)-epinephrine over the unnatural (+)-enantiomer. © 1996 Wiley-Liss, Inc.
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  • 46
    ISSN: 0899-0042
    Keywords: fluorogenic alkyldiacylglycerols ; albumin glycerolipid interaction ; microbial lipases ; lipoprotein lipase ; cutinase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new type of fluorogenic alkyldiacyl glycerols was synthesized and used as fluorogenic substrates for the analysis of lipase activities and stereoselectivities. These compounds contain perylene as a fluorophore and the trinitrophenylamino (TNP) residue as a quencher. Both substituents are covalently bound to the ω-ends of the sn-2 and sn-1(3) acyl chains, respectively. Upon glycerolipid hydrolysis, the residues are separated from each other thus allowing determination of lipase activity by the continuous increase in fluorescence intensity which is caused by dequenching. Using enantiomeric pairs of these compounds, we were able to analyze lipase stereoselectivity depending on the reaction medium. Mixtures of enantiomeric fluorogenic alkyldiacyl glycerols, selectively labelled with pyrene or perylene as fluorophores, can be used for a dual-wavelength “stereoassay” of lipases. Since absorption and emission maxima of both labels are clearly separated, hydrolysis of the respective enantiomeric substrates can be determined simultaneously, and the difference in the rates of hydrolysis can be taken as a parameter for the stereopreference of a lipase. Hydrolysis rates measured with perylene-substituted lipids are generally lower than those obtained with the pyrene analogs. Thus, with a mixture of perylene and pyrene-substituted lipids, we observe a higher apparent stereoselectivity of lipases since we measure a combination of stereo- and substrate selectivity. In the presence of albumin, all microbial lipases tested so far exhibit stereopreference for the sn-1 glycerol position. In our assay, the apparent stereoselectivities are highest if in the presence of albumin, the sn-1 position carries pyrene and the sn-3 position is substituted with perylene. The lipase stereoselectivity assay described here requires the simultaneous measurement of the fluorescence intensities at two different wavelengths in a single cuvette and can thus be carried out using existing and cheap instrumentation that was developed for the fluorimetric analysis of Ca++ concentrations. © 1996 Wiley-Liss, Inc.
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  • 47
    ISSN: 0899-0042
    Keywords: hydrolysis ; carboxylesterase ; bioconversion ; plasma concentration ; pharmacokinetics ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The in vitro and in vivo stereoselective hydrolysis characteristics of the mutual prodrug FP-PPA, which is a conjugate of flurbiprofen (FP) with the histamine H2-antagonist PPA, to reduce gastrointestinal lesions induced by FP were investigated and compared with those of FP methyl ester (rac-FP-Me) and FP ethyleneglycol ester (rac-FP-EG). The rac-FP derivatives were hydrolyzed preferentially to the (+)-S-isomer in plasma and to the (-)-R-isomer in liver and small intestinal mucosa. Interestingly, in the gastric mucosa, the stereoselectivity of hydrolysis of (-)-R-FP-PPA was opposite from that of rac-FP-Me and rac-FP-EG, which suggested that the stereoselective hydrolysis of FP-PPA was helpful in reducing gastric damage induced by (+)-S-FP. However, hydrolysis of all rac-FP derivatives was found to be catalyzed by carboxylesterases in the gastric mucosa. The stereoselective disposition of FP enantiomers early after intravenous administration of rac-FP-PPA could be explained by the stereoselective formation of (-)-R-FP from rac-FP-PPA in the liver. (-)-R-FP-PPA was completely hydrolyzed to form (-)-R-FP in vivo, while 78% of (+)-S-FP-PPA was hydrolyzed to (+)-S-FP, with a corresponding decrease in the area under the curve. Twenty-five percent of (+)-S-FP-PPA might be eliminated as the intact prodrug or its metabolites other than FP. The most important bioconversion of FP-PPA occurred in plasma, and additional hydrolysis of the R-enantiomer in liver resulted in the stereoselectivity observed following both i.v. and p.o. administration. © 1996 Wiley-Liss, Inc.
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  • 48
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    Chirality 8 (1996), S. 525-530 
    ISSN: 0899-0042
    Keywords: temazepam ; 3-O-methyltemazepam ; 3-O-ethyltemazepam ; stereoselectivity ; nucleophilic substitution ; racemization ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomeric 3-O-methyltemazepam and 3-Oethyltemazepam were highly stereoselectively substituted by the 3-methoxy group of methanol in acidic anhydrous methanol and by the 3-ethoxy group of ethanol in acidic anhydrous ethanol, respectively. The stereoselectivity of the homonucleophilic substitution reactions was determined by circular dichroism spectropolarimetry and gas chromatography-mass spectrometry. In anhydrous solutions containing 0.5 M D2SO4 at 50°C, for example, the stereoselectivity was ∼63:1 for enantiomeric 3-O-methyltemazepam in CD3OD and ∼94:1 for enantiomeric 3-O-ethyltemazepam in C2D5OD. The high stereoselectivity at C3 position was primarily due to the presence of a methyl group at N1 position. © 1996 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.
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  • 49
    ISSN: 0899-0042
    Keywords: felodipine ; retention model ; micellar mobile phases ; chiral resolution ; CHIRAL-AGP ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A retention model for the chiral separation of an uncharged solute, felodipine, on CHIRAL-AGP, using a micellar mobile phase is proposed. The model assumes the presence of two stereoselective sites and each enantiomer was found to interact with different sites. Addition of a chiral aliphatic alcohol, (+)-(S)-2-octanol, preferentially interacted with the binding site for (-)-(S)-felodipine. The monomeric form of the micellar agent (Tween® 20) competed with the enantiomers for the adsorption sites, and the formation of a 1:1 complex between the enantiomers and the micelles was assumed. The retention of the solutes was effectively controlled by adding small quantities (〈1.63 × 10-3 M) of the nonionic detergent Tween 20 to the mobile phase. Baseline separation was achieved by addition of 1.0 mM n-octylamine to the mobile phase; 8.14 × 10-4 M Tween 20 in phosphate buffer pH 7.0. The separation factor (α = 1.74) was unaffected by the detergent concentration in the presence of 1.0 mM n-octylamine. © 1995 Wiley-Liss, Inc.
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  • 50
    ISSN: 0899-0042
    Keywords: tetrahydroisoquinoline alkaloids ; optical isomers ; enantiomeric separation ; β-cyclodextrin ; high-performance liquid chromatography ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Tetrahydroisoquinoline alkaloids, which are known to be present not only in plants but also in animals, including mammals, can be considered as condensation products of 2-phenylethylamines (e.g., catecholamines) with aldehydes (e.g., acetaldehyde) or 2-oxo acids (e.g., pyruvic acid). In this study the possibility of separating the optical isomers of several tetrahydroisoquinolines by high-performance liquid chromatography was investigated. For isosalsoline, tetrahydropapaveroline and laudanosoline a good enantiomeric separation could be achieved by applying β-cyclodextrin-bonded silica as stationary phase in connection with various mobile phases. With respect to laudanosoline, the addition of β-cyclodextrin as chiral selector to the mobile phase using a C18 reversed-phase column as stationary phase revealed an even higher resolution when compared with the chiral columns. All tested tetrahydroisoquinolines which could be well separated into enantiomers bear a hydroxyl group at carbon atom 7 as a common structural feature. Those alkaloids substituted with a methoxy group on position 7 instead of a hydroxyl group (e.g., salsolidine) failed to be resolved into their optical isomers. Therefore, the presence of a hydroxyl group on C7 of the aromatic ring seems to be conducive to steric discrimination. However, the separation results for 1-carboxysalsolinol were unsatisfactory although this molecule possesses a 7-hydroxyl group. In this case the existence of a carboxyl group on C1 reduced the chiral recognition and thus the enantiomeric resolution. © 1995 Wiley-Liss, Inc.
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  • 51
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    Chirality 7 (1995), S. 40-43 
    ISSN: 0899-0042
    Keywords: enantioselectivity ; pig liver esterase ; hydrolysis ; oxazepam ; chiral stationary phase HPLC ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A widely utilized pig liver esterase preparation has been found to be derived essentially exclusively from the cytosolic fraction of pig livers. Esterases in cytosol and microsomes prepared from a fresh pig liver hydrolyzed the S- and R-enantiomers of racemic oxazepam 3-acetate (rac-OXA) with specific activity ratios of approximately 2.3:1 and 1:62, respectively. Product formations were analyzed by chiral stationary phase high-performance liquid chromatography. The commercial pig liver esterase preparation showed greater activity toward S-OXA than did the esterases in the cytosolic fraction prepared from fresh pig liver. The results established that (i) esterases contained in microsomes and cytosol of pig liver have opposite enantioselectivity in the hydrolysis of rac-OXA and (ii) the commercial pig liver esterase preparation has a cytosolic origin. © 1995 Wiley-Liss, Inc.
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  • 52
    ISSN: 0899-0042
    Keywords: ibuprofen ; loxoprofen ; CS-670 ; loxoprofen and CS-670 trans-alcohols ; enantiomer ; diastereoisomer ; chiral HPLC ; chiral recognition ; NMR ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An optical resolution of the amide derivatives of ibuprofen and the carbamate-alkylester derivatives of the trans-alcohol metabolite of loxoprofen and an analogous compound, CS-670, was studied by chiral high-performance liquid chromatography (HPLC). The chiral columns SUMIPAX OA-4000 and OA-4100 were used to investigate the enantiomeric separation behavior of these derivatives using both reversed and normal mobile phases. A better separation factor (α) of the amide and the carbamate ester derivatives was obtained in the normal mobile phase than in the reversed mobile phase HPLC. In addition, the recognition mechanisms of both amide and carbamate ester enantiomers were investigated by 1H-nuclear magnetic resonance (NMR). It is suggested that the important driving forces for the enantiomeric separation are the formation of hydrogen bonding and the charge transfer complex between these derivatives and an active site of the chiral stationary phase. © 1995 Wiley-Liss, Inc.
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  • 53
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    Chirality 7 (1995), S. 34-39 
    ISSN: 0899-0042
    Keywords: 1,4-benzodiazepines ; oxazepam ; 3-O-methyloxazepam ; 3-O-ethyloxazepam ; stereoselectivity ; racemization ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Kinetics of acid-catalyzed heteronucleophilic substitution and racemization of enantiomeric MeOX in ethanol and enantiomeric EtOX in methanol were studied by quenching reaction products at various times by neutralization. Enantiomeric contents of remaining substrate and reaction product were determined by chiral stationary phase high-performance liquid chromatography. The experimental procedure allowed the determination of the stereoselectivity (i.e., the enantiomeric ratio of a substitution product formed from an enantiomerically pure substrate) involved in the heteronucleophilic substitution reactions. The stereoselectivity was found to vary between 58:42 and 87:13, depending on the acid concentration, substrate, solvent, and temperature. The enantiomeric purity of remaining substrates was identical to that of the starting substrate, indicating that the enantiomeric substrates did not undergo a ring-opening reaction. The results provided additional evidence supporting the mechanism proposed earlier in acid-catalyzed stereoselective heteronucleophilic and homonucleophilic substitutions and the resulting racemization of enantiomeric 3-alkoxy-1,4-benzodiazepines in alcoholic solvents. © 1995 Wiley-Liss, Inc.
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    Chirality 7 (1995) 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 55
    ISSN: 0899-0042
    Keywords: enantioselective HPLC ; cellulose benzoates ; enantiomeric resolution ; warfarin ; ketoprofen ; lorazepam ; lormetazepam ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to generalize the recently described method for the insolubilization of polysaccharide derivatives to benzoates of cellulose, five mixed 10-undecenoate/benzoates of this polysaccharide have been prepared and linked to allyl silica gel by means of a radical reaction. The chiral recognition ability of the resulting materials when used as high-performance liquid chromatographic chiral stationary phases was evaluated using heptane/2-propanol and heptane/chloroform mixtures as mobile phases. Chirality 9:145-149, 1997. © 1997 Wiley-Liss, Inc.
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  • 56
    ISSN: 0899-0042
    Keywords: chiral separation ; β-cyclodextrin derivatives ; capillary electrophoresis ; vicinal diols ; borate complexation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Capillary electrophoretic enantioseparation of compounds containing vicinal diol groups have been investigated using different β-cyclodextrin (CD) derivatives and borate as a background electrolyte. Both native β-CD and several β-CD derivatives are examined. Chiral recognition is attributed to both enantioselective inclusion of the analyte into the chiral cavity of the CD and complexation with borate. The influence of concentration of the chiral selector, pH, and organic modifiers on the resolution was studied. Four diols were baseline separated. Chirality 9:153-156, 1997. © 1997 Wiley-Liss, Inc.
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  • 57
    ISSN: 0899-0042
    Keywords: chiral high-performance liquid chromatography ; polysaccharides ; structure-retention relationships ; amides ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of enantiomeric amides have been chromatographed on three amylose-based chiral stationary phases (CSPs): amylose tris(3,5-dimethylphenylcarbamate) (AD-CSP), amylose tris (S-phenylethylcarbamate) (AS-CSP), and amylose tris(R-phenylethyl-carbamate) (AR-CSP). The relative retentions and enantioselectives of the solutes on the three CSPs were compared and basic structure-retention relationships developed to describe the chromatographic results. The data indicate that for these solutes the observed elution order was a function of the chirality of the amylose backbone, while the magnitude of the enantioselective separations was affected by the chirality of the carbamate side chain. Chirality 9:173-177, 1997. © 1997 Wiley-Liss, Inc.
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    Chirality 7 (1995), S. 74-81 
    ISSN: 0899-0042
    Keywords: polymorphism ; pseudoracemate ; optical resolution ; crystallisation ; solid solution ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solubility behaviour of praziquantel enantiomeric system at -15°C, 4°C, and 15°C is presented. This system exhibits both a pseudoracemate behaviour and one which leads to the formation of enantiomerically pure crystals. For solutions with lower enantiomeric purity, the pseudoracemate behaviour is more prominent. Solid solutions are formed when the mixtures of the enantiomers are crystallised. The phase diagram shows a complete absence of a eutetic. For mixtures with higher enantiomeric purity, pure crystals of the excess enantiomer are obtained when the solution is crystallised. Temperature dependency of this polymorphism is not significant. With these phase diagrams, a method is suggested to obtain enantiomerically pure isomers of praziquantel from a partially resolved mixture. © 1995 Wiley-Liss, Inc.
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  • 59
    ISSN: 0899-0042
    Keywords: chiral selectors ; natural product derived hosts ; optically active hosts ; clathrate hosts ; bulky diols ; inclusion compounds ; host-guest complexes ; chiroselective cocrystallization ; crystal packing analysis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New chiral host molecules 1 and 2 involving a bulky terpenoid unit and aromatic ethyne spacer groups were synthesized using Pd-catalyzed cross-coupling reactions of (+)-2α-ethynyl-2β-hydroxybornane (4) or (+)-2α-ethynyl-2β-hydroxyfenchane (5) with 9,10-dibromoanthracene. Host compounds 1 and 2 from crystalline inclusions with 1-methoxy-2-propanol (3) in 1:1 and 1:2 stoichiometry, respectively. In the case of 1, complete enantiomer resolution (ee 〉 99%) of 3 is effected in one cocrystallization step. However, constitutional isomer 2 failed in the enantiomer separation of 3, which might be explained due to the different crystal lattice buildup of these cocrystals. Chirality 9:203-210, 1997. © 1997 Wiley-Liss, Inc.
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    Chirality 9 (1997), S. 233-237 
    ISSN: 0899-0042
    Keywords: non-covalent molecular imprinting ; Scatchard plot ; batch rebinding ; site specificity ; boc-amino acids ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A potential problem associated with molecularly imprinted polymer (MIP) sorbents is that of cross-reactivity. In this study three MIPs (imprinted with Boc-L-phenylalanine, Boc-L-alanine, Boc-L-glutamic acid) plus a non-imprinted control were prepared and examined for their ability to bind differentially the enantiomers of boc-protected phenylalanine in an effort to quantify cross-reactivity and to develop a predictive model. Batch rebinding studies showed a degree of predictability for a number of MIP-ligand pairs, but other combinations showed unexpectedly high levels of cross-reactivity. Despite the general acceptance of heterogeneity of MIP binding sites, many previous studies report linear Scatchard plots, which is indicative of binding site homogeneity. The data from this study produced curves, clearly demonstrating heterogeneity. The theoretical and practical implications of this heterogeneity are discussed. Chirality 9:233-237, 1997. © 1997 Wiley-Liss, Inc.
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  • 61
    ISSN: 0899-0042
    Keywords: chiral polymethine dyes ; circular dichroism ; helicity ; helicity rules ; cis-peak ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis, chiroptical properties, and quantum-mechanical calculations of the monomethine dye 3 and of the trimethine dyes 7 and 11 are reported. In 3 and 11, the chromophore is forced into a twisted all-Z-conformation by steric interaction of the end groups in the former and the presence of a t-butyl group in the mesoposition of the latter, which is manifest in the UV/Vis spectra not only in the reduced intensity of the longest wavelength absorption, but also in the occurrence, at shorter wavelength, of a “cis-peak.” Chiral substitution of the end groups serves as chiral anchor to discriminate between otherwise enantiomeric forms and makes them amenable to chiroptical investigation. The results are in agreement with theoretically calculated chiroptical data based on helically twisted cyanine chromophores. They support the contention that not only the sense of the helix, but also its length determines the sign of the associated Cotton effect. Chirality 9:243-249, 1997. © 1997 Wiley-Liss, Inc.
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  • 62
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    Chirality 7 (1995), S. 90-95 
    ISSN: 0899-0042
    Keywords: antithrombotic nipecotamides ; platelet aggregation inhibitors ; resolution of optical isomers ; stereoisomer synthesis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereoisomers of α,α′-bis[3-(N,N-diethylcarbamoyl)-piperidino]-p-xylene (1) were synthesized. Rac ethyl nipecotate was resolved by diastereomeric (-)-D- and (+)-L-tartrate salt formation. The enantiomeric esters were hydrolyzed to the corresponding nipecotic acids, which were then converted into t-BOC derivatives. Treatment of the latter with diethylamine/isobutyl chloroformate and removal of the t-BOC protecting group afforded (R)- and (S)-N,N-diethylnipecotamides. Condensation of the latter with α,α′-dibromo-p-xylene gave (R,R)- and (S,S)-1. The meso-diastereomer was obtained by stereospecific synthesis in addition to our earlier procedure involving fractional crystallization of the diastereomeric mixture obtained by synthesis. The latter was resolved earlier into 1A,1B, and 1C using chiral high-performance liquid chromatography (HPLC). Based on the stereospecific synthesis now achieved, 1A and 1B are assigned the configurations, (R,R) and (S,S) respectively, and 1C is assigned the meso configuration. The (R,S) structure of the latter is also confirmed by X-ray crystallography. © 1995 Wiley-Liss, Inc.
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  • 63
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    Chirality 7 (1995) 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 64
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    Chirality 7 (1995), S. 121-127 
    ISSN: 0899-0042
    Keywords: cyclic formyl-monoterpenes ; tricyclic formyl-sesquiterpenes ; homogeneous catalyst ; rhodium-phosphine catalyst ; platinum-phosphine-tin(II)chloride catalyst ; hydroformylation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hydroformylation of (+)-(R)-limonene (1), (+)-(1R)-isolimonene (2), camphene (3), and (+)-β-cedrene (4) resulted in the regiospecific formation of the corresponding linear aldehyde in the presence of homogeneous platinum and, in some cases, rhodium catalysts. The epimeric composition could be influenced slightly by optically active catalysts formed with chiral bidentate phosphines. The relative configurations of newly formed stereogenic centers were analyzed by 1H and 13C NMR. The primarily formed aldehydes of (+)-8(15)-cedren-9-ol (5) and γ-gurjunene (6) underwent cyclization, giving a mixture of epimers of tetracyclic terpenes. Despite the moderate diastereoselection, the products are of practical importance due to their highly regioselective formation. © 1995 Wiley-Liss, Inc.
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