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  • Chemistry  (10)
  • 2010-2014
  • 1990-1994  (10)
  • 1965-1969
  • 1993  (10)
  • 1
    Electronic Resource
    Electronic Resource
    Degradation of Cephaloridine on Alkaline Hydrolysis (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 1619-1625 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A kinetic study on the alkaline hydrolysis of cephaloridine (1) at pH 10.5 and 37° was carried out using ion-pair reversed-phase HPLC. The main resulting degradation products, the 7-epimer 2 of 1, the Δ2-isomer 3 of 1, and the 3-methylidene compound 4 were identified. The presence of a pyridinio group at C(31) results in a slightly increased formation constant for the 3-methylidene compound 4 and the 7-epimer 2, and introduces a new reaction: the isomerization of the double bond at C(3) in the dihydrothiazine ring to C(2).
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760418
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  • 2
    Electronic Resource
    Electronic Resource
    The Terpenoids of Maytenus boaria MOL. (Celastraceae) (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 2537-2543 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The known triterpenoids β-amyrin (= olean-12-en-3β-ol; 22), lupeol (= lup-20(29)-en-3β-ol; 17), betulin (= lup-20(29)-ene-3β,28-diol; 18), lup-20(29)-ene-3β,30-diol (20), oleanolic acid (= 3β-hydroxyolean-12-en-28-oic acid; 21), and betulonic acid (= 3-oxolup-20(29)-en-28-oic acid; 19), together with epicatechol (= cis-2-(3,4-dihydroxyphenyl)-3,4-dihydro-2H-1-benzopyran-3,5,7-triol; 23), 5′-O-methylgallocatechol (= trans-2-(3,4-dihydroxy-5-methoxyphenyl)-3,4-dihydro-2H-1-benzopyran-3,5,7-triol; 24), and 4-hydroxybenzaldehyde were isolated from the aerial parts of Maytenus boaria (MOL.). Additonally, the eight 4,C4-dihydro-β-agarofuran sesquiterpenoids 1-8, one of them a diol with a (4R)-configuration, and compound 9 were present in the extract. The structures of these compounds were established by spectroscopic and chemical means.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760712
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  • 3
    Electronic Resource
    Electronic Resource
    HPLC and 1H-NMR Studies of Alkaline Hydrolysis of Some 7-(Oxyiminoacyl)cephalosporins (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 2789-2802 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Alkaline hydrolysis (pH 10.5) of the three 7-(oxyiminoacyl)cephalosporins 1a-c (cefuroxime, ceftazidime, and ceftriaxone) was studied at 37° using HPLC and 1H-NMR techniques. The 7-epicephalosporin 2, the 3-methylidene compound 3, and the 6-epimer 4 of the 3-methylidene compound 3 were identified for each cephalosporin as the major degradation products under the conditions used; ceftazidime (1b) yielded also the Δ2-isomer 5b (Scheme 1). A kinetic scheme was developed to account for the production of these compounds, and the different kinetic constants involved in the process were calculated. The experimental results show that the presence of a pyridinio group at position C-C(3) favours the appearance of the Δ2-isomer, which was detected mainly in cephalosporins bearing an ester function at C(4). The presence of an oxyimino group at C—CONH—C(7) facilitates epimerization at C(7) (→ 2), whereas that of an electron-withdrawing group at C—C(3) results in a increased formation constant for the 3-methylidene compound 3. The 3-methylidene compounds 3a-c produced by the three cephalosporins on cleavage of the β-lactam ring all underwent epimerization at C(6) to yield the corresponding 6-epimer 4.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760805
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  • 4
    Electronic Resource
    Electronic Resource
    Synthesis and molecular weights of metal poly(vinyl acetate)s, 5.Part 4: cf. ref.5. (1993)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 194 (1993), S. 3377-3383 
    Details
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Metal vinyl acetate (MVAc) colloids were obtained by cocondensation at 77 K of the monomer with several metals such as: Pd, Ag, Au, Zn, Cd, Ga, In, Ge, Sn, Sb and Bi. Some of these colloids are stable for months at room temperature (Au, Ag, Pd). The colloids were polymerized with different amounts of initiator (2,2′-azoisobutyronitrile) at 65°C, and a wide range of viscosity-average molecular weights (104 〈 M̄v 〈 106) were obtained depending upon the metal used. The particle size of these clusters dispersed in the polymer matrix is around 1400 Å. The polymers are stable even at 320°C, and the metal content is ranging from 0,01 to 0,96 wt.-%. The color of the polymers depends upon the metal used.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1993.021941216
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  • 5
    Electronic Resource
    Electronic Resource
    Computer simulation and theory for free energies in dilute near-critical solutions (1993)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 39 (1993), S. 1985-1994 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Computer simulation results are presented for the residual chemical potential μ1r, local solvent density enhancement Δρloc, and average solute potential energy u1 of a solute molecule immersed in afluid very close to its critical point. The simulation algorithm involves the Monte Carlo method in conjunction with free energy perturbation ideas. A number of ideas related to the feasibility of computer simulations in near-critical solvents are investigated. Due to long-range correlations near the critical point, it is not clear that simulations in this regime can be expected to yield accurate results using ensemble sizes of the magnitude typically used in simulations. Examination of system size and number of sampling steps on the simulations shows that the accuracy of the results depend to a large extent on the nature of the thermodynamic property being investigated. The solute chemical potential is suited particularly to simulation since it depends primarily on short-range structure in the system. A comparison of the simulation results with integral equation theory calculations shows both approaches agree well with each other.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690391209
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  • 6
    Electronic Resource
    Electronic Resource
    Capillary and slit-die flow of two liquid-crystalline polymers. The effect of pressure on viscosity (1993)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 31 (1993), S. 347-356 
    Details
    ISSN: 0887-6266
    Keywords: viscosity of melts by capillary and slit die methods ; liquid crystal polymers, viscosities, comparison of capillary and slit die result for ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Viscosity measurements on two commercial thermotropic liquid crystalline polymers (LCPs) (a copolyester and a polyesteramide) were performed using both capillary and slit die methods. The results are in agreement with those presented in the literature for the same LCPs, when the measurements are carried out with a L/D = 30 die; these results are affected more by Bagley corrections for the polyesteramide than for the copolyester. For both LCPs, viscosities measured in the slit die are lower than those obtained by capillary rheometry. Nevertheless, the difference is much bigger in the case of the polyesteramide, for which a reduction by a factor of 3 is observed. This is not due to the preshear or thermal history, since the same piston-barrel system and thermal treatment were applied in both types of measurements. Depending on the polymer sample and temperature, concave and convex curvatures were observed in the pressure profiles during the slit flow. From these curvatures, the pressure coefficient α of viscosity was evaluated. However, in the case of the polyesteramide the convex curvature leads to a negative coefficient (the viscosity decreases with pressure, instead of increasing). This result may be explained by envisaging a change in the structure of the melt along the slit length. © 1993 John Wiley & Sons, Inc.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1993.090310313
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  • 7
    Electronic Resource
    Electronic Resource
    The transfer coefficient of the electrochemical reduction of cephalosporins and cefamycins (1993)
    Weinheim : Wiley-Blackwell
    Electroanalysis 5 (1993), S. 325-331 
    Details
    ISSN: 1040-0397
    Keywords: Cephalosporin ; Cefamycin ; Reduction ; Voltammetry ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrochemical reduction at a mercury electrode of three cephalosporins and one cefamycin bearing a common R3 substituent was studied. The first one-electron transfer of the reduction process was found to be the rate-determining step in all four cases. In addition, the process took place via species adsorbed on the electrode. The analysis of the voltammetric peaks obtained in the absence of diffusion allowed us to establish the variation of the apparent transfer coefficient, αap, throughout the peak. This was found to be a linear function of the faradic current, which is consistent with the assumption that the free energy of adsorption of the antibiotics is a linear function of the coverage. In addition, the transfer coefficient was found to be markedly dependent on the nature of substituent R-and on the character of the cephalosporin or cefamycin.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/elan.1140050410
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  • 8
    Electronic Resource
    Electronic Resource
    Viscosity of a copolyesteramide in the nematic state (1993)
    Basel : Wiley-Blackwell
    Die Makromolekulare Chemie, Rapid Communications 14 (1993), S. 539-544 
    Details
    ISSN: 0173-2803
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/marc.1993.030140902
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  • 9
    Electronic Resource
    Electronic Resource
    Thermodynamic characterization of tautomeric equilibria by multinuclear magnetic resonance. Application to 3-hydroxypyridine (1993)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 31 (1993), S. 552-556 
    Details
    ISSN: 0749-1581
    Keywords: 13C NMR ; 15N NMR ; Variable temperature ; Tautomeric equilibria ; 3-Hydroxypyridine ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A marked temperature dependence of the 13C and 15N chemical shifts of 3-hydroxypyridine in D2O (pD = 7.0) was observed. This dependence is explained in terms of thermally induced equilibrium shifts between the neutral form and the zwitterionic form of 3-hydroxypyridine. A method was developed to characterize the thermodynamics of the equilibria that are rapid on the NMR time scale, using the observed average 13C and 15N chemical shifts at different temperatures. This method was applied to the thermodynamics of the tautomerization equilibria of 3-hydroxypyridine. The equilibrium constants and the standard enthalpy change of the tautomerization equilibrium obtained by 13C and 15N methods agree very well with the results obtained by other spectroscopic methods.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260310607
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  • 10
    Electronic Resource
    Electronic Resource
    Alkaline hydrolysis of cephaloridine: An 1H—NMR study. Temperature dependence of the rate constants (1993)
    New York, NY : Wiley-Blackwell
    International Journal of Chemical Kinetics 25 (1993), S. 865-874 
    Details
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A kinetic study on the basic hydrolysis of cephaloridine at pD= 10.5 was carried out by using the 1H—NMR technique. Epimerization at H7, a nucleophilic attack of hydroxyl ion on the β-lactam carbonyl group followed by the release of the pyridine molecule, and isomerization of the double bond at position 3 in the dihydrothiazine ring were the major reactions observed.Based on the results obtained, it should be emphasized that the presence of a pyridine group at 3′ results in a slightly increased formation constant for the exo methylene compound relative to other cephalosporins with different substituents at that position.The activation energy for the epimerization constant and the cleavage of the β-lactam ring at pD 10.5 was 21.2 kcal/mol. © 1993 John Wiley & Sons, Inc.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/kin.550251007
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