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  • 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
    Kinetic and thermodynamic parameters for Schiff-rase formation between 5′-deoxypyridoxal and hexylamineSupported by the CICYT and the universidad de Alcalá de Henares (1990)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 73 (1990), S. 1991-1998 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermodynamic parameters involved in the formation of Schiff bases between 5′-deoxypyridoxal and hexylamine were determined at different pH values and a constant ionic strength (0.1 M). The overall and individual rale constants of formation and hydrolysis at 10, 15, 20, 25, and 30° were also determined. The enthalpy of the overall formation process was found to be negative at all the pH values assayed except the neutral, while its entropy was always positive. The results obtained show the great relevance of the phosphate group at C(5′) to the stabilization of the Schiff bases of pyridoxal 5′-phosphate.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19900730723
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  • 3
    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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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    Kinetic Study on Stability of Schiff Base of Pyridoxal 5′-Phosphate and Leucine in Water Media with Cationic Surfactants (1992)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 75 (1992), S. 1029-1038 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We studied the stability of the Schiff bases formed between pyridoxal 5′-phosphate (PLP) and leucine in the presence of (hexadecyl)trimethylammonium bromide (CTAB) over a wide pH range by determining the kinetic constants of formation and hydrolysis of these compounds. The results show that the stability of the Schiff bases is increased by the presence of CTAB as a result of increased rates of formation and decreased hydrolysis rate constants. The ionic head groups of CTAB favour the formation of the bases, while its hydrophobic rests protect the imine double bond from hydrolysis. This model system permits one to obtain partially hydrophobic media with no need for any non-aqueous solvents.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19920750406
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  • 6
    Electronic Resource
    Electronic Resource
    Theoretical Calculations of β-Lactam Antibiotics. Part VI. AM1 calculations of alkaline hydrolysis of clavulanic acid (1994)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 77 (1994), S. 1557-1569 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The gas-phase basic hydrolysis of clavulanic acid (a) was studied by using the AM1 semi-empirical method. The results obtained show that the hydroxyethylidene side chain at C(2) is pivotal to the stability of the different reaction products involved. The products with an open oxazolidine ring are more stable than those with a closed ring fused to the β-lactam ring. This behaviour differs from that of penicillins and cephalosporins where the most stable degradation products are those with an intact thiazolidine or dihydrothiazine ring, respectively, fused to the β-lactam ring. The different chemical reactivity of clavulanic acid relative to penicillins and cephalosporins could explain the disparate behaviour of the latter two types of compound towards β-lactamases. Once the acyl-enzyme intermediate of clavulanic acid has been formed, it can evolve with cleavage of the oxazolidine ring to form a difficult to deacylate compound.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19940770611
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  • 7
    Electronic Resource
    Electronic Resource
    Ordered odd-number terpolyamides, 2.Part 1: cf4 Crystal structure (1990)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 191 (1990), S. 2587-2597 
    Details
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A combined X-ray diffraction and electron microscopy study was carried out on both the solution-grown crystals and oriented films of nylons 3/5/7 and 7/5/3. These regular odd-odd-odd terpolyamides have such sequence of amide groups along the chain that they have to be aligned parallel in order to form every hydrogen bond without strain. It was found that a layered α-structure with hydrogen-bonded sheets made up of parallel chains in fully extended conformation is adopted in both cases. These results are not necessarily generalized to odd-numbered nylons where an antiparallel alignement of chains is compatible with the formation of hydrogen bonds. Chain-folded lamellar crystals obtained from solution were investigated by electron diffraction and their surface topology examined by polymer decoration with polyethylene. A bimodal orientation of hydrogen-bonded sheets within the lamella is assumed to occur in order to account for the obtained decorating patterns. Either way, chain foldings are not contained in such sheets but intersect them at an angle of about 60°.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1990.021911107
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  • 8
    Electronic Resource
    Electronic Resource
    Regular odd-number terpolyamides, 1. Synthesis (1991)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 192 (1991), S. 21-34 
    Details
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In order to improve our comprehension of the crystalline structure of polyamides, two regular odd terpolyamides containing 3-aminopropanoyl, 5-aminopentanoyl and 7-aminoheptanoyl units were prepared by the active ester polycondensation method. The choice of the sequencing order in the starting compounds (called here heterotrimers) and the various routes leading to them are discussed. It was found that both the reactions and the purification of intermediates were most conveniently performed by combining the heterodimer of the lower amino acids with the higher amino acid. The polycondensation was carried out through a multiple step process either in solution or in the solid-state at a temperature below 200°C. 13C NMR spectra showed that no rearrangement during the polycondensation occurs under these conditions. The terpolyamides have sufficiently high molecular weights to provide good films for crystallographic studies. The two terpolyamides of inversed order do not cocrystallize even when coprecipitated from a solution, probably because of their different crystalline structures.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1991.021920103
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  • 9
    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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  • 10
    Electronic Resource
    Electronic Resource
    The interaction of poly(methacrylate) radicals with diphenyldiacetylenes (1994)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 32 (1994), S. 3147-3151 
    Details
    ISSN: 0887-624X
    Keywords: polymerization of methyl methacrylate ; diphenylbutadiynes ; ESR spectra ; radical stabilization ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The free radical polymerization of methyl methacrylate (MMA) in the presence of p,p′- disubstituted diphenylbutadiynes was studied. Both the rate and degree of polymerization are somewhat lowered by the presence of the diynes, but the propagating radicals were stabilized giving clear ESR signals of the interacted polyMMA radicals at the polymerization temperature of 70°C. The magnitude of the interaction depended on the electron density of the diynes; in the cases of diphenylbutadiyne and dimethoxycarbonyldiphenylbutadiyne, the intoraction was more enhanced showing ESR signals with smaller spectra widths and increasing the number of radicals with the polymerization time, while in the cases of electron donor-substituted diynes the interaction was weaker and the radical concentration remained constant during the polymerization. These systems are considered to be examples of the stabilization of transient radicals by the direct interaction of radicals with additives without formation or breaking of chemical bonds. No diacetylenic group was found in the polyMMA obtained. © 1994 John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pola.1994.080321615
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