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  • kinetics  (17)
  • molecular recognition  (16)
  • asymmetric syntheses  (12)
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  • 1
    ISSN: 0947-6539
    Keywords: asymmetric syntheses ; alkenylations ; SAMP/RAMP hydrazones ; selenyl aldehydes ; sphingosine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: L-threo-sphingosine and its D-erythro isomer (1) are subunits of many glycosphingolipids, gangliosides and ceramides. This paper describes the highly diastereo- and enantioselective synthesis of both isomers (de, ee 〉 98%). The key steps in the synthesis are the aldol reaction of the SAMP hydrazone (S)-2 with racemic α-phenylselenylpentadecanal 3, the diastereoselective triacetoxyborohydride reduction of ketone 5 and exclusive (E) C—C double bond formation in the elimination of hydroxyl and selenyl moieties promoted by methanesulfonyl chloride. Mesylate 8 was then readily converted via the 1,3-O-acetonide-protected azidosphingosine 9 to L-threo-sphingosine. Conversion to the known 1-O,2-N-diacetyl-protected sphingosine 13 with subsequent Mitsunobu inversion of the C3—OH centre afforded the D-erythro-sphingosine epimer.
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  • 2
    ISSN: 0947-6539
    Keywords: kinetics ; peroxyl radicals ; pulse radiolysis ; radicals ; superoxide radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxyl radicals were generated radiolytically and reacted with a number of benzene derivatives (PhH). In the presence of oxygen, the hydroxycyclohexadienyl radicals thus formed were converted into their corresponding peroxyl radicals. Pulse radiolysis has shown the oxygen addition (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO2· to yield phenols, alongside some ring-fragmentation products (product-forming reaction, p). The rate constants for the forward and reverse reactions (kf and kr) and the corresponding stability constants K (= kf/kr) were determined for the hydroxy-cyclohexadienyl radicals derived from anisole, toluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoate ion, phenylalanine, and terephthalate ion. The constants kf lie between 8 × 108 (anisole) and 1.6 × 107 dm3 mol-1 s-1 (terephthalate ion), and kr between 7.5 × 104 (toluene) and 3.4 × 103 s-1 (terephthalate ion). The stability constants lie between 2.6 × 104 (benzene) and 3.3 × 103 dm3 mol-1 (phenylalanine). The rate constants for the product-forming reactions kp are between 5.5 × 103 (anisole) and 3.4 × 102 s-1 (benzoate). For the peroxyl radical derived from phenylalanine, a bond dissociation energy of 5.5 kcal mol-1 has been derived. A number of hydroxy-cyclohexadienyl radicals (e.g., those derived from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) react too slowly to allow the equilibrium constant to be determined by means of pulse radiolysis. These reactions have rate constants kf in the order of 5 × 106 dm3 mol-1 s-1, except for nitrobenzene where the reaction is too slow for measurement. The rate constants kr are below 500 s-1, and the product-forming reaction is too slow to be detected by pulse radiolysis. γ-Radiolysis of N2O/O2(4:1)-saturated aqueous solutions of benzonitrile gave dimeric compounds (e.g., dicyanobiphenyls) in low yield, alongside the three isomeric phenols; this again proves the low reactivity of its hydroxycyclohexadienyl radical toward oxygen.
    Additional Material: 6 Ill.
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  • 3
    ISSN: 0947-6539
    Keywords: exchange processes ; kinetics ; NMR spectroscopy ; zirconium complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have shown that it is possible to inhibit the transfer of magnetization in a system with several exchanging sites in dynamic equilibrium, as in a mixture of cis- and trans-ZrCl4L2 with excess free ligand L (L = (CH3O)3PO). The forward and backward reaction rates involving two selected sites can be studied while the effect of competing exchange processes is “quenched”. This can be achieved either by selective inversion of the magnetization of the two chosen sites in the course of the reaction interval, or alternatively by inversion of the magnetization of all other sites in the exchange network. The rate of exchange from the free to the cis site was determined to be kcis←free = 0.018 s-1. In the usual methods, this process would tend to be overshadowed by the almost two hundred times faster competing exchange process from the cis to the trans site (Ktrans←cis = 3.32 s-1).
    Additional Material: 6 Ill.
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  • 4
    ISSN: 0947-6539
    Keywords: copper proteins ; cross-reactions ; electron-transfer reactions ; kinetics ; metalloproteins ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron self-exchange rate constant for the Type 1 blue copper protein umecyanin from horseradish roots has been determined as 6.1 × 103 M-1 S-1 at pH 7.5, I = 0.100 M, 25°C by an NMR line-broadening method. The value obtained is one of the lower self-exchange rate constants determined for this class of protein; this is attributed to the presence of positively charged residues near to the electron-transfer site. The self-exchange rate constants calculated by means of a Marcus analysis of data for the cross-reactions (25°C) of umecyanin with azurin and cytochrome c551 (both from Pseudomonas aeruginosa) are substantially less at 8.0M-1 S-1 and 13.9M-1S-1, respectively, and are independent of pH in the range 7.0-8.0, I = 0.100M. The discrepancy between the self-exchange rate constants obtained by these two different methods can be rationalised if it is assumed that umecyanin reacts with the two proteins employed in the cross-reaction studies through the same site, but that this site is different from that used for the self-exchange process. A comparison of the primary structure of umecyanin with those of other Type 1 copper proteins has revealed that a glutamine rather than a methionine is likely as the fourth ligand of Cu at the active site. Other comparisons are made with stellacyanin, and the electron-transfer reactivity of the two proteins is discussed.
    Additional Material: 6 Ill.
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  • 5
    ISSN: 0947-6539
    Keywords: amino alcohols ; asymmetric syntheses ; chiral auxiliaries ; enantiomeric resolution ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically active tert-leucinol is an important building block in asymmetric synthesis. However, the (R) enantiomer particularly has so far remained difficult to obtain, mainly because of the laborious synthesis of the precursor amino acid, (R)-tert-leucine. Here we present a new, classical resolution of racemic tert-leucinol, which allows straightforward preparation of each, but especially the (R) enantiomer, in good yields and high optical purities. The feasibility of the synthesis of useful derivatives is demonstrated by transformation into the corresponding (R)-4-tert-butyl-2-oxazolidinone.
    Additional Material: 1 Tab.
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  • 6
    ISSN: 0947-6539
    Keywords: alkynes ; asymmetric syntheses ; cyclopropanes ; spiro compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variety of chiral, nonracemic 2-alkoxy-1-alkynylcyclopropanes 7 were synthesized in good to very good yields from enantiomerically pure glycidol derivatives (glycidol tosylate, epichlorohydrin) by boron trifluoride promoted addition of lithium trimethylsilylacetylide followed by protection of the secondary hydroxyl group and finally a diastereoselective γ-elimination. The 2-ethoxy derivative (S,R)-7 b was deprotonated with n-butyllithium, and the resulting 1-lithio-2-ethoxy derivative (S,R)-20 functionalized by treatment with oxygen followed by tosyl chloride. Protodesilylation and catalytic hydrogenation smoothly furnished 1-ethenylcyclopropyl sulfonates, which underwent a clean Pd0-catalyzed SN2′-type substitution with dimethyl propargylsodiummalonate to give the (E)-configurated enyne (R,E)-26 with a methylenecyclopropane end group. A diastereoselective Pauson-Khand reaction completed the sequence to give the enantiomerically pure spirocyclopropaneannelated bicyclo[3.3.0]octane derivative 31.
    Additional Material: 3 Tab.
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  • 7
    ISSN: 0947-6539
    Keywords: hydrogen bonds ; molecular recognition ; polyelectrolyte multilayers ; surface chemistry ; thin films ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Functional polyanions were prepared by copolymerization of sulfopropyl acrylate and sulfopropyl methacrylate with monomers bearing triaminopyrimidine or barbituric acid functionalities, respectively. Functionalized polyelectrolyte multilayers were assembled from these copolymers by stepwise alternating adsorption with poly(choline methacrylate). These multilayers are suited for molecular recognition of substrates that are complementary to the functional groups incorporated. Thus, multilayers containing triaminopyrimidine moieties selectively bind barbituric acid, and vice versa, when exposed to solutions of the 1:1 complex of barbituric acid and triaminopyrimidine. The molecular recognition process was monitored by UV/Vis spectroscopy, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and photoelectron spectroscopy (XPS). Remarkably, after successful recognition and binding of the complementary substrates to the multilayers, the stepwise layering could be continued.
    Additional Material: 6 Ill.
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  • 8
    ISSN: 0947-6539
    Keywords: cobalt ; cytochrome c ; electron transfer ; kinetics ; volume profile ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Kinetics and Thermodynamics of the Reversible Outer-Sphere Electron-Transfer Reactions between Horse Heart Cytochrome cII/III and [Co(phen)3]3+/2+ and [Co(bpy)3]3+/2+ were studied in detail, in particular as a function of temperature and pressure. It was possible to construct a volume profile for both reactions from the pressure data. The transition state was found to be halfway between the reactant and product states on a volume basis in all studied systems. This is in agreement with the λ≠ parameter estimated from the Marcus theory. For all the systems investigated, the differences in the activation volumes are in good agreement with the reaction volumes determined from spectrophotometric and electrochemical measurements at elevated pressure, and from the difference in the partial molar volumes of the metal complexes. The activation and reaction volumes of the bipyridine system are significantly smaller than those of the corresponding phenanthroline and terpyridine systems. A detailed mechanistic analysis is presented. The results show that the different kinetic and thermodynamic techniques employed complement one another.
    Additional Material: 7 Ill.
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  • 9
    ISSN: 0947-6539
    Keywords: amino alcohols ; asymmetric syntheses ; dihydroxylations ; diol cleavage Horner-Wittig reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spiroepoxide 1 was prepared from quinidine and converted into β-amino alcohol 3 (86% over two steps). Dihydroxylation of enantiopure oxazatricylic olefin (E)-4 provided diastereomeric diols 5a and 5b. Stereospecific conversion of 1,2-secondary, tertiary diol 5b into tetracyclic spiroepoxide 6 was accomplished in high yield by a one-pot tosylation-cyclization procedure. 1,2-Diol cleavage with NaIO4 in 80% acetic acid afforded the new tricyclic dehydrorubanone 7, containing the 4-oxa-7-azatricyclo[4.3.1.03,7]-decan-2-one core structure. Similarly, acetylated rubanone 9 was prepared on a 20 g scale. Reduction with NaBH4 in the presence of CeCl3 provided rubanols 10a and 10b (1:1.1). Horner-Wittig reaction of 9 with diethyl cyanomethylphosphonate was (Z)-selective, furnishing unsaturated nitrile (Z)-13. Conversion into the α,β-unsaturated aldehyde (Z)-14 and reduction afforded enartiopure allylic alcohol (Z)-12, which is a new isomer of the key quinidine metabolite 15.
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  • 10
    ISSN: 0947-6539
    Keywords: electron transfer ; indene ; kinetics ; rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of the indenyl complex [Rh(η5-C9H7)(η4-cod)] has been investigated in the context of structural effects induced by the transfer of one electron. The reduction of this complex occurs in two steps, leading first to the radical anion and then to the highly frangible dianion. Both species eliminate the indenyl anion. In the presence of free cyclooctadiene, the related cleavage leading to the indenyl anion and bis-cyclooctadiene rhodium fragments now follows a Michaelis-Menten-type mechanism involving precoordination of one extra COD ligand to the initial radical anion. These results suggest the modification of the hapticity of the indenyl ligand in connection with 17- and 19-electron metalcentered intermediates.
    Additional Material: 3 Ill.
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