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  • 1
    ISSN: 1434-4475
    Keywords: 4-Benzoyl-5-phenylfuran-2,3-dione ; Dibenzoylacetic acid-N-carboxyamides ; Oxazin-2,4-dione ; Keto-enole tautomerism ; Semiempirical quantum chemical calculations ; Crystal structure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary 4-Benzoyl-5-phenylfuran-2,3-dione (1) and the urethanes2 combine under loss of carbon monoxide yielding the open chain dibenzoylacetic acid derivatives3 and4. 3a, b only can be cyclized to the oxazinone5. The keto-enole tautomerism3 ⇌4 is further investigated with aid of semiempirical quantum chemical calculations, based upon the molecular geometry of3a, deduced from an X-ray study.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Monatshefte für Chemie 129 (1998), S. 1285-1292 
    ISSN: 1434-4475
    Keywords: Keywords. 3-Acetyl-4-methylaminopyridin-2-one; Hydrogen bridge bond; 4-Hydroximinopyridin-2-one; 3-(1-Hydroxyethyliden)-piperidin-2 ; 4-dione; Isoxazolo[4 ; 3-c]pyridinone; Isoxazolo[4 ; 5-c]pyridinone..
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung.  Bei der Umsetzung von 4-Methylamino-5,6-dihydropyridinon mit Acetylchlorid entstand ausschließlich das 3-Acetylderivat. Bei der alkalischen Hydrolyse der Methylaminogruppe entstand ein Gemisch zweier Hydroxyderivate. Diese reagieren mit Hydroxylamin zum Isoxazolo[4,5-c]pyridinon, während das 4-Methylaminoanaloge in Abhängigkeit vom pH-Wert zum Isoxazolo[4,3-c]pyridinon oder zum Isoxazolo[4,5-c]pyridinon cyclisiert. Die Konfiguration der letzteren Verbindungen wurde durch NMR-Messungen gesichert..
    Notes: Summary.  Reaction of 4-methylamino-5,6-dihydropyridinone with acetyl chloride yielded exclusively the 3-acetyl derivative. When the methylamino group of the latter was removed by alkaline hydrolysis, a mixture of two hydroxy derivatives was formed. Those cyclized upon treatment with hydroxylamine exclusively to the isoxazolo[4,5-c]pyridinone, whereas the 4-methylamino analogue yielded, depending on pH, mainly the isoxazolo[4,3-c]pyridinone or the isoxazolo[4,5-c]pyridinone. The configurations of the latter compounds were established by NMR experiments..
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  • 3
    ISSN: 1434-4475
    Keywords: Keywords. 3 ; 4-Diphenylcyclobut-3-ene-1 ; 2-diones; 13C NMR; IR; AM1 calculations; Correlation analysis.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung.  Die Infrarotabsorptionsbanden der C*O Streckschwingung und die chemischen Verschiebungen der 13C NMR Signale wurden für eine Reihe von neun substituierten 3,4-Diphenylcyclobut-3-en-1,2-dionen in CHCl3, CDCl3 und teilweise in CCl4 gemessen. Die AM1-Ladungsdichten, Bindungsordnungen und Bindungswinkel wurden für die Optimalgeometrie berechnet. Zwischen den arithmetischen Mitteln der symmetrischen und asymmetrischen C*O-Streckschwingung, den Hammettschen σ-Konstanten, den Geschwindigkeitskoeffizienten und Aktivierungsenthalpien der basenkatalysierten Ringspaltung sowie den AM1-Sauerstoffladungsdichten und Bindungsordnungen der Carbonylgruppen wurden gute bis befriedigende Korrelationen gefunden. Die Resultate zeigen, daß die Faktoren, die die Reaktivität dieser Verbindungen kontrollieren, dieselben sind wie jene, die die strukturellen und IR-spektroskopischen Eigenschaften bedingen. Die Korrelation zwischen den Wellenzahlen der symmetrischen und der asymmetrischen C*O-Streckschwingung hat einen Anstieg von a=1.069, was auf einen starken Kreuzkonjugationseffekt im Cyclobutendionring hinweist. Die Korrelation der Wellenzahlenseparation der symmetrischen und asymmetrischen C*O Streckschwingungen (Δv(C*O)) für verschiedene zyklische Diketone läßt einen Winkel von γ=83° zwischen den Bindungsvektoren der beiden Carbonylgruppen des Cyclobutendionringes erwarten, der ausgezeichnet mit dem durch die semiempirische AM1-Rechnung erhaltenen übereinstimmt.
    Notes: Summary.  The infrared absorption bands of the C*O stretching vibration and the 13C NMR chemical shifts were measured for a series of nine substituted 3,4-diphenyl-cyclobut-3-ene-1,2-di-ones in CHCl3, CDCl3, and partially in CCl4. The AM1 charge densities, bond orders, and valence angles were calculated for the optimized geometry. Good to satisfactory mutual correlations were found between the arithmetic means of the wave numbers of symmetric and asymmetric C*O stretching vibrations, Hammettσ constants, rate coefficients, and enthalpies of activation of the base-catalyzed ring fission as well as the AM1 oxygen charge densities and bond orders at the carbonyl groups. The results show that the factors controlling the reactivity of these compounds and the structural and IR spectroscopic properties of the C*O groups must be the same. The correlation between the wave numbers of symmetric and asymmetric C*O stretching vibrations affords a slope of a=1.069 which points to the existence of a strong cross-conjugation effect in the cyclobutenedione ring. The correlation of the wave number separation of the absorption bands of symmetric and asymmetric C*O stretching vibrations (Δv(C*O)) for various cyclic diketones enabled the prediction of the angle γ=83° between the bond vectors of the two carbonyl groups in the cyclobutenedione rings, which is in satisfactory agreement with the value calculated by the semiempirical AM1 method.
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  • 4
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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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: Semi-empirical (AM1) calculations on the 12 isomeric products and the corresponding transition states of ketene-1-azabutadiene [4 + 2] and [2 + 2] cycloaddition reactions as a model system for the reaction of ketenes with 4-iminobenzylfuran-2,3-diones are presented. A [4 + 2] type of reaction of the ketene C=C double bond leading to a six-membered lactam compound is found to be highly favoured both thermodynamically and kinetically. [2 + 2] Cycloadducts generally have significantly higher activation energies. Reactions involving the formation of a carbon-nitrogen bond proceed in most cases via attack of the nitrogen lone pair leading to a zwitterionic intermediate. Depending on the respective cycloadduct, both two-step and concerted, albeit asynchronous, processes were obtained. Based on the structures of the various transition states, some predictions with respect to substituent effects are made. Similarities to and differences from the analogous reaction of ketenimines with oxa-1,3-dienes are discussed.
    Additional Material: 3 Ill.
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  • 6
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: AM1 calculations for the tautomerism of the three isomeric hydroxypyridines are reported and compared with recent ab initio results. Intrinsic stabilities of the various tautomers are predicted by AM1 with an accuracy comparable to or even better than the best available ab initio calculations. Solvation is accounted for by the supermolecule approach and also a continuum model for solvent effects. Except for 3-hydroxypyridine, AM1 correctly accounts for the observed shift of the tautomeric equilibria due to hydration. Energetics of hydrogen bonding are reasonably described by this semiempirical method. With respect to structures, bifurcated hydrogen bonds are preferred by AM1. In addition, for 2-hydroxypyridine and its lactam tautomer self-association in addition to barriers to proton transfer are considered. With respect to tautomerization transition states, AM1 shows serious shortcomings. Compared with both experimental and ab initio results, barriers to proton transfer are considerably overestimated by AM1, especially in the hydrated and associated species. For the dimers AM1 predicts an unsymmetrical transition state which, however, is only slightly lower in energy than the symmetrical structure with two negative eigenvalues of the force constant matrix. Despite several attempts, the transition state for proton transfer in the doubly hydrated species could not be located.
    Additional Material: 3 Ill.
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  • 7
    ISSN: 0894-3230
    Keywords: γ-lactones ; base-catalysed hydrolysis ; reactivity-structure correlations ; isobenzofuranones ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate coefficients were measured for the base-catalysed hydrolysis of a series of substituted 3-(phenoxy- or thiophenoxymethylene)-(Z)-1(3H)-isobenzfuranones (3-phenoxy- or thiophenoxymethylenephthalides) in 70% (v/v) aqueous dioxane at 30.0 °C, in addition to the carbonyl stretching frequencies in chloroform and tetrachloromethane following deconvolution and band separation, when required. The Hammett reaction constants for the alkaline hydrolysis of the 3-/4-substituted phenoxy and thiophenoxy series are ca 0.75 and 1.10, respectively. These results are related to electrostatic field and resonance effects. Successful correlations between the carbonyl stretching frequencies and substituent constants and the rates of alkaline hydrolysis were found. Computational studies using the semi-empirical AM1 method correctly model both the details of the mechanistic pathway and the substituent effects. © 1998 John Wiley & Sons, Ltd.
    Additional Material: 4 Ill.
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  • 8
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Ab initio MP2/6-31G*//MP2/6-31G* and semiempirical AM1 and PM3 calculations on a series of differently substituted α-oxo-ketenes are used to investigate E/Z-isomerism and rotational barriers in these molecules. Sterically crowded derivatives are found to exist solely as s-E conformers. The unusual stability of these derivatives thus can be attributed to their inability to adopt the s-Z conformation required for the normal α-oxo-ketene reactions. With respect to structures and energies, the PM3 method (especially in the case of highly crowded molecules) is found to be less reliable than AM1. Ab initio HF/3-21G and PM3 vibrational frequencies appear to be of sufficient accuracy for a distinction between s-Z and s-E conformers. In this respect, the AM1 method appears less reliable. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
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  • 9
    ISSN: 0170-2041
    Keywords: Calculations, AM1 and INDO, S-CI ; Cyanomethylene compounds ; Pyrazolones ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (Dicyanomethylene)pyrazolones 1a, b are treated with some indoles and pyrroles. The resulting disubstituted malono-nitriles 2a-f, 4a-e, 6 yield cyanomethylene compounds 3a-f, 5a-e, 7 by irreversible thermal or photochemical decomposition. The compounds exist in different isomeric forms. Structures are proven by NMR spectroscopy and X-ray analysis. UV-Vis data are discussed by means of AM1 and INDO/S-CI calculations.
    Additional Material: 4 Ill.
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  • 10
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Equilibrium twist angles, rotational barriers around essential single bonds, and preferred conformations for over 60 conjugated organic molecules were calculated using the semiempirical AM1 (Austin model 1) method. Comparison with ab initio and experimental data shows that AM1 can be applied quite successfully to conformational problems of this type. Relatively large errors are, however, found for compounds in which lone pair-hydrogen and especially lone pair-lone pair interactions are decisive for their conformational behavior. AM1-calculated rotational barriers in conjugated molecules, however, are found to be much too low. Moreover, AM1 does not seem to even correctly reproduce the trend of rotational barriers within a series of structurally related compounds. E/Z energy differences obtained by AM1 are also frequently considerably too low. In contrast to rotational barriers, their trends and thus conformational preferences, however, are quite satisfactorily calculated by this method. Ionization potentials obtained by AM1 are too high by about 0.5 eV. However, trends are quite well predicted.
    Additional Material: 1 Ill.
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