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  • 1
    Electronic Resource
    Electronic Resource
    Circular dichroism of axially chiral methaqualone, 3-Aryl-2-mercapto- and 3-aryl-2-alkylthio-4(3H)-quinazolinones: conformational dependence of CD and assignment of absolute configuration (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 253-261 
    Details
    ISSN: 0899-0042
    Keywords: chiroptical properties ; Cotton effect ; atropisomerism ; quantum-mechanical calculation ; AM1 ; CNDO/S ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rotational strengths calculated on the basis of quantum-mechanically obtained minimum energy geometries were used to determine the absolute configurations of axially chiral 3-aryl-4(3H)-quinazolinones from the sign of the observed Cotton effects (CEs). For the spectral data, CNDO/S calculations were used; for the geometries, ab initio (RHF/6-31G) and semiempirical (AM1) theories were used. Oscillator and rotational strengths of all excited states down to 200 nm were compared to experimental absorption and circular dichroism (CD) data. It was found that the sign of the 1Lb Cotton effects obtained for the enantiomers of methaqualone and derivatives of 3-aryl-2-alkylthio-4(3H)-quinazolinones can be correlated unambiguously with the absolute configuration. Furthermore, the sign of the Cotton effect of the π-π* transition of the thiocarbonyl chromophore of 3-aryl-2-mercapto-4(3H)-quinazolinones is suitable for a successful stereochemical correlation. Chirality 10:253-261, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Helically twisted chiral cyanine dyes: Influence of chromophore length on observed and calculated rotatory strengths (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 9 (1997), S. 243-249 
    Details
    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.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Circular dichroism and electronic structure calculations on naproxen (1992)
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 5 (1992), S. 748-754 
    Details
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The circular dichroism (CD) spectrum of naproxen exhibits bisignate behaviour in the 210-250 nm region with pronounced temperature dependence, indicating a dynamic solution behaviour. According to semi-empirical (PM3, AM1) calculations, four conformations have to be considered in order to describe the potential energy surface of the molecule. These conformations, all chiral, differ in the relative orientation of the carboxyl group with respect to the naphthalene moiety, which could explain the differences observed in the CD spectra. Electronic structure calculations employing CNDO/S and coupled oscillator theory suggest that the two oppositely signed CD bands of naproxen are not the two components of a couplet but result from excitations of the naphthalene 1Ba and 1Bb states that are coupled to the carboxyl ππ* transition. The four conformations are attributed to two different coupling patterns; implications with respect to observed spectra are discussed.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/poc.610051107
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  • 4
    Electronic Resource
    Electronic Resource
    Chiral Tri- and Pentamethinium Cyanine Dyes with 1,2,3,4-Tetrahydro-6-methylquinolyl End Groups: UV/Vis and CD Spectroscopy and Structure Correlations (1996)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1996 (1996), S. 1823-1831 
    Details
    ISSN: 0947-3440
    Keywords: Cyanine dyes, chiral ; Dyes ; Chirality ; Circular dichroism ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: UV/Vis and CD spectra of the chiral trimethinium cyanine dyes T2 to T5 and the pentamethinium cyanine dyes P2 to P5 have been studied for possible correlations between chiroptical properties and molecular structure. With a non-planar conjugated π-system as a consequence of steric interaction between the two end groups and the polymethine chain and with the methyl substituents in the 2- or 3-ring positions acting as chiral anchors, the inherent chirality of these dyes manifests itself in the CD spectra which follow the same pattern throughout: monosignate CD absorptions in the long-wavelength cyanine band region and bisignate in the aromatic-end-group region. A correlation between the sense of twist of the polymethine chain and the observed and calculated sign of the cyanine band is established, which allows the assignment of the absolute configuration for the 2-, but not for the 3-methyl substituted dye derivatives. Neither in the trimethinium nor in the pentamethinium dyes do the calculated energies, geometries or chiroptical properties indicate any interaction between the two end groups.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199619961116
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  • 5
    Electronic Resource
    Electronic Resource
    Synthesen mit Isoxazolinen, 5. 1,3-Asymmetrische Induktion bei der Reduktion 3,5-disubstituierter 2-Isoxazoline; diastereoselektive Synthese und Konfigurationszuordnung von γ-Aminoalkoholen (1980)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1980 (1980), S. 101-121 
    Details
    ISSN: 0170-2041
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Syntheses via Isoxazolines, 51.  -  1,3-Asymmetric Induction in the Reduction of 3,5-Disubstituted 2-Isoxazolines; Diastereoselective Synthesis and Configurational Assignment of γ-Amino Alcohols2)The cycloaddition of nitrile oxides to alkenes, followed by reduction of the ensuing 2-isoxazolines 3, yields γ-amino alcohols 4 in a regioselective manner. The stereoselectivity of the reduction step is examined; it is highest with LiAlH4 as the reducing agent. 1,3-Asymmetric induction by isoxazoline ring substituents leads to β-4: α-4 ratios of ca. 85:15 for 5-methyl and ca. 95:5 for 5-phenyl. (CH3)2S-BH3, less selectively, furnishes 60: 40, Na-Hg and Na/ethanol reduction give rise to 40:60 diastereomer mixtures. From these, two amino alcohols of the α-series are isolated. 3a is reduced by NaBH3CN to give cis/trans-isoxazolidines 11.  -  The relative configurations of β- and α-4 and, consequently, the steric course of these reductions, are established by 1H- and 13C-NMR studies of 4 as well as of cyclic derivatives (azetidine 7, tetrahydro-1,3-oxazine 10). In addition, conformations of the free amino alcohols 4 are suggested on the basis of 13C-NMR data.
    Notes: Durch Cycloaddition von Nitriloxiden an Alkene und Reduktion der dabei entstehenden 2-Isoxazoline 3 werden regioselektiv γ-Aminoalkohole 4 aufgebaut. Die Stereoselektivität der Reduktion wird untersucht; sie ist am grüßten mit LiAlH4 als Reduktionsmittel. Die 1,3-asymmetrische Induktion durch Substituenten am Isoxazolin-Ring führt bei 5-Methyl zum Verhältnis β-4: α-4 = 85:15, bei 5-Phenyl zu ca. 95:5. Reduktion mit (CH3)2S-BH3 ergibt wenig selektiv ca. (60:40)-, mit Na-Hg oder Na/Ethanol (40:60)-Diastereomerengemische, aus denen zwei Aminoalkohole der α-Reihe isoliert werden. NaBH3CN reduziert 3a zu cis/trans-Isoxazolidinen 11.  -  Die relativen Konfigurationen für β- sowie α-4 und damit der sterische Verlauf der verschiedenen Reduktionen werden durch 1H- und 13C-NMR-Untersuchungen von 4 sowie cyclischer Derivate (Azetidin 7, Tetrahydro-1,3-oxazin 10) bestimmt. Aus den 13C-NMR-Daten ergeben sich auch Hinweise auf die Konformationen der freien Aminoalkohole 4.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.198019800109
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  • 6
    Electronic Resource
    Electronic Resource
    Synthesen mit Isoxazolinen, 6. Asymmetrische Induktion bei der Reduktion von 2-Isoxazolinen zu γ-Aminoalkoholen mit zwei und drei benachbarten Stereozentren2,3) (1980)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1980 (1980), S. 122-139 
    Details
    ISSN: 0170-2041
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Syntheses via Isoxazolines, 61). - Asymmetric Induction in the Reduction of 2-Isoxazolines to Yield γ-Amino Alcohols with Two or Three Vicinal Stereocentres2,3)The substituted 3-aminopropanols 3 and 5 with two or three adjacent chiral centres are prepared from alkenes and nitrile oxides via LiAlH4 reduction of intermediate isoxazolines 2 and 4, respectively, in a regio- and stereoselective mode. 4-Alkyl groups give rise to β-3 in ca. 40-70% stereo-selectivity. cis-4,5-Disubstitution leads to an excess of 80 - 90% of β-5, in the case of the trans-compound 4b to an excess of 45% of β-5. 1,3-Asymmetric induction here exceeds 1,2-induction, which points to Li…O complexation during the hydride-transferring step. LiAlH4 reduction of 4,5-dimethylisoxazolines 4a and 4b shows approximate additivity of substituent effects (ΔΔG
    Notes: Die substituierten 3-Aminopropanole 3 und 5 mit zwei oder drei benachbarten Chiralitätszentren werden regio- und stereoselektiv aus Alkenen und Nitriloxiden durch LiAlH4-Reduktion der Isoxazolin-Zwischenprodukte 2 bzw. 4 erhalten. 4-Alkylgruppen bewirken bei der Reduktion zu β-3 ca. 40- bis 70proz. Stereoselektivität, bei cis-4,5-Disubstitution wird ein überschuß von 80-90%, bei trans-Stellung (4b) ein solcher von ca. 45% an β-5 erhalten. Die 1,3-asymmetrische Induktion übertrifft dabei die 1,2-Induktion, was mit Li…O-Komplexierung beim Hydrid-Additionsschritt gedeutet wird. Bei der LiAlH4-Reduktion der 4,5-Dimethylisoxazoline 4a und 4b wird näherungsweise Additivität der Substituenteneffekte (ΔΔG
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.198019800110
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