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  • 1
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermische Cyclisierungen von Dienen, Eninen, Enonen und ähnlichen ungesättigten Systemen illustrieren das präparative Potential der intramolekularen En-Reaktion. Eine Fülle substituierter, anellierter und überbrückter Ringsysteme, darunter auch Naturstoffe, sind so auf regio- und stereoselektive Weise zugänglich. Zahlreiche Beispiele dienen dazu, die Möglichkeiten, Grenzen und gemeinsamen Gesetzmäßigkeiten dieser Cyclisierungs- und der dazu inversen Ringöffnungs-Reaktion zu umreißen. Manche dieser Cyclisierungen sind in industrieller Größenordnung durchführbar.
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  • 3
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Zahlreiche Beispiele intramolekularer Cycloadditionen von 1,3-Dienen, Nitronen und Azomethiniminen bezeugen den präparativen Wert dieser Varianten zur regio- und stereoselektiven Synthese anellierter sowie überbrückter Ringsysteme. Die Gemeinsamkeiten, Unterschiede und Grenzen dieser Reaktionstypen werden systematisch umrissen.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Eine rasch wachsende Fülle von Ergebnissen bezeugt die Leistungsfähigkeit asymmetrischer Diels-Alder- und En-Reaktionen zur Synthese enantiomerenreiner Verbindungen. Der vorliegende Aufsatz erfaßt systematisch und kritisch eine Auswahl stereoseitendifferenzierender Prinzipien unter Einbeziehung neuester, spektakulärer Fortschritte. Die „chirale Information“ entstammt im allgemeinen kovalent gebundenen Hilfsgruppen; einige Hilfsstoffe sind kristallin, preiswert und bequem aus natürlichen Quellen (Monoterpenen, Hydroxysäuren, Aminosäuren, Steroiden und Zuckern) zugänglich. Durch nicht-destruktive Chiralitätsübertragung entstehen bis zu vier Asymmetriezentren mit voraussagbarer relativer und absoluter Konfiguration. Abspaltung der Hilfsgruppe (teils unter effizienter Rückgewinnung des Hilfsstoffs) von den diastereomerenreinen Addukten ergibt eine Reihe polyfunktioneller, enantiomerenreiner Synthesebausteine. Ihr Potential zeigt sich bei Synthesen von physiologisch interessanten chiralen Naturstoffen wie Prostaglandinen, Antibiotica, Terpenoiden, Shikimisäure und α-Allokaininsäure.
    Additional Material: 10 Tab.
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  • 5
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metallo-En-Reaktionen waren bis vor wenigen Jahren kaum bekannt. Ihre intramolekularen Varianten entwickelten sich nun in jüngster Zeit auf beeindruckende Weise. Der strategische Einsatz stereoselektiver Magnesium-En-Cyclisierungen ermöglicht ergiebige Synthesen zahlreicher, strukturell sehr verschiedener Naturstoffe (z. B. Sesquiterpene marinen und pflanzlichen Ursprungs, Alkaloide, Riechstoffe, insekteneigene Abwehrstoffe, Fungitoxine). Ein brillantes Beispiel ist die Synthese des olfaktisch interessanten, aber schwer zugänglichen Tricyclovetivens (+)-Khusimon, die 20 Jahre Arbeit zum Aufbau dieser Substanzklasse in den Schatten stellt. Palladium-, Platin- und Nickel-katalysierte Versionen befinden sich noch im Anfangsstadium der Erforschung, zeigen aber bereits ein faszinierendes Spektrum von Möglichkeiten. Ein nahezu 100% stereospezifischer C-O-→C-Pd-→C-C-Chiralitäts-Transfer erschließt einfache und selektive cis- und trans-Anellierungen. Die milden Reaktionsbedingungen sind mit vielen funktionellen, u. a. stickstoffhaltigen Gruppen kompatibel; damit eröffnen sich interessante Perspektiven zur Herstellung von Heterocyclen (z. B. Alkaloiden). Abfangreaktionen der cyclisierten σ-Metall-Zwischenprodukte unter Einschiebung von Kohlenstoffmonoxid führen zu anellierten Cyclopentanonen und Cyclopentenonen, wobei „gleichzeitig“ und stereokontrolliert vier CC-Bindungen gebildet werden. Die hier und im folgenden vorgestellten Beobachtungen bieten eine Grundlage zur weiteren Erforschung und Anwendung dieser leistungsfähigen Methode in der Organischen Synthese.
    Additional Material: 11 Tab.
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  • 6
    ISSN: 0570-0833
    Keywords: Cycloaddition ; Synthetic methods ; Intramolecular reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Numerous examples of intramolecular cycloaddition of 1,3-dienes, nitrones, and azomethine imines attest the preparative value of this variant for regioselective and stereoselective synthesis of annelated and bridged ring systems. The common features, differences, and limitations of these types of reaction are systematically reviewed.
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  • 7
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0570-0833
    Keywords: Intramolecular reactions ; Ene reaction ; Synthetic methods ; Cyclization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermal cyclizations of appropriate dienes, enynes and related unsaturated systems, some of them carried out on an industrial scale, demonstrate increasingly the preparative power of the intramolecular ene reaction. A variety of substituted, fused and bridged ring systems, including natural products, are thus easily accessible in a regio- and stereo-selective manner. Numerous examples are discussed systematically illustrating the possibilities, limitations, and common features of this cyclization reaction and its reverse ring-opening process.
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  • 9
    ISSN: 0570-0833
    Keywords: Synthetic methods ; Cycloaddition ; Diels-Alder reaction ; Ene reaction ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rapidly accumulating evidence shows the value of asymmetric Diels-Alder and ene reactions for the syntheses of enantiomerically pure molecules. This article presents a systematic and critical treatment of various stereoface-differentiating principles, including very recent spectacular advances. The chiral information is mainly provided by covalently bound auxiliaries, some of which are crystalline, inexpensive, and readily available from naturally occurring monoterpenes, hydroxy acids, amino acids, steroids, and sugars. Non-destructive transfer of chirality leads to the efficient creation of up to four chiral centers with predictable relative and absolute configurations. Regenerative cleavage of the auxilary group from the diastereomerically pure adducts furnishes a range of polyfunctional, optically pure building blocks. Their synthetic potential is illustrated by strategic applications to the syntheses of physiologically interesting, chiral natural products such as prostaglandins, antibiotics, terpenoids, shikimic acid, alkaloids, and kainoids.
    Additional Material: 10 Tab.
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  • 10
    ISSN: 0570-0833
    Keywords: Metallo-ene reaction ; Ene reaction ; Synthetic methods ; Catalysis ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metallo-ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio- and stereoselective magnesium-ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)-khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium-, platinum-, and nickel-catalyzed versions of the metallo-ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific C—O→C—;Pd-→ C—C chirality transfer permits simple and selective, cis- or trans-annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ-metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon-carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis.
    Additional Material: 11 Tab.
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