1,721,046 research outputs found
Derivatisierungsreaktionen von Cinchona-Alkaloiden und eine Cinchona-Alkaloid-katalysierte asymmetrische Synthese von 2,5-Diarylphopholanen
No full textThis thesis consists of two parts: The first part deals with the synthesis of structurally diverse cinchona alkaloid derivatives via nucleophilic addition of organometallic reagents. An addition of organolithium reagents leads to 2´-alkylated products. The reaction with Grignard reagents leads to new 4´-substituted cinchona alkaloid derived aminals via conjugate addition. Direct core-alkylation by means of organometallic reagents is an easy way to access structural analogues of the cinchona alkaloids. Several functional groups of the alkaloid do not partake in this reaction and thus remain within the reaction products, providing a handle for further functionalization, or suggesting use of those derivatives as organocatalysts or as ligands for transition metals. The second part of this thesis describes an asymmetric catalytic total synthesis of (2R,5R)-diphenylphospholanic acid (Fiaud’s acid; 1-hydroxy-1r-oxo-2c,5t-diphenylphospholane). We have developed a nickel-catalyzed cross-coupling reaction of thiophene with aryl Grignard reagents to form (E,E)-1,4-diarylbutadienes. Trialkylphosphines as well as N-heterocyclic carbenes serve as ligands for nickel. Under these conditions we were able to synthesize various diarylbutadienes in yields of up to 75%. Submitted to a McCormack cycloaddition reaction, these butadienes reacted to give meso-2,5-diarylphosphol-3-enamides. The yield of the phosphacycles varied between 24 and 84%, depending on the steric hinderance of the aryl substituent. An organocatalytic asymmetric desymmetrization of the meso-phospholenes with cinchona alkaloids acting as bifunctional catalysts in acetonitrile solution gave the corresponding 2-phospholenes in quantitative yields with an enantiomeric excess of up to 91%. The 2,5-diphenyl-2-phospholeneamide was hydrolyzed to the phospholenic acid in aqueous acid. Diastereoselective reduction of the unsaturated acid with sodium in liquid ammonia and ammonia sulphate as proton source gave the phospholanic acid in quantitative yield in a diastereomeric ratio of trans/cis = 7.7:1. Crystallization from methanol gave the pure trans-diastereomer of Fiaud’s phospholanic acid with an enantiomeric excess of >98%
Derivatisierungsreaktionen von Cinchona-Alkaloiden und eine Cinchona-Alkaloid-katalysierte asymmetrische Synthese von 2,5-Diarylphopholanen
No full textThis thesis consists of two parts: The first part deals with the synthesis of structurally diverse cinchona alkaloid derivatives via nucleophilic addition of organometallic reagents. An addition of organolithium reagents leads to 2´-alkylated products. The reaction with Grignard reagents leads to new 4´-substituted cinchona alkaloid derived aminals via conjugate addition. Direct core-alkylation by means of organometallic reagents is an easy way to access structural analogues of the cinchona alkaloids. Several functional groups of the alkaloid do not partake in this reaction and thus remain within the reaction products, providing a handle for further functionalization, or suggesting use of those derivatives as organocatalysts or as ligands for transition metals. The second part of this thesis describes an asymmetric catalytic total synthesis of (2R,5R)-diphenylphospholanic acid (Fiaud’s acid; 1-hydroxy-1r-oxo-2c,5t-diphenylphospholane). We have developed a nickel-catalyzed cross-coupling reaction of thiophene with aryl Grignard reagents to form (E,E)-1,4-diarylbutadienes. Trialkylphosphines as well as N-heterocyclic carbenes serve as ligands for nickel. Under these conditions we were able to synthesize various diarylbutadienes in yields of up to 75%. Submitted to a McCormack cycloaddition reaction, these butadienes reacted to give meso-2,5-diarylphosphol-3-enamides. The yield of the phosphacycles varied between 24 and 84%, depending on the steric hinderance of the aryl substituent. An organocatalytic asymmetric desymmetrization of the meso-phospholenes with cinchona alkaloids acting as bifunctional catalysts in acetonitrile solution gave the corresponding 2-phospholenes in quantitative yields with an enantiomeric excess of up to 91%. The 2,5-diphenyl-2-phospholeneamide was hydrolyzed to the phospholenic acid in aqueous acid. Diastereoselective reduction of the unsaturated acid with sodium in liquid ammonia and ammonia sulphate as proton source gave the phospholanic acid in quantitative yield in a diastereomeric ratio of trans/cis = 7.7:1. Crystallization from methanol gave the pure trans-diastereomer of Fiaud’s phospholanic acid with an enantiomeric excess of >98%
Rutheniumkatalysierte Hydratisierung von terminalen Alkinen und deren Einsatz in Synthesestrategien
No full textIn the context of this work the ruthenium-catalyzed anti-Markovnikon-hydration of terminal alkynes was optimized. This covers the reaction conditions (catalyst loading, concentration, solvent/reaction media, equivalents of water, additives and exchange of the counterion) as well as the variation of the ruthenium complex via ligand exchange. The most aktivst catalyst was the [cyclopentadienylruthenium(II)(2-diphenylphosphino-6-(2,4,6-triphenylphenyl)pyridine)(triphenylphosphino)(acetonitrile)]hexafluorophosphate complex. Beside the optimization of the catalysis the ruthenium-catalyzed anti-Markovnikov-hydration was used in synthetic strategies to result aldols and poly-1,3-diols and also poly-1,4-diols. The poly-1,4-diols led via a sequential synthesis from simple base-chemicals to poly-1,4-diols with high structural complexity and natural related compounds. The hydration of propargyl alcohols to aldols, respectively 1,3-diols was more challenging in comparison to the hydration of 1,4-diols. But finally, it was possible to synthesize the massoialactone and a derivative of the family of the polyacetate secondary pyrons
Microwave-assisted catalytic synthetic methods: hydroalkoxylation, transfer hydrogenation and hydrogen autotransfer alkylation
No full textIn dieser Arbeit wurden neue selektive und effiziente katalytische Reaktionen untersucht, die durch Erhitzen auf sehr hohe Temperaturen mittels Mikrowellenstrahlung stark beschleunigt werden: Dazu gehörten die Synthese neuer Substrate für die in der Arbeitsgruppe entwickelte Hydroalkoxylierung nicht aktivierter Alkene, die Neuentwicklung einer asymmetrischen Transferhydrierung nicht aktivierter C-C-Doppelbindungen und die Entwicklung einer Wasserstoffautotransfer-Alkylierung von Pyrrolen.For this thesis new selective and efficient catalytic reactions were studied, that are greatly accelerated by microwave heating to high temperatures: This included the synthesis of new substrates for the previously developed hydroalkoxylation of non activated alkenes, the development of an asymmetric transfer hydrogenation of non activated C-C-double bonds and the development of a hydrogen autotransfer alkylation of pyrrols
Applications of the ruthenium-catalyzed anti-Markovnikov hydration of terminal alkynes in organic synthesis
No full textAnwendungen der Ruthenium-katalysierten anti-Markovnikov-Hydratisierung (AMH) terminaler Alkine werden aufgezeigt. Die Homologisierung terminaler Alkine via AMH und Bestmann-Ohira Alkinylierung wurde etabliert. Mit dieser Methode sind auch 13C-markierte Alkine leicht zugänglich, die in 13C-Click-Reaktionen eingesetzt wurden. Eine enantioselektive Synthese von 10-Hydroxystearinsäure via AMH als Schlüsselschritt wurde ebenfalls realisiert.Applications of the ruthenium-catalyzed anti-Markovnikov hydration (AMH) of terminal alkynes in organic synthesis were explored. They comprise a general homologation of terminal alkynes via AMH to aldehydes followed by Bestmann-Ohira alkynylation. 13C-labeled alkynes are readily accessed by that method and the latter were used in 13C-click reactions. The implementation of AMH as key-step in an enantioselective total synthesis of 10-hydroxystearic acid is also described
Catalytic hydration of terminal alkynes: mechanistic investigation of CpRu<sup>+</sup>-based catalyst systems and systematic activity screening
Full text linkIn dieser Arbeit wurden die aktivsten Cyclopentadienyl-Ruthenium-basierten Katalysatorsysteme für die anti-Markovnikov-Hydratisierung terminaler Alkine zu Aldehyden einer vergleichenden mechanistischen Studie unterzogen und dabei die Möglichkeit einer Abweichung vom üblichen Mechanismus via Ru(II)-Vinylidenintermediat überprüft. Darüber hinaus wurde eine effiziente Mikrowellen-basierte Screeningstrategie entwickelt, anhand der neue Markovnikov- und anti-Markovnikov-selektive Hydratisierungskatalysatoren identifiziert werden können.In the present thesis the most active cyclopentadienyl-ruthenium-based catalyst systems for anti-Markovnikov hydration of terminal alkynes to aldehydes were subjected to a comparative mechanistic study examining the possibility of a deviation from the conventional mechanism via Ru(II)-vinylidene intermediate. In addition, an efficient microwave-based screening strategy was developed that allows for identification of new Markovnikov and anti-Markovnikov selective hydration catalysts
The cyclization of 2-allylphenols to 2-methylcoumarans as an example for the asymmetric metal-catalyzed hydroalkoxylation of non-activated alkenes
No full textIn der vorliegenden Arbeit werden Metallalkoxide als schwach Lewis-saure Katalysatoren für die intramolekulare Hydroalkoxylierung von 2-Allylphenol zu 2-Methylcumaranen bei Temperaturen von > 200 °C vorgestellt. Für das gezielte Erhitzen wurde ein Mikrowellenreaktor verwendet.Erstmals gelang die asymmetrische Hydroalkoxylierung nicht-aktivierter Alkene. Mit einem chiralen Titancarboxylatkomplex als Katalysator konnten Enantioselektivitäten von bis zu 87% ee bei Reaktionstemperaturen von 220–240 °C erzielt werden.In this thesis metalalkoxides are introduced as weak Lewis-acidic catalysts for the cyclization of 2 allylphenols to 2-methylcoumarans at elevated temperature levels (> 200 °C). All catalytic cyclization were carried out in a microwave reactor.For the first time an asymmetric hydroalkoxylation of non-activated alkenes was realized. Using a chiral titanium carboxylate complex as catalyst, enantioselectivities up to 87% ee were achieved at reaction temperatures of 220–240 °C
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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