Le développement de catalyseurs efficaces à base de ruthénium a contribué à l’essor de la métathèse des oléfines. Cette réaction catalysée par des complexes à base de tungstène, rhénium ou molybdène, était appliquée, jusqu’à présent, à des oléfines non fonctionnalisées. L’utilisation de précurseurs à base de ruthénium a permis d’élargir les domaines d’applications de la métathèse grâce à leur tolérance élevée à de nombreuses fonctions organiques. En effet, les esters méthyliques d’huiles végétales peuvent être valorisés en bases chimiques par la réaction de métathèse en présence de catalyseurs à base de ruthénium. Nous reportons, ici, l’étude de la réactivité des complexes de ruthénium en éthénolyse de l’oléate de méthyle ainsi que la conception et la caractérisation de nouveaux systèmes. Nous exposerons la synthèse de nouveaux catalyseurs de ruthénium contenant des ligands carbènes aminoalkyle cycliques et l’évaluation de leurs performances. Nous montrerons que ces précatalyseurs présentent des propriétés catalytiques intéressantes pour l’éthénolyse de l’oléate de méthyle. Ces ligands ont l’avantage d’être modulables, ce qui nous offrent de nombreuses possibilités en terme d’originalité, tout comme la fonctionnalisation du ligand isopropoxybenzylidène qui permet l’immobilisation des catalyseurs dans les liquides ioniques. Par ailleurs, des tentatives de coordination d’autres ligands, tels que le fluorénylidène et le silylène, susceptibles de conduire à des complexes de ruthénium efficaces seront abordées. / The development of efficient ruthenium based catalysts has contributed to the development of olefin metathesis. This reaction catalyzed by tungsten, rhenium or molybdenum complexes, was applied, until now, to non-fonctionalized olefins. The use of ruthenium precursors allowed to broaden range of applications of olefin metathesis owing to their high tolerance to various organic functions. Indeed, methyl esters of vegetable oils can be converted into chemical base by metathesis in the presence of ruthenium based catalysts. In this study, we report ruthenium complexes reactivities for ethenolysis of methyl oleate as well as the design and the characterization of new systems. We describe the synthesis of new ruthenium catalysts containing cyclic alkyl(amino)carbenes and the evaluation of their performances. We show that these precatalysts display interesting catalytic properties for the ethenolysis of methyl oleate. Modifications of the carbenes ligands architecture provide the possibility to create novel catalysts as the functionalization of the isopropoxybenzylidene ligand which allowed immobilization of catalysts in ionic liquid. Besides, attempts to coordinate other ligands, such as fluorenylidène and silylene, able to afford active complexes will be discuss.

Die Grundlage vieler zukünftiger Flüssigszintillator-Neutrinoexperimente (SNO+, Daya Bay, LENA) ist das Lösungsmittel Lineare-Alkyl-Benzene (LAB, C6H5CnH2n+1, n = 10 - 13). Zusammen mit dem weit verbreiteten Szintillator 2,5-Diphenyloxazole (PPO) ist es ein farb- und geruchsloses Detektormaterial mit hohem Flammpunkt. Im Vergleich zu toluol- oder xylolbasierten Szintillatoren ist LAB+PPO preiswert und nicht gesundheitsschädlich. Die Eigenschaften von LAB machen es ebenfalls interessant für die Anwendung an nELBE, die Neutronenfugzeitanlage im Helmholtz-Zentrum Dresden - Rossendorf. Ein neuer Ansatz zur Bestimmung der Lichtausbeute im niederenergetischen Bereich (bis 2 MeV) wird vorgestellt. Kombiniert wurden Messungen mit (quasi) monoenergetischen Gammastrahlungs-Prüfstrahlern und einem in dieser Arbeit aufgebauten Compton-Spektrometer. Letzteres ermöglicht die Bestimmung der Lichtausbeute bis zu 5 keVee. Der Birks-Parameter wurde für eine Lösung von LAB + 3 g/l PPO sowie für den Flüssigszintillator NE-213 bestimmt. Die relative Lichtausbeute in Bezug auf letzteren konnte mit diesen Messmethoden ebenfalls ermittelt werden. Zur spektralen Analyse des Lumineszenzlichtes wurden Messungen an Fluoreszenz- und UV/VIS- Spektrometern durchgeführt. Die Pulsformdiskriminationsfähigkeit auf LAB basierenden Szintillatoren wurde während eines Flugzeitexperiments in einem gemischten n-gamma-Feld eines Cf(252)-Prüfstrahlers ermittelt. Dabei kamen unterschiedliche Algorithmen der semi-analogen und digitalen Pulsformdiskrimination zum Einsatz. / Linear alkyl benzene (LAB, C6H5CnH2n+1, n = 10 - 13) is the proposed solvent for the SNO+, the Daya Bay Neutrino and LENA experiment. In solution with the commonly used scintillator PPO it is a colourless, odourless and cheap liquid scintillator with a high fash point and low health hazard compared to toluene based ones. The properties of LAB make this scintillator interesting also for nELBE, the neutron time-of-fight facility at Helmholtz-Zentrum Dresden - Rossendorf. A new approach to measure the light yield in the low-energy range using a combination of quasi-monoenergetic photon sources and a Compton-spectrometer is described. The latter allows the measurement of the light yield down to 5 keVee (electron equivalent). The Birks- Parameter was determined for a homemade solution (LAB + 3 g/l PPO) and for NE-213. The light yield (relative to this standard scintillator) was confrmed by measurements using a fuorescence spectrometer. The ability of pulse-shape-discrimination in a mixed n-gamma- field of a Cf(252) source was tested using different digital and semi-analogue techniques.

No description available.

Nesta tese foram desenvolvidas metodologias sintéticas para a preparação de compostos orgânicos de telúrio empregando reagentes organometálicos. Primeiramente, foi estudada a abertura tipo SN2 de lactonas, por organoiltelurolatos de lítio ou magnésio para obtenção dos respectivos ácidos telurocarboxílicos. Os organoiltelurolatos foram preparados a partir da reação do telúrio elementar com o organometálico desejado, evitando a manipulação de diteluretos alquílicos que apresentam odores desagradáveis. As reações de abertura de lactonas são de fácil execução e levam aos produtos em bons rendimentos (60 - 88 %). Além disso, há a possibilidade de transformação funcional dos produtos para alcoóis, a partir de uma redução in situ, ou para acil derivados, por meio da esterificação de Steglich. Os alcoóis obtidos são precursores sintéticos das respectivas espécies dilitiadas. Adicionalmente, foram preparados teluretos de alquil arila funcionalizados a partir da reação entre alquiltelurolatos de lítio com iodetos de arila catalisada por CuI (5 mol %). Também foi explorada a influência de ligantes, o que levou a um aumento do rendimento das reações testadas. Também foi estudada a reatividade de n-butilteluretos vinílicos e arílicos com reagentes organometálicos. Ou seja, foi estudada a reação de troca telúrio/metal com compostos organoilzinco e organoilmanganês. As reações com compostos de organoilzinco apresentaram resultados insatisfatórios, devido à baixa seletividade de transferência de grupos ligados ao zinco. As reações com compostos de manganês apresentaram bons resultados, com seletividade na transferência de ligantes e bons rendimentos dos produtos de captura dos compostos de organoilmanganês com eletrófilos. Foi estudada também a reação de acoplamento cruzado entre compostos de aril Grignard e teluretos de butil-vinila na presença de cloreto de manganês e iodeto de cobre (MnCl2/CuI). A reação ocorreu com elevada estereosseletividade e com rendimentos de moderados a bons. / In this PhD thesis synthetic methodologies for the preparation of organic compounds of tellurium employing organometallic reagents were developed. Initially, the SN2 type opening reaction of lactones by means of lithium or magnesium organotellurolates leading to the corresponding carboxylic acids was studied. The metal organotellurolates were prepared by reacting elemental tellurium with organolithium or Grignard reagents, avoiding the manipulation of bad smelling dialkylditellurides. The opening reactions occured easily, leading to the products in good yields (60 - 88 %). The tellurocarboxylic acids were transformed into alcohols by in situ reduction or into acyl derivatives, by a Steglich esterification. The obtained alcohols are precursors of the corresponding dilithium species by reaction with n-butyllithium. Additionally, functionalized arylbutyltellurides were prepared by reacting lithium alkyltellurolates with functionalized aryl iodides in the presence of CuI (5 mol %). The influence of ligands was also explored, leading to improved yields. It was also investigated the reactivity of vinyl butyltellurides with organometallic reagents. The tellurium/metal exchange reaction with organozinc and organomanganese compounds was studied. The reactions with organozinc compounds presented unsatisfactory results, in view of the low selectivity transfer of the groups linked to zinc. The reaction with organomanganese compounds presented better results in view of the fast Te/Mn exchange reaction, selectivity in the ligand transfer and good yields of the organomanganese capture products with electrophiles. It was also studied the cross-coupling reaction between aryl Grignard reagents and vinyl butyl tellurides in the presence of manganese chloride and copper iodide (MnCl2/CuI). The reaction occuried with high stereosselectivity and in moderate to good yields.

Nesta tese foram desenvolvidas metodologias sintéticas para a preparação de compostos orgânicos de telúrio empregando reagentes organometálicos. Primeiramente, foi estudada a abertura tipo SN2 de lactonas, por organoiltelurolatos de lítio ou magnésio para obtenção dos respectivos ácidos telurocarboxílicos. Os organoiltelurolatos foram preparados a partir da reação do telúrio elementar com o organometálico desejado, evitando a manipulação de diteluretos alquílicos que apresentam odores desagradáveis. As reações de abertura de lactonas são de fácil execução e levam aos produtos em bons rendimentos (60 - 88 %). Além disso, há a possibilidade de transformação funcional dos produtos para alcoóis, a partir de uma redução in situ, ou para acil derivados, por meio da esterificação de Steglich. Os alcoóis obtidos são precursores sintéticos das respectivas espécies dilitiadas. Adicionalmente, foram preparados teluretos de alquil arila funcionalizados a partir da reação entre alquiltelurolatos de lítio com iodetos de arila catalisada por CuI (5 mol %). Também foi explorada a influência de ligantes, o que levou a um aumento do rendimento das reações testadas. Também foi estudada a reatividade de n-butilteluretos vinílicos e arílicos com reagentes organometálicos. Ou seja, foi estudada a reação de troca telúrio/metal com compostos organoilzinco e organoilmanganês. As reações com compostos de organoilzinco apresentaram resultados insatisfatórios, devido à baixa seletividade de transferência de grupos ligados ao zinco. As reações com compostos de manganês apresentaram bons resultados, com seletividade na transferência de ligantes e bons rendimentos dos produtos de captura dos compostos de organoilmanganês com eletrófilos. Foi estudada também a reação de acoplamento cruzado entre compostos de aril Grignard e teluretos de butil-vinila na presença de cloreto de manganês e iodeto de cobre (MnCl2/CuI). A reação ocorreu com elevada estereosseletividade e com rendimentos de moderados a bons. / In this PhD thesis synthetic methodologies for the preparation of organic compounds of tellurium employing organometallic reagents were developed. Initially, the SN2 type opening reaction of lactones by means of lithium or magnesium organotellurolates leading to the corresponding carboxylic acids was studied. The metal organotellurolates were prepared by reacting elemental tellurium with organolithium or Grignard reagents, avoiding the manipulation of bad smelling dialkylditellurides. The opening reactions occured easily, leading to the products in good yields (60 - 88 %). The tellurocarboxylic acids were transformed into alcohols by in situ reduction or into acyl derivatives, by a Steglich esterification. The obtained alcohols are precursors of the corresponding dilithium species by reaction with n-butyllithium. Additionally, functionalized arylbutyltellurides were prepared by reacting lithium alkyltellurolates with functionalized aryl iodides in the presence of CuI (5 mol %). The influence of ligands was also explored, leading to improved yields. It was also investigated the reactivity of vinyl butyltellurides with organometallic reagents. The tellurium/metal exchange reaction with organozinc and organomanganese compounds was studied. The reactions with organozinc compounds presented unsatisfactory results, in view of the low selectivity transfer of the groups linked to zinc. The reaction with organomanganese compounds presented better results in view of the fast Te/Mn exchange reaction, selectivity in the ligand transfer and good yields of the organomanganese capture products with electrophiles. It was also studied the cross-coupling reaction between aryl Grignard reagents and vinyl butyl tellurides in the presence of manganese chloride and copper iodide (MnCl2/CuI). The reaction occuried with high stereosselectivity and in moderate to good yields.

There is an increasing trend towards advancing the understanding and development of ethylene oligomerization catalysts, both in academia and industry. The metal of choice in this chemistry is invariably chromium, which has shown great versatility in selective trimerization/tetramerization, non-selective oligomerization and polymerization of ethylene. While much success has been achieved in ethylene trimerization, the same con not be said about tetramerization catalysis. Aminophosphine based ligands have demonstrated their ability towards selective 1-octene production, however, the popular PNP catalyst is able to achieve only 70% selectivity. In order to explore the possibility of developing and enhancing the selectivity of chromium based ethylene tetramerization catalyst, this thesis work was undertaken. The ligand systems we chose for our work were bidentate aminophosphine based (PN(CH2)nNP), which has yielded interesting selective oligomerization. Subtle modifications were found to result in drastic changes in selectivity, from tetramerization (PN(CH2)3NP) to trimerization (PN(CH2)2NP). We managed to successfully develop the first truly selective (over 90%) 1-octene catalyst with polymer-free behavior. Further modifications on the ligand framework, where one atom of Si was used to link the two NP units, resulted in non-selective oligomerization, in which case we determined that the oxidation-state of chromium is a key player. We explored other modifications on our selective ligands in which one of the arms on the bidentate ligand was replaced with a base-donor amine, phosphine or pyridine, and resulted in interesting selectivity changes. The final modification that we tested was a novel N(CH2)2P ligand and found it to be a highly active, non-selective oligomerization catalyst.

Nanostructured materials play a vital role in almost all aspects of science and technology in the 21st century. The materials include nanoparticles, nanofilms, biological membranes etc. whose physicochemical properties are size-dependent. Thin films have wide range of applications in various branches of science. One of the efficient methods to form miniaturized structures for device applications is to fabricate nanostructured films on different substrates. Surfactant assembly on metallic and non-metallic surfaces based on self assembly and Langmuir-Blodgett technique offers a unique way to form thin films at molecular levels. The process of formation of unimolecular assemblies gives the flexibility of tuning the properties of underlying substrates for various applications including wetting characteristics, lubrication, passivation, mimicking biological phenomena etc. Towards this direction, self assembled monolayers (SAMs) of alkanethiols on gold and silver surfaces have been studied comprehensively for the past two decades. The reported literature on short chain length thiol-based monolayers is however, limited since the formation using conventional methods yield poor quality monolayers. The short chain length monolayers are useful in various applications like tribology, layer-by-layer assemblies, biosensors etc. Hence, it is essential to reproducibly form SAMs of various chain lengths and understand their properties. The present study is related to the formation of SAMs of alkanethiols and diselenides on gold and silver surfaces to form ordered and well-oriented monolayers. Monolayers of varying chain lengths (CH3(CH2)nSH where n = 3, 5, 7, 9, 11, 15) have been formed on gold and silver surfaces using different methods, (1) adsorption from neat thiols; (2) adsorption under electrochemical control and (3) adsorption from alcoholic solutions of the thiols. The characteristics features of the SAMs have been followed based on three different aspects, (i) structure and stability of the methylene groups (ii) interfacial characteristics involving the end group and the solvent and (iii) metal-head group interactions. The structure and stability of the monolayers have been followed based on vibrational spectroscopy and electrochemistry under different environment including thermal perturbations. The stability of the SAMs at different temperatures and subsequent changes associated with the orientation / packing has been monitored both in the dry state using reflection absorption infrared spectroscopy (RAIRS) and under electrochemical conditions using cyclic voltammetry and impedance analysis. Monolayers adsorbed from neat thiols show superior quality in terms of stability and structural arrangement. Short chain thiols with n = 3, 5, 7 show substantial stability when the adsorption is carried out from neat thiols. Figure 1 shows the RAIR spectra of hexanethiol SAM on gold adsorbed by three different procedures. Monolayers adsorbed under potential control behave very similar to the monolayers adsorbed from neat thiol as for as stability and structural orientation are concerned. Monolayers prepared using conventional methods of adsorption from alcoholic solutions are of inferior quality in terms of stability and arrangement especially for the short chain lengths. This is likely to be due to the fact that monolayers prepared using conventional methods may have intercalated solvent molecules within the monolayer assembly that degrade the integrity of the SAM leading to poor quality. The blocking characteristics of the monolayers for diffusing redox couple have been followed by determining the heterogeneous electron transfer rate constant using electrochemical techniques. The spectroscopic data and the electrochemical data follow similar trend indicating the superior quality of monolayer adsorbed from neat thiol in terms of stability as compared to conventionally prepared monolayers. Figure 1. RAIR spectra of hexanethiol-SAMs on Au(111) surface at 25C. The monolayers are formed by adsorption (A) from neat thiol (B) under potential control and (C) from alcoholic solution of the thiol. Wavenumber (cm-1) The interfacial characteristics of the monolayers (effect of end group functionality on the solvent properties) have been monitored on the basis of capacitance, contact angle and atomic force microscopy- measurements. Well-organized monolayers behave like good capacitors with relatively low values of double layer capacitance in presence of a liquid electrolyte as compared to the expected values based on known thickness and dielectric constant of the SAMs. This behavior can be explained by invoking the depletion of water density at the methyl terminated SAM-water interface where the solvent properties are different from that of bulk. Variation of one such property, dielectric constant, has been mapped using force measurement based on AFM. Dielectric constant of water changes from the bulk value of 78 to a low value as given in figure 2. This cross-over occurs within a span of 1-3 nm depending on the chain length of the thiol. Of the three procedures used, the ones based on the use of neat thiol and electrochemical adsorption result in well-oriented alkyl chains followed by highly oriented methyl terminal groups. This is responsible for the high hydrophobic nature of the interface and the subsequent observation of interfacial water properties. The SAMs prepared from ethanol fail to show the hydrophobic effects. Hydrophilic monolayers (NH2 terminated monolayers) fail to show depletion of water density at the interface indicating the importance of end group functionality in altering the interfacial characteristics of the monolayer. Figure 2. Spatial variation of dielectric permittivity of water at the hexanethiol SAM - water interface. The SAM is formed on gold (111) surface; (a) from ethanolic solution of the thiol (b) under electrochemical control (c) from neat thiol. The origin on the x-axis is the position of the methyl groups of SAM and the direction towards right side is in to the bulk water. The well-oriented SAMs have been used to follow the adsorption of a biopolymer. Zein protein is a prolamine of maize and is projected to be a biocompatible coating for food products and food containers. Hence, it is essential to prepare impermeable coatings of zein with different surface wetting properties. The adsorption of zein on highly ordered SAMs with hydrophobic or hydrophilic end group functionality has been studied and the orientation of the protein followed using spectroscopy, microscopy and electrochemistry. It is observed that zein shows higher affinity towards hydrophilic than hydrophobic surfaces with small foot print size on the Figure 3. Orientation of zein protein on hydrophilic and hydrophobic SAM as deciphered from the experimental data. hydrophilic surface resulting in large surface coverage. Figure 3 shows the schematics of zein deposits on hydrophilic and hydrophobic SAM surfaces determined based on spectroscopy, quartz crystal microbalance and electrochemical studies. The AFM shows cylindrical, rod-like and disc-like features of zein on hydrophilic surfaces that form the base units for the growth of cylindrical structures of zein. The published literature on the studies on SAMs on silver surfaces reveals that there is no consensus on the structure of the monolayers on silver. This may be due to the difficulty in getting pristine oxide-free surfaces in the case of silver and this is likely to affect the monolayer quality. Hence, it is decided to prepare SAMs of alkanethiols on silver and study their characteristics. Subtle differences between the monolayers adsorbed from neat thiol and from alcoholic solutions of thiols have been observed in terms of stability and permeability. Atomic force microscopic studies illustrate the presence of depletion of water at the SAM-aqueous interface. Diselenide-based monolayers have been formed on gold to understand the head group-substrate interactions on the monolayer properties. The disorder observed on short chain diselenide-based monolayers formed from alcoholic solutions can be eliminated by adsorption from neat compounds as described for the thiols. A preliminary account on the stability of SAMs under hydrodynamic conditions has been given based on rotating disc electrode voltammetry. It is observed that the SAMs get well-ordered when the electrode is rotated at a fast rate leading to the hypothesis that the monolayer assembly gets annealed as a function of the rotation rate. The thesis is planned as follows: Chapter 1 gives general introduction about organic thin films with particular emphasis on self-assembled monolayers on gold and silver, their characteristics in terms of stability, interfacial properties and adsorption behaviour. Chapter 2 deals with the experimental methodologies and schematics used for the preparation and characterization of the monolayers. Chapter 3 is on the contribution of alkyl spacer to the stability of the monolayers studied using spectroscopy and electrochemistry. Chapter 4 deals with the interfacial properties of the SAMs in presence of aqueous medium. In order to emphasize the importance of the terminal functional groups, adsorption of zein has been demonstrated on surfaces of controlled wettablity. Chapter 5 explains the formation and stability of monolayers of short and long chain alkyl diselenides on gold surfaces. Chapter 6 gives the structural and interfacial characteristics of alkanethiol monolayers on silver surfaces. The stability and subsequent changes of alkanethiol monolayers under hydrodynamic conditions has been discussed in the appendix section.(For fig pl refer pdf file.)

In solution, the environment around the solute is determined solely by the solvent molecules, which are present closer to the solute. This interaction between solute and solvent shell is very crucial for equilibrium structure and reactivity of the solute. In the thesis, first we have investigated control of solvent polarity on the excited structure of 2,2,2 trifluroacetophenone and later effect of electronic excitation on the solvent shell organization has been described. It has been reported in literature that the lowest energy triplet configuration of 2,2,2 trifluroacetophenone corresponds to n,π* state. There are some other reports in favor of the probable existence of 2,2,2 trifluroacetophenone in n,π* lowest triplet state. Thus, transient absorption and time resolved resonance Raman spectroscopic methods have been used along with theoretical calculations to investigate the discrepancy in the assignment of the lowest triplet state configuration It has been observed that the lowest triplet state of 2,2,2 trifluroacetophenone is indeed nπ* and there is a solvent polarity induced change in triplet state energy ordering and structure changes. The absorption spectrum of hydrated electron is broad and structureless. So, it was though that the broadening feature could be attributed to homogeneous and inhomogeneous broadening. Transient resonance Raman spectrum of the water bending mode in presence of hydrated electron has been recorded at different excitation wavelengths. Interestingly, it has been observed that, peak position of water bending mode in presence of hydrated electron alters with change of excitation wavelength. A model has been proposed based on the experimental data.

The present dissertation covers the total synthesis of viridiofungin triesters and studies toward the total synthesis of xeniolide F. In both cases, sigmatropic rearrangements of α-allyloxy substituted α,β-unsaturated esters are employed: for the viridiofungin ester synthesis a [2,3]-Wittig rearrangement and for the xeniolide F synthesis a catalytic asymmetric Claisen rearrangement CAC. For both rearrangement reactions the historical development, main characteristic and important variations are discussed. The viridiofungin triester synthesis represents a convergent and highly flexible route toward these natural products. The [2,3]-Wittig rearrangement allowed the diastereoselective synthesis of the polar head group with two adjacent stereogenic centers. The E-configured double bond was formed by a Julia-Kocienski olefination. During the studies toward the total synthesis of xeniolide F a new, diastereoselective strategy for the generation of allyl vinyl ethers with E-configured vinyl ether double bond was established employing rhodium catalyzed OH-insertion and an E-selective Horner-Wadsworth-Emmons olefination. Under the conditions of the catalytic asymmetric Claisen rearrangement (CAC) this highly substituted allyl vinyl ether rearranged diastero- and enantioselectively to the corresponding a-keto ester. This example clearly illustrates the high potential of the CAC as synthetic tool for natural product synthesis. / In der vorliegenden Arbeit wird die Totalsynthese von Tirestern der Viridiofungine A, A2 und A4 sowie die Synthese eines Schlüsselintermediates für die Totalsynthese von Xeniolid F dargestellt. In beiden Fällen wird ausgehend von einem α-allyloxysubstituierten α,β-ungesättigten Ester eine Umlagerungsreaktion als Schlüsselschritt eingesetzt: im Falle der Viridiofunginester eine diastereoselektive [2,3]-Wittig-Umlagerung, bei den Arbeiten zur Totalsynthese von Xeniolid F eine diastero- und enantioselektive, katalytische Claisenumlagerung. Für beide Umlagerungsreaktionen werden ausführlich die theoretischen Hintergründe sowie die historische Entwicklung und wichtige Varianten besprochen. Mit der Viridiofungintriestersynthese wird eine konvergente und bezüglich der lipophilen Seitenkette sehr flexible Syntheseroute vorgestellt. Die [2,3]-Wittig-Umlagerung konnte dabei erfolgreich für die diastereoselektive Synthese der hochsubstituierten, polaren Kopfgruppe der Viridiofunginester mit zwei benachbarten stereogenen Zentren (davon eines quartär) eingesetzt werden. Zur Bildung der E-konfigurierten Doppelbindung wurde die Julia-Kocienskie-Olefinierung ausgenutzt. Bei den Arbeiten zur Totalsynthese von Xeniolid F wurde eine neuartige Strategie zur diastereoselektiven Synthese eines Allylvinylethers mit E-konfigurierter Vinyletherdoppelbindung eingesetzt. Die Horner-Wadsworth-Emmons-Olefinierung (HWE-Olefinierung) generierte dabei E-selektiv die Vinyletherdoppelbindung. Das für die HWE-Olefinierung benötigte Phosphonat wurde durch rhodiumkatalysierte OH-Insertion aus einem Allylalkohol und einem Diazaphosphonoacetat hergestellt. Der hochsubstituierte Allylvinylether wurde unter den Bedingungen der katalytisch asymmetrischen Claisenumlagerung umgesetzt und führte mit exzellenter Diastereo- und Enantioselektivität zum entsprechenden α-Ketoester. Anhand dieses Beispiels konnte das Potential der katalytisch asymmetrischen Claisenumlagerung zum Aufbau von hochfunktionalisierten Bausteinen für die Naturstoffsynthese verdeutlicht werden.

No description available.