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Carbon-carbon bond formation is one the fundamental reactions in organic synthesis. The quest for the development of new and more efficient processes for the construction of this bond has been an ongoing focus for years. The transformations that permit the use of simple precursors to access complex structural architectures in the absence of stoichiometric quantities by-products are highly desirable. Hydrogen is a cheapest and cleanest reductant available to the mankind. The catalytic hydrogenation has been widely utilized in the industry, however the construction of the carbon-carbon bond under hydrogenative conditions has been achieved only for alkene hydroformylations and Fisher-Tropsh process and limited to the use of carbon monoxide. The extension of the hydrogenative carbon-carbon bond formations beyond aforementioned processes would be of a great significance to the synthetic community. The overview of allene use in the metal catalyzed reactions to achieve carbonyl and imine allylation and vinylation is presented in Chapter 1. The following chapter vii discusses the development of metal catalyzed hydrogenative and transfer hydrogenative coupling of allenes and carbonyl compounds to afford allylation products. These studies have resulted in the development of the first carbonyl allylation from the alcohol oxidation level. Chapter 3 discusses efforts towards achieving highly enantioselective hydrogenative coupling of alkynes to carbonyl compounds. / text

L'objectif de ce travail était d'accéder au squelette des alcaloïdes de type Aspidosperma et Strychnos à partir d'arylcyclohexa-2,5-diènes. Ces derniers sont d'abord synthétisés par réaction de Birch alkylante, puis ont été désymétrisés dans un premier temps par des réactions de Michael. Cette réaction fournit la cétone de Büchi, le noyau tétracyclique des alcaloïdes Aspidosperma en seulement en 6 étapes et un rendement global de 17%. Dans un second temps, la réaction d'amination oxydante catalysée par des métaux (Pd, Cu) a été développée. Cette réaction a permis un accès rapide au squelette pentacyclique d’aza-aspidospermanes et au squelette tétracycliques des alcaloïdes de type Strychnos. En parallèle, nous avons décrit une approche vers le squelette pentacyclique de la mossambine et la strychnine. / The aim of this work was to access the skeleton of the Aspidosperma and the Strychnos alkaloids using arylcyclohexa-2,5-dienes as common synthetic precursors. Initially, these arylcyclohexadienes were synthesized through Birch reductive alkylation reactions. The desymmetrization of these cyclohexadienes was developed via the Michael addition reaction, providing the Büchi ketone, the tetracyclic core of Aspidosperma alkaloids, in only 6 steps and 17% overall yield. On the other hand, we described the oxidative amination reaction catalyzed by metals (Pd, Cu). The palladium oxidative amination reaction allowed a fast access to the pentacyclic framework of aza-aspidospermanes and the tetracyclic framework of the strychnos. In parallel, we have described an approach toward the pentacyclic skeleton of mossambine and strychnine.

[pt] Em processos de lixiviação sulfúrica de concentrados minerais contendo magnésio, é comum que este metal permaneça em solução após as etapas de neutralização e precipitação seletiva de impurezas. Sob a perspectiva de integração mássica desse material, é possível utilizar o MgSO4.7H2O obtido como um precursor na formação de MgO que, por sua vez, pode ser empregado nas etapas de neutralização dos fluxogramas de processo em substituição aos insumos tradicionalmente empregados e adquiridos pela indústria. Desta forma, os processos de decomposição redutora se apresentam como uma alternativa para a redução da temperatura de processamento do MgSO4.7H2O, incorporando aos produtos um melhor desempenho quanto à reatividade. Dentro desse contexto, o presente trabalho tem como objetivo apreciar o processo de decomposição redutora deste sal na presença de hidrogênio, entender o comportamento do sistema reacional e caracterizar os produtos reacionais, via MEV/EDS, ICP/OES e FTIR. O estudo termodinâmico através do software Medusa confirmou a alta solubilidade entre o sulfato e o magnésio e o software HSC certificou que a presença de um agente redutor é significante, pois há uma diminuição considerável de temperatura do processo. Já o estudo experimental da decomposição em atmosfera redutora indica que é possível transformar o referido sulfato no óxido de interesse em temperaturas abaixo de 800 graus Celsius o que, por sua vez, caracteriza-se como uma redução de pelo menos 400 graus Celsius quando comparado com o mesmo processo em atmosfera não redutora. A reatividade do produto final foi classificada como adequada, de acordo com a metodologia proposta, atingindo a faixa de pH básico dentro do intervalo de tempo apropriado (abaixo de 120 s). Com isso, o estudo indica que o referido processo unitário pode se configurar, de fato, como uma alternativa para a diminuição da temperatura de decomposição do MgSO4.7H2O. / [en] In sulfuric leaching processes of mineral concentrates containing magnesium, it is common for this metal to remain in solution after the steps of neutralization and selective precipitation of impurities. Removing the liquors can easily be achieved by crystallization of the sulfate in the heptahydrate form. From the perspective of the mass integration of this material, it is possible to use the MgSO4.7H2O obtained as an intermediate in MgO formation which, in return, can be used in the neutralization steps of the process flowcharts instead of the inputs traditionally used and acquired by the industry. To this end, the resulting oxide must have a high surface area and, consequently, a high acid reactivity. Considering the simple decomposition of the magnesium salt, the temperature which this reaction occurs is high so that such properties are incorporated into the product. In this way, the reductive decomposition processes are presented as an alternative for the reduction of the processing temperature of the MgSO4.7H2O, incorporating a better performance in the reactivity to the products. In this context, the present work aims to evaluate the process of reductive decomposition of this salt in the presence of hydrogen, to understand the behavior of the reaction system and to characterize the reaction products, via MEV/EDS, ICP/OES and FTIR. The experimental study of the decomposition in reducing atmosphere indicates that it is possible to transform said sulfate into the oxide of interest at temperatures below 800 Celsius degrees which, in return, is characterized as a reduction of at least 400 Celsius degrees when compared to the same process in a non-reducing atmosphere. The reactivity of the final product was classified as adequate, according to the proposed methodology, reaching the basic pH range within the appropriate time interval (below 120s). Thus, the study indicates that the said unitary process can in fact be configured as an alternative for the decrease in the decomposition temperature of MgSO4.7H2O.

Sei $K$ eine endliche Erweiterung von $QQ_p $ und sei $R$ der Robba-Ring mit Koeffizienten in $K$ sein, die mit einem absoluten Frobenius-Lift $phi$ ausgestattet sind. Sei $F$ der Fixköper von $K$ unter $phi $ und sei $G$ eine verbundene reduktive Gruppe über $F$. Diese Arbeit untersucht $G$-$ (phi,nabla)$-Module über $R$, nämlich $(phi,nabla)$-Module über $R$ mit einer zusätzlicher $G$-Struktur. In Kapitel 3 konstruieren wir einen gefilterten Faserfunktor aus der Darstellungskategorie von $G$ auf endlich-dimensionalen $F$-Vektorräumenbis zur Kategorie von $QQ$-gefilterten Modulen über $R$, und beweisen, dass dieser Funktor spaltbar ist. In Kapitel 4 beweisen wir eine $G$-Version des $p$-adischen lokalen Monodromie-Satzes. In Kapitel 5 beweisen wir eine $G$-Version des logarithmischen lokalen Monodromie-Satzes unter bestimmten Annahmen. Als Anwendung fügen wir jedem $G$-$(phi,nabla)$-Modul eine Weil-Deligne-Darstellung der Weil-Gruppe $W_{kk((t))} $ in $G(K^{nr})$ an, wobei $kk$ der Restklassenkörper von $K$, und $K^{nr}$ die maximal unverzweigte Erweiterung von $K$ ist. / Let $K$ be a finite extension of $QQ_p$ and let $R$ be the Robba ring with coefficients in $K$, equipped with an absolute Frobenius lift $phi$. Let $F$ be the fixed field of $K$ under $phi$ and let $G$ be a connected reductive group over $F$. This thesis investigates $G$-$(phi,nabla)$-modules over $R$, namely $(phi,nabla)$-modules over $R$ with an additional $G$-structure. In Chapter 3, we construct a filtered fiber functor from the category of representations of $G$ on finite-dimensional $F$-vector spaces to the category of $QQ$-filtered modules over $R$, and prove that this functor is splittable. In Chapter 4, we prove a $G$-version of the $p$-adic local monodromy theorem. In Chapter 5, we prove a $G$-version of the logarithmic $p$-adic local monodromy theorem under certain assumptions. As an application, we attach to each $G$-$(phi,nabla)$-module a Weil-Deligne representation of the Weil group $W_{kk((t))}$ into $G(K^{nr})$, where $kk$ is the residue field of $K$, and $K^{nr}$ is the maximal unramified extension of $K$.

Selon une vision populaire, l’évolution serait un processus de « progrès » qui s’accompagnerait d’un accroissement de la complexité moléculaire des êtres vivants. Cependant, les programmes de séquençage des génomes ont révélé l’existence d’espèces dont les lignées ont, au contraire, subi une réduction massive de leur génome. Ainsi, chez les cyanobactéries Prochlorococcus et Pelagibacter ubique, certaines lignées ont subi une réduction de 30% de leur génome. Une telle évolution « à rebours », dite évolution réductive, avait déjà été observée pour des bactéries endosymbiotiques, pour lesquelles la sélection naturelle n’est pas assez efficace pour éliminer les mutations délétères comme les pertes de gènes. Cela vient notamment du fait que ces bactéries endosymbiotiques subissent, à chaque reproduction de leur hôte, une réduction drastique de leur taille de population. Cette explication semble peu plausible pour des cyanobactéries marines comme Prochlorococcus et Pelagibacter, qui ont un mode de vie libre et qui font partie des bactéries les plus abondantes des océans. D’autres hypothèses ont ainsi été proposées pour expliquer l’évolution réductive comme l’adaptation à un environnement stable et pauvre en nutriments, des forts taux de mutation, mais aucun de ces hypothèses ne semble capable d’expliquer toutes les caractéristiques génomiques observées. Dans cette thèse, nous nous intéressons au cas de l’évolution réductive chez Prochlorococcus, pour laquelle de nombreuses séquences et données sont disponibles. Deux approches sont utilisées pour cette étude : une analyse phylogénétique des génomes de Prochlorococcus, et une approche théorique de simulation où nous testons différents scénarios évolutifs pouvant conduire à une évolution réductive. La combinaison de ces deux approches permet finalement de proposer un scénario plausible pour expliquer l'évolution réductive chez Prochlorococcus. / Given a popular view, evolution is an incremental process based on an increase of molecular complexity of organisms. However, some organisms have undergo massive genome reduction like the endosymbionts. In this case the reduction can be explained by the Muller’s ratchet due to the endosymbiont lifestyle with small population and lack of recombination. However, in some marine bacteria, like Prochlorococcus et Pelagibacter, lineage have undergo up to 30% of genome reduction. Their lifestyle is almost the opposite to the one of the endosymbionts and reductive genome evolution can not be easily explicable by the Muller’s ratchet. Some other hypothesis has been proposed but none can explain all the observed genomic characteristics. In the thesis, I am interested in the reductive evolution of Prochlorococcus. I used two approaches: a theoretical one using simulation where different scenarios are tested and an analysis of Prochlorococcus genomes in a phylogenetic framework to determine the causes and characteristics of genome reduction. The combination of these two approaches allows to propose an hypothetical evolutive history for the reductive genome evolution of Prochlorococcus.

Foi estudada a degradação do pentaclorofenol (PCP) em reator aneróbio horizontal de leito fixo (RAHLF) de volume de 2000 mL. O reator foi inoculado com microrganismos oriundos de reatores aneróbios não previamente adaptados a PCP. Atividade microbiana foi monitorada através de técnicas clássicas na presença do organoclorado na faixa de 2,0 a 13 mg/L de PCP. O reator apresentou eficiência de 97% na remoção de DQO e completo desaparecimento do composto de PCP em todas as concentrações testadas. A fração orgânica foi consumida totalmente na primeira terça parte do reator de acordo com os valores determinados de ácidos voláteis, DQD e PCP. Não foi verificada inibição da atividade de culturas microbianas. Os exames microscópicos, fluorescência e varredura, permitiram verificar o predomínio de microrganismos pertencentes ao Domínio Archea. As técnicas moleculares PCR, ARDRA e hibridação slot-blot confirmaram o predomínio do Domínio Archaea e possibilitaram a verificação de alterações na diversidade das populações após adição de 2 mg PCP/L. Conclui-se que o reator sem prévia adaptação do inóculo foi eficiente para o tratamento do PCP, e os microrganismos relacionados às Archaea metanogênicas acetocláticas podem estar envolvidas na degradação deste composto. / The degradation of pentachlorophenol (PCP) was studied in a 2000 mL. Horizontal Anaerobic Immobilized System (HAIS). The reactor was inoculated with microorganisms obtained from an anaerobic reactor without previous adaptation to the PCP. The microbial activity was evaluated by using classic techniques in order to . monitor its behavior during the HAIS fed with a range of PCP between 2.0 to 13 mg/L. The reactor presented 97% of efficiency in the removal of COD and complete decrease of PCP in alI concentrations tested. The total consumption of organic fraction took place mainly in the first third part of the reactor according the values of volatile fatty acids, COD and PCP obtained. Microbial inhibition was not verified in during HAIS operation. Microscopic examinations allowed certifying the Archaea Domain predominance according the morphologies observed. The molecular techniques polimerase chain reaction (PCR), ARDRA and slot-blot hibridation confirmed the predominance of Archaea Domain and alIowed verifying some changes in the population\'s diversity under additions of 2mg PCP/L. The efficiency of PCP decreased in the anaerobic reactor was related to the presence of Archaea Domain, especially the acetoclastic methanogens, whose where probably involved with the organochlorine compound degradation.

Die toxische sowie kanzerogene Verbindung 1,2-Dichlorpropan (DCP) ist weit verbreitet in Industrie und Landwirtschaft. Die Verbindung zeigt eine geringe chemische Reaktivität, ist nur mäßig wasserlöslich und unter aeroben Bedingungen weitestgehend beständig gegenüber mikrobiellen Abbauprozessen in der Umwelt. Als Folge reichert sich DCP in Grundwässern, Sedimenten und Böden an und gefährdet über die Nahrungskette die Gesundheit von Mensch und Tier. Um DCP effizient und ökonomisch zu unbedenklichen Verbindungen abzubauen, wurden mikrobielle Mischkulturen aus belasteten Sedimenten angereichert und in einen Wirbelschichtreaktor überführt. Dieses Verfahren ermöglichte eine kontinuierliche anaerobe Dechlorierung von DCP zu Propen. Grundsätzlich stellen biologische Abbauverfahren, bei denen komplexe mikrobielle Mischpopulationen eingesetzt werden, einen vielversprechenden Weg zur Transformation chlororganischer Verbindungen dar. Jedoch liegen üblicherweise nur wenige Informationen über die Zusammensetzung der betreffenden Populationen vor, so daß eine Optimierung bzw. effiziente Steuerung des Prozesses erheblich erschwert wird. Gegenstand der vorliegenden Arbeit war die Bestimmung der mikrobiellen Zusammensetzung der DCP-dechlorierenden Bioreaktorpopulation. Aufgrund der bekannten Limitierungen klassisch-mikrobiologischer Nachweisverfahren wurde eine Kombination mehrerer molekulargenetischer Methoden eingesetzt, die auf der vergleichenden Sequenzanalyse ribosomaler RNA beruhten. Die Untersuchungen zeigten, daß die Bakterienpopulation des Reaktors außerordentlich divers zusammengesetzt war und im wesentlichen aus bislang nicht-kultivierten Arten bestand. Es dominierten "Grüne nicht-schwefelhaltige Bakterien" (green nonsulfur bacteria) sowie Grampositive Bakterien mit niedrigem GC-Gehalt. Die Archaea hingegen waren fast ausschließlich durch zwei bekannte methanogene Spezies vertreten, Methanosaeta concilii sowie Methanomethylovorans hollandica. Der Vergleich der gewonnenen rDNA-Daten mit denen anderer Lebensräume ergab, daß Süßwasserhabitate, in denen chlororganische Verbindungen reduktiv umgesetzt werden, offenbar eine spezifische Populationsstruktur aufweisen. Es konnten spezifische 16S rDNA-Gruppen definiert werden (SHA-Cluster), die auch nach längerem Reaktorbetrieb noch nachgewiesen werden konnten. Darüber hinaus wurden Dehalobacter restrictus- sowie Dehalococcoides ethenogenes-ähnliche Bakterien in der DCP-dechlorierenden Bioreaktorpopulation gefunden. Beide Spezies sind in der Lage, chlororganische Verbindungen unter Verwendung von Wasserstoff als alleinigem Elektronendonor reduktiv zu dechlorieren. Es ist davon auszugehen, daß Dehalobacter und Dehalococcoides spp. aufgrund ihrer Physiologie an der reduktiven Umsetzung des DCPs beteiligt sind. Die Untersuchung der Population über einen längeren Zeitraum zeigte überdies, daß Bakterien der Gattung Dehalobacter überproportional angereichert und daraufhin zur dominierenden Spezies im Reaktor wurden. Dieser Befund läßt auf eine zentrale Rolle von Dehalobacter spp. bei der Transformation von DCP zu Propen schließen. Konsequenterweise führte die Zugabe von Wasserstoff zum Reaktor zur einer deutlichen Steigerung des DCP-Umsatzes. Dehalobacter und Dehalococcoides spp. sowie die anderen durch SHA-Cluster repräsentierten Bakterien stellen potentielle Indikatororganismen für die DCP-Transformation im Reaktor dar. Ein kontinuierliches Monitoring dieser Bakterien würde zu einer effizienteren Steuerung des Dechlorierungsprozesses und damit zu einer Optimierung des Verfahrens führen. / The toxic and carcinogenic compound 1,2-dichloropropane (DCP) is widely used in industry and agriculture. DCP shows a low chemical reactivity. It is only moderately soluble in aqueous systems and almost recalcitrant to microbial degradation under aerobic conditions. As a consequence DCP accumulates in groundwater, sediments and soil, thus endangering humans and animals via the food chain. To efficiently transform DCP to harmless organic compounds microbial mixed cultures have been enriched from sediments and were subsequently transferred into a fluidized bed bioreactor. This process allowed a continuous anaerobic dechlorination of DCP to propene. Bioreactor processes using complex microbiota represent a promising technology for transformation of chlorinated compounds. However, the composition of the used population is usually unknown, hence hindering both optimization and control of the degradation process. Subject of this work was the analysis of the microbial diversity of the DCP-dechlorinating bioreactor population. Conventional culture-dependent microbiological methods are often limited if used for the analysis of complex communities. Therefore, a combination of different molecular methods based on comparative 16S rRNA analysis was applied. It was found that the bioreactor population was highly diverse and consisted mainly of as yet-uncultured bacteria. Members of the green nonsulfur bacteria and the gram-positive bacteria with low G+C content dominated the consortium. In contrast the archaea were represented by only two species, Methanosaeta concilii and Methanomethylovorans hollandica. The comparison of the rDNA data with those of other biotopes revealed that reductively dechlorinating freshwater habitats show a specific community structure. 16S rDNA-clusters were defined, which could still be detected after a longer operation time of the bioreactor. Furthermore, Dehalobacter restrictus- and Dehalococcoides ethenogenes-like bacteria were found in the DCP-dechlorinating bioreactor population. Both species are capable of reductive dechlorination using hydrogen as the sole electron source. Therefore, it could be assumed that these bacteria were also involved in the dechlorination of DCP. The investigation of the bioreactor population for a longer period of time revealed that Dehalobacter-like bacteria were significantly enriched and subsequently became the most frequently found bacterium within the bioreactor. This indicates a major role of Dehalobacter spp. within the transformation process of DCP to propene. Consequently, the addition of hydrogen to the bioreactor led to an increase of the DCP transformation rate. Dehalobacter und Dehalococcoides spp. as well as the bacteria represented by the specific SHA-clusters are possibly suitable as indicator organisms for the transformation of DCP within the bioreactor. A continuous monitoring of these bacteria would lead to a more efficient control and hence, to an optimization of the transformation process.

Les travaux de recherche présentés ici ont pour objectif la synthèse de fongicides innovants de la manière la plus éco-compatible qu’il soit. Pour cela, plusieurs axes de réflexion ont été étudiés. Tout d’abord, la synthèse d’un intermédiaire de fongicides bien connu, l’o-crésol1, a été réalisée à partir d’un substrat naturel, le salicylaldéhyde2. Les conditions expérimentales ont été éco-conçues afin de correspondre au mieux aux exigences de la chimie verte. Des solvants et des réactifs classés verts ont été utilisés, comme l’éthanol, l’eau et l’acide acétique3 par exemple, ainsi que des techniques alternatives comme l’activation par micro-ondes (Schéma 1). Dans un second temps, des synthèses éco-compatibles de nouveaux fongicides ont été réalisées. Le salicylaldéhyde, substrat naturel de référence, a été mis à réagir avec différents alcools afin de réaliser des réactions d’éthérifications réductrices. Les molécules obtenues sont composées d’une partie phénolique portant un groupement alcoxyle en position 2. Ce groupement alcoxyle se compose d’une longueur de chaîne variable ayant une influence sur les propriétés fongicides des produits synthétisés (Schéma 1). Troisièmement, des réactions d’homo- et d’hétéro-couplages pinacoliques ont été réalisées entre divers aldéhydes, aromatiques et aliphatiques, afin d’obtenir des diols vicinaux, appelés pinacols, symétriques ou non (Schéma 1). Dernièrement, les propriétés fongicides et anti-oxydantes de toutes les molécules obtenues de manière stable, référencées ou non, ainsi que les substrats de départ, ont été testés. Les tests microbiologiques ont été réalisés sur une gamme de diverses souches fongiques. / The purpose of this research is to synthesize innovative fungicides by the most ecocompatible way. Several hypotheses have been investigated. First, an extensively studied molecule, intermediate of fungicides, o-cresol1, has been achieved from a natural substrate, salicylaldehyde2. The experimental conditions have been eco-designed in the aim to best match green chemistry requirements. Green solvents and reactants have been used, as ethanol, water and acetic acid3 for example, as well as alternative technologies like microwaves activation (Schema 1). Secondly, novel fungicides green syntheses have been carried out. Salicylaldehyde, a natural substrate model, reacted with various alcohols in the aim to perform reductive etherification reactions. In this way, products were composed of phenolic part with an alkoxyl group at the 2nd position. These alcoxyl groups have various chain lengths, playing a key role in fungicidal ability (Schema 1). Then, homo and hetero pinacol coupling reactions have been achieved with various aromatic or aliphatic aldehydes to obtain symmetrical and asymmetrical vicinal diols, called pinacols (Schema 1). At last, fungidal and antioxidant properties of all stable obtained molecules, referenced or not, as well as starting materials, have been evaluated. Microbiological studies have been carried out with various fungal strains.

Le couplage croisé entre les esters allyliques et les composés carbonylés promu par le diiodure de samarium est une méthode efficace pour la formation de liaisons carbone-carbone. Une approche « umpolung » de réaction entre un composé carbonylé électrophile et un allylsamarien nucléophile, obtenu sans la réduction préalable d’espèces de type π-allylmétal de transition intermédiaires, fournirait une nouvelle voie plus simple pour la construction de cette liaison dans des conditions douces.Les esters allyliques de type dihydropyranyle se sont montrés d’excellents substrats dans les réactions d'allylation, d’aldéhydes ou de cétones, induites par le diiodure de samarium évitant ainsi l'utilisation de catalyseurs au palladium ou d’un autre additif. En série glycal, la nature et la configuration du substituant en position C-4 jouent un rôle très important à la fois sur le rendement et sur la structure des produits modifiés. Les couplages réducteurs directs ont lieu régiosélectivement en C-3 avec une stéréochimie relative 3,4-trans. La même stratégie appliquée aux dérivés Neu5Ac2en de l’acide sialique a permis d’obtenir par réactions à basse température, les produits de couplage en C-2 avec des rendements quantitatifs et une parfaite régio- et stéréosélectivité. Cette transformation donne un nouvel accès facile, rapide et très efficace aux α-C-sialosides. / The cross-coupling of allylic esters and carbonyl compounds promoted by samarium diiodide is an efficient method for the formation of carbon-carbon bonds. An umpolung approach, reaction between a carbonyl electrophile and an allyl samarium nucleophile, without the prior reduction of the intermediate π-allyl transition metal complexes, would provide a simple route for this bond construction under mild conditions.Dihydropyranyl allylic esters have been found to be excellent substrates for carbonyl allylation reactions mediated by samarium diiodide – without the use of palladium catalysts or any other additive. In glycal series, the nature and the configuration of the substituent at the C-4 position play crucial roles both on yields and structures of the modified products. The direct reductive couplings take place regioselectively at C-3 with a 3,4-trans relationship. Applied to Neu5Ac2en derivatives of sialic acid, the same strategy furnished the coupling products at the anomeric position in quantitative yields and with a perfect regio- and stereoselectivity. This transformation provides an easy, rapid and efficent access to α-C-sialosides.