1,721,441 research outputs found
Meanderings of a Curious Chemist
No full textInternational audienceThis account relates briefly the personal trajectory of the author and the thought processes that went into solving synthetic problems and developing new reactions more generally useful in organic synthesis. 1 Preamble and early education 2 In France with Sir Derek Barton 3 Chemistry at Ecole Polytechnique and at the ICSN 4 Final remark
Synthesis of 22-oxaspiro[4.5]decane CD-ring modified analogs of 1 alpha,25-dihydroxyvitamin D3
No full textIn search of analogs of 1α,25-dihydroxyvitamin D3 featuring a dissociation of calcemic and other activities, a series of stereoisomeric 19-nor-22-oxa derivatives, characterized by a spiro[4.5]decane cyclic system instead of the classical CD-ring system, have been synthesized in an enantioselective way
Recent advances in the asymmetric catalytic synthesis of chiral 3-hydroxy and 3-aminooxindoles and derivatives: Medicinally relevant compounds
Get PDFSeveral oxindole derivatives, of natural or synthetic origin, have been identified as medicinally appealing compounds, with a plethora of bioactivities reported. Chiral 3-hydroxy and 3-aminooxindole scaffolds have captured the attention of several research groups, due to their importance in drug discovery. In this review, we systematically address the wide variety of asymmetric catalytic methodologies employed in the preparation of these relevant chiral scaffolds, present in many biologically active compounds and/or natural products. Special focus will be given to the nature of the catalyst used
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Sulfur Betaines from S-Propargyl Xanthates. Unusual Chemistry from a Simple Functional Group
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Variations on the Author
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Nouvelles voies d'accès aux composés organoborés par la chimie des xanthates
No full textL’étude de la chimie radicalaire, et plus particulièrement de la chimie du groupement dithiocarbonate, aussi appelé xanthate, constitue le cœur des recherches effectuées dans le laboratoire. La chimie radicalaire des xanthates, particulièrement puissante pour former des liaisons C-C, est caractérisée par ses conditions douces, l’absence de métaux de transition et sa tolérance fonctionnelle.C’est tout d’abord cette chimie et la manière dont elle s’inscrit dans le paysage de la chimie radicalaire qui sera abordé (Chapitre I).Les travaux menés durant cette thèse se sont focalisés sur l’emploi de cette chimie pour synthétiser des composés organoborés. Ces composés sont donc ensuite décrits à travers leur intérêt pharmaceutique, dans la chimie des matériaux et avant tout comme intermédiaires de synthèse (Chapitre II).Une fois, la chimie des xanthates et celle des esters boroniques replacées dans leur contexte, les premiers travaux visant à la synthèse d’esters boroniques primaires sont décrits (Chapitre III). Cette méthode repose sur la stabilisation d’un radical en alpha d’un groupement boryle. Elle a donc pu être étendue à la synthèse d’esters boroniques secondaires, la synthèse des xanthates nécessitant cette fois-ci d’alterner les substituants portés par le bore (Chapitre IV). Enfin, la synthèse d’esters boroniques tertiaires selon la même logique a été testée, avec cependant quelques limitations en termes de rendement (Chapitre V).Afin de palier à ces limitations, l’emploi d’un cycle tendu a été préféré aux simples chaînes alkyles. La synthèse d’esters cyclobutylboroniques a donc pu être effectuée avec de bons rendements et, dans certains cas, une bonne diastéréosélectivité (Chapitre VI).D’autres xanthates, dont l’addition repose sur la stabilisation du radical par le bore, ont été ciblés avec peu de succès. La formation de gem-silylboronates par la chimie des xanthates et néanmoins possible (Chapitre VII).La chimie des xanthates permet également d’effectuer des réactions d’allylation à partir d’éther allyliques. En insérant un groupement boryle dans l’agent allylant, la synthèse modulaire d’alcènes borylés de géométrie (Z) a été réalisée (Chapitre VIII).Cette méthode a également pu être employée pour la synthèse de silanes alcéniques, cibles qui jusqu’ici avaient été problématiques (Chapitre IX). Enfin, quelques projets annexes ont permis d’explorer d’autre alliances entre la chimie des xanthates et certaines transformations ioniques plus classiques (Chapitre X).Our group focuses mainly on radical chemistry and especially the chemistry of the dithiocarbonates (also called xanthates). The chemistry of xanthates is an extremely powerful tool for the intermolecular construction of C-C bonds. It proceeds under mild conditions, involves no transition metals, and possesses broad functional group tolerance. This chemistry and the way it fits in the larger picture of radical chemistry is discussed first (Chapter I).The research works carried out during this PhD focused on the use of xanthates for the synthesis of boron containing molecules. These compounds, their bioactivity, their synthesis and use in organic synthesis is briefly reviewed next (Chapter II).Once the chemistries of xanthates, boronic esters and their alliance have been presented, the synthesis of primary boronic esters is described (Chapter III). This method relies on the stabilization of a radical adjacent to a boryl group and can be thus extended to the synthesis of secondary boronic esters. In order to access the starting xanthates, it is necessary to modulate the ligand on the boron depending on the reaction (Chapter IV). Finally, tertiary boronic esters were also obtained through the same logic, with some limitations, however (Chapter V).In order to alleviate these limitations, ring strain was used to destabilize the initial radicals. This led to the synthesis of cyclobutylboronic esters through modular successive xanthate additions, with good yields and in some cases, a good diastereoselectivity (Chapter VI).Other α-boryl xanthates were also targeted in order to broaden the structure accessible. In this process, the synthesis of gem-silylboronates was attempted with success (Chapter VII).Xanthates can be used in allylation reactions starting from allylic ethers. By inserting the boryl group into the allylating agent, a convergent and diastereoselective synthesis of alkenylboronates was accomplished (Chapter VIII).This method was then extended to the synthesis of alkenylsilanes (Chapter IX). Finally, some other projects were conducted to probe for new alliances between xanthate chemistry and classical ionic transformations (Chapter X)
Nouvelle application de la chimie radicalaire des xanthates
No full textMes travaux de recherche doctorale sont principalement liés à la chimie radicalaire des xanthates et à ses applications. Ces travaux de recherche sont réalisés sous la supervision du Prof. Samir Z. Zard au Laboratoire de Synthèse Organique de l'Ecole Polytechnique. Ce manuscrit est composé de trois chapitres et d'une partie expérimentale.Dans le chapitre 1, le concept général et le mécanisme de la chimie radicalaire des xanthates ont été brièvement présentés. Nous avons résumé une variété de méthodes de synthèse des xanthates primaires, secondaires et tertiaires. De plus, nous avons résumé brièvement certaines applications des xanthates, y compris les additions radicales intra et intermoléculaires, la cyclisation radicale intramoléculaire et la synthèse de dérivés d'acides aminés et de produits naturels basés sur la puissante chimie radicalaire des xanthates.Dans le chapitre 2, nous avons décrit un bref aperçu des différentes stratégies de synthèse de l'eptazocine. De nouvelles voies de synthèse de l'eptazocine ont été présentées, basées sur le puissant processus d'addition-transfert dégénéré de xanthates comme étape clé. Bien que l'eptazocine n'ait pas pu être synthétisée avec succès par trois voies différentes impliquant la chimie radicalaire des xanthates comme étape clé, le squelette central de l'eptazocine a été construit, en particulier sur le centre de carbone quaternaire benzylique.Dans le chapitre 3, nous avons développé une voie pratique, convergente et bon marché pour obtenir des 1,3-Dithiane-2-ones, des 1,3-Dithiolan-2-ones et des 1,2-Dithiolanes, basée sur l'addition radicale de xanthates à des oléfines portant un groupe partant convenablement situé. Avec les produits d'addition portant des xanthates obtenus par addition radicale de xanthates et d'acétates d'allyle facilement disponibles, une réaction inhabituelle a eu lieu, conduisant à la formation de lactones inattendues. Cette méthode présente certains avantages: (a) les synthèses sont très courtes, utilisent des matières premières et des réactifs peu coûteux et sont extrêmement flexibles; (b) une grande variété de groupes fonctionnels peut être tolérée et les conditions de réaction sont douces; (c) elle est économique en atomes puisque les deux atomes de soufre des xanthates se retrouvent dans les produits.Dans la section expérimentale (au chapitre 4), les 98 produits des chapitres 2 et 3 ont été préparés par les méthodes générales correspondantes et caractérisés par 1H NMR, 13C NMR, IR, HRMS et point de fusion pour les produits solides.My PhD research works are mainly related to the radical chemistry of xanthates and involved its application. These research works are accomplished under the supervision of Prof. Samir Z. Zard in the Laboratoire de Synthèse Organique in Ecole Polytechnique. This manuscript consists of three chapters and an experimental section.In chapter 1, the general concept and mechanism of the radical chemistry of xanthates have been introduced briefly. It has summarized a variety of methods for the synthesis of primary, secondary and tertiary xanthates. In addition to, we have summarized briefly some applications of xanthates including the intra-and intermolecular radical additions, intramolecular radical cyclization, the synthesis of amino acid, ketones derivatives and natural products based on the powerful radical chemistry of xanthates.In chapter 2, we have described a brief overview of various strategies to synthesize the eptazocine. Some new synthesis routes to the eptazocine have been presented based on the powerful degenerate addition-transfer process of xanthates as the key step. Although the eptazocine could not be synthesized successfully by three different routes involving the radical chemistry of xanthates as the key step, the core skeleton of the eptazocine has been constructed, especially on the benzylic quaternary carbon center.In chapter 3, we have developed a practical, convergent, and cheap route to 1,3-Dithiane-2-ones, 1,3-Dithiolan-2-ones and 1,2-Dithiolanes, based on the radical addition of xanthates to olefins bearing a suitably located leaving group. With adducts bearing xanthates obtained by radical addition of easily available xanthates and allyl acetates, an unusual reaction has been taking place leading to the formation of the unexpected lactones. This method has some advantages: (a) the syntheses are very short, use inexpensive starting materials and reagents, and are extremely flexible; (b) a broad variety of functional groups can be tolerated and the reaction conditions are mild (c) it is atom economical since both sulfur atoms of the xanthates end up in the products.In the experimental section (in chapter 4), the 98 products in Chapter 2 and 3 were prepared by the corresponding general methods and characterized by 1H NMR, 13C NMR, IR, HRMS and melting point for the solid products
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