1,723,732 research outputs found
Computational Investigation on the Origin of Atroposelectivity for the Cinchona Alkaloid Primary Amine-Catalyzed Vinylogous Desymmetrization of N-(2- t-Butylphenyl)maleimides
Full text linkMechanistic studies clarifying how chiral primary amines control the stereochemistry of vinylogous processes are rare. We report a density functional theory (DFT) computational study for the comprehension of the reaction mechanism of the vinylogous atroposelective desymmetrization of N-(2-t-butylaryl)maleimide catalyzed by 9-amino(9-deoxy)epi-quinine. Our results illustrate how the origin of the atroposelectivity was realized by the catalyst through steric and dispersion interactions. The role of N-Boc-l-Ph-glycine was crucial for the formation of a closed transition-state geometry and the activation of both reaction partners
Organocatalytic Enantioselective Formation of Atropisomers
No full textAsymmetric synthesis of atropisomers is an emerging and challenging field successfully addressed by organocatalysis. The versatility of chiral organic molecules in catalyzing the enantioselective formation of axially chiral compounds is reflected in the enormous number of different classes of atropisomeric structures achievable via a wide range of strategies. This chapter reports an in-depth analysis of covalent and noncovalent organocatalytic methodologies for the synthesis of important axially chiral organic scaffolds, highlighting selected examples of those fundamental strategies that are largely employed in atroposelective synthesis such as desymmetrization, kinetic and dynamic kinetic resolution, alkylation or arylation, cycloaddition, and central-to-axial chirality transfer to name a few
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Organocatalytic asymmetric conjugate additions of oxindoles and benzofuranones to cyclic enones
Full text linkThe asymmetric catalytic synthesis of densely functionalized molecules that contain vicinal quaternary and tertiary stereocenters is a challenge for modern chemical methodology. Here we show that a chiral primary amine, derived from natural molecules, efficiently catalyzes the stereoselective conjugate addition of oxindoles to cyclic enones, leading directly to valuable chiral scaffolds. Proof-of-concept for extending the method to benzofuranone derivatives is also provided. © Georg Thieme Verlag Stuttgart New York
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