1,721,029 research outputs found
Linear aminopolyhydroxylated structures through rapid domino assembly of a highly functionalized heterotricyclic system and its selective cleavage
No full textThis paper presents a new approach to linear aminopolyhydroxylated structures, based on the rapid assembly of a highly functionalized heterotricyclic system and its cleavage. The complex tricyclic structure is obtained by a sequence of two mild tandem reactions: the first involves a base-promoted nitroaldol-intramolecular cyclization between an activated primary nitroalkane and an aldehyde bearing a leaving group on the alpha-carbon atom to give 4-hydroxy-2-isoxazolines-2-oxides. In the second tandem process, the 4-hydroxy group was used as an anchor to link an olefinic residue (either an allyl group or a vinylsilane), which then gave rise to a spontaneous intramolecular cycloaddition with the nitrone moiety of the 2-isoxazoline-2-oxide to afford a complex tricyclic structure, These two tandem processes could be condensed into a single three-component domino reaction, which starting from the three acyclic substrates realizes a big jump in molecular complexity through the selective formation of five new bonds and four new chiral centers all concerted as a dynamic continuum in which every event depends on all the others, The second part of this project dealt with the cleavage of the tricyclic structure to unmask the target linear compounds, The richness of functionalities that were selectively installed on the tricyclic structures was screened for possible cleavages. Methods to reduce the exocyclic ester group, hydrogenolyse the bicyclic nitroso acetal, and hydroxydesylilate the cyclic silyl ether were found. In addition, during the early attempts of linearization of the tricyclic compounds, unexpected clean fragmentations of the products were observed, some of them affording synthetically useful new products. An array of interesting linear aminopolyhydroxylated structures was obtained by the combination of one or two cleavage steps from the tricycle, with overall very good acyclic selectivity, efficiency, and atom economy
Reaction of allylzinc reagents and zinc enolates of ketones with alpha-amidoalkylphenyl sulfones
No full textComputational 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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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