1,720,973 research outputs found
Catalysis in water: synthesis of beta-amno amides by Sc(III) promoted condensation of silylketene pyridylthioacetal and imines
No full textThe scandium (III) catalysed condensation of silylketene thioacetals and pyridylthioacetals with imines in pure water was studied. The reaction of the phenylthioacetal derivatives brings always to b-aminoesters; the reaction of pyridylthioacetals leads to the stereoselective formation of b-lactams in organic solvents, while in the presence of a large amount of water affords b-amino amides in good yields. That represents a new, easy, one-pot catalytic synthesis in water of an interesting class of compounds, direct precursors of 1,3-diamino hydroxy compounds. The possibility of recovering and recycling the catalytic systems was also briefly investigated
Synthesis of new chiral cyclic 1,2-diamines and their evaluation as catalysts for enantioselective Diels-Alder reactions
No full textThe synthesis of new, enantiomerically pure cyclic 1,2-diamine structures was realized through a zinc promoted coupling of chiral bis-imines easily prepared starting from a chiral biphenyl dicarboxylic acid. The catalytic properties of such novel compounds were preliminarily studied; the trifluoroacetate salts of these chiral 1,2-diamines were shown to be able to catalyze the Diels–Alder reaction between cyclopentadiene and a,b-unsaturated aldehydes with good exo stereoselectivity and enantioselectivities up to 71%
A convenient, highly stereoselective, metal-free synthesis of chiral amines
No full textA low cost, efficient, metal-free highly stereoselective reduction of ketimines to chiral amines was developed. Different imines bearing a very cheap and removable chiral auxiliary were reduced simply by trichlorosilane in the presence of N,N-dimethyl-formamide, often in quantitative yield and complete control of the absolute stereochemistry, to afford highly enantiomerically enriched amines
Stereoselective synthesis and characterization of new enantiomerically pure phosphoric acids
No full textStarting from (R)-6,6′-dimethyldiphenyl-2,2′-dicarboxylic acid, a novel class of enantiomerically pure cyclic dialkyl phosphates was synthesized and properly characterized. The absolute configuration was determined by 2D NOESY experiments. The catalytic behavior of the new chiral Bronsted acids was investigated in the stereoselective addition of a silyl keteneacetal to aldimines. The Mannich-type reaction was promoted in up to 94% yields and enantioselectivities up to 55%. On the basis of preliminary molecular mechanic calculations, a model of stereoselection was also proposed to explain the sense of the enantioselectivity observed in the reaction
Organocatalytic synthesis of dipyrromethanes by the addition of N-methylpyrrole to aldehydes
No full textThe reaction of aldehydes with N-methylpyrrole carried out in the presence of catalytic pyrrolidinium tetrafluoroborate at room temperature affords the corresponding dipyrromethanes and minor amounts of tripyrranes. This new, mild, and convenient process represents the first organocatalytic synthesis of dipyrromethanes. The products are obtained in chemical yields similar to those obtained with existing methods, which, however, require either a much larger excess of heterocycle (when aldehydes are employed as starting materials), or more drastic reaction conditions (when aldehyde equivalents are used as starting materials) than those employed here. A mechanism is proposed to explain the course of this reaction. (c) 2006 Elsevier Ltd. All rights reserved
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
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