1,720,956 research outputs found
Substituent dependence of the diastereofacial selectivity in iodination and bromination of glycals and related cyclic enol ethers
No full textThe stereochemical course of the electrophilic iodination and bromination of tri-O-benzyl-D-glucal under various conditions has been compared to that of substituted dihydropyrans 2-5. IN3 addition in acetonitrile affords trans-alpha -iodoazides (80-87%), besides small amounts of trans-beta -adducts, in the presence or the absence of benzyloxy substituents at C-3 or C-4, and in agreement with bridged iodonium ion intermediates. In contrast, the diastereofacial selectivity of bromine addition in dichloroethane going through open bromo oxacarbenium ions depends strongly on the substituents. Whereas the trans-alpha -dibromides are the main (85-95%) adducts in the absence of C-4 and C-5 substituents, in their presence a moderate to exclusive selectivity for cis-alpha -addition (60-99%) is observed. The predominance of trans-alpha -addition is again observed whatever the substituents when the bromination is carried out in the same solvent but with a tribromide ion salt, supporting a concerted addition of the two bromine atoms under these conditions. Finally, bromine addition in methanol exhibits a completely different behavior with the nonselective formation of trans-alpha- and trans-beta -methoxybromides and a small dependence on the substituents. In agreement with the absence of azide trapping of any cationic intermediate, it is concluded that these brominations which do not go through an ionic intermediate are concerted additions of bromine and methanol with very loose rate- and product-determining transition states. Finally, the substituent conformation at C-4 influences drastically the stereoselectivity in all these brominations. Evidence for alpha -anomeric control of the nucleophile approach at C-4 is given by the highly predominant formation of alpha -adducts, except in the methanolic bromination. The factors determining the versatile selectivity of the electrophile approach are discussed in terms of PPFMO theory and of the special mechanisms of glycal reactions
Preassociation, free-ion, and ion-pair pathways in the electrophilic bromination of substituted cis- and trans-stilbenes in protic solvents
No full textRates and products of electrophilic bromination of ring-substituted cis-and trans-stilbenes have been investigated in acetic acid, trifluoroethanol, ethanol, methanol, and water-methanol mixtures. The mY(Br) relationships (linear for nucleophilic solvents only, with m = 0.8), the deviations of the two nonnucleophilic solvents from the mY(Br) plots (Delta(AcOH) and a Delta(TFE) positive, negative, or negligible), the kinetic solvent isotope effects (k(MeOH)/k(MeOD) = 1.1 - 1.6), the chemoselectivity (predominant dibromide, DB, or solvent-incorporated adducts, MA), and the high dependence of the stereochemistry on the solvent and the substituents (from stereoconvergency to stereospecificity) are discussed and interpreted in terms of a mechanistic scheme, analogous to the Jencks scheme for aliphatic nucleophilic substitutions, in which preassociation, free-ion, and ion-pair pathways compete, In particular, the stereochemical outcome of these reactions is consistent with a marked change in the nucleophilic partners of the product-forming ionic intermediate arising from different ionization routes. Return, i.e. change in the rate-limiting step from ionization to product formation, is shown to be related to substituent-dependent, but not solvent-dependent, bromine bridging
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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