1,721,026 research outputs found
CHELATION-CONTROLLED ENANTIOSELECTIVE SYNTHESIS OF KEY INTERMEDIATES FOR THE PREPARATION OF CARBAPENEM ANTIBIOTICS PS-5 AND 1-BETA-METHYL-PS-5
No full textReaction of the E silyl ketene acetal derived from (1S,2R)-N-methylephedrine butyrate (7) with TiCl4 and β-alkoxy aldehyde 5 gave the aldol product in 75% yield as a 78:11:11 mixture of stereoisomers. In agreement with a chelated transition structure model, the major isomer is syn (2R,3S). The condensation product was transformed into 3, a key intermediate for the preparation of (+)-PS-5 [8:1 trans-cis; ee(trans) = 75%]. The stereoselectivity of the aldol reaction was substantially improved by using α-methyl-β-alkoxy aldehyde 6, which was synthesized in 91% enantiomeric excess and 50% overall yield starting from (1S,2R)-N-methylephedrine propionate E silyl ketene acetal 13. In this case the reaction of silyl ketene acetal 7 with TiCl4 and aldehyde 6 gave the aldol product in 70% yield as a single isomer out of the eight possible isomers. The result is in agreement with a chelated transition structure model and with a kinetic resolution-matched pair mechanism. The condensation product was transformed into 4 and 18, key intermediates for the preparation of 1β-methyl-PS-5 (ee > 99%). Via a different approach, the reaction of enantiomerically pure aldehyde (S)-(+)-6, prepared from (R)-(-)-methyl 3-hydroxy-2-methylpropionate, with TiCl4 and achiral E or Z silyl ketene acetals 19 or 20 gave the aldol product in high yield respectively as a 99:1:0:0 or 96:3:1:0 mixture of stereoisomers. The major isomer 21, obtained in agreement with a chelated transition structure model, was transformed into the key intermediate 18 via the same reaction sequence
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
CHELATION CONTROLLED ALDOL ADDITIONS OF THE ENOLSILANE DERIVED FROM TERT-BUTYL THIOACETATE - A STEREOSELECTIVE APPROACH TO 1-BETA-METHYLTHIENAMYCIN
No full textThe tert-butyldimethylenolsilane derived from tert-butyl thioacetate is a useful reagent for highly stereoselective chelation-controlled additions to chiral α-alkoxy and α-methyl-β-alkoxy aldehydes where the corresponding acetate fails. The aldol product deriving from the TiCl4 mediated addition to α-methyl-β-alkoxy aldehyde 5 was elaborated In high yield to give the bicyclic β-lactam 11, an Intermediate for the preparation of 1β-methylthienamycin. Problems associated with the partial racemlzation of aldehyde 5 during the TiCl4 mediated condensations have not yet been completely resolved
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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