1,720,957 research outputs found
Perruthenate ion. Another metal oxo species able to promote the oxidative cyclization of 1,5-dienes to 2,5-disubstituted cis-tetrahydrofurans
No full textPerruthenate ion, from tetrapropylammonium perruthenate (TPAP), in the presence of tetrabutylammonium periodate (TBAPI) as reoxidant catalyses the stereospecific and stereoselective oxidative cyclization of 1,5-dienes to cis-2,5-disubstituted tetrahydrofurans in good to moderate yields. NMO also works as co-oxidant in the process but is less effective and at least 0.7 eq. TPAP are required. Acidic conditions promote the formation of THF diols. 1,5-Dienes with two terminal double bonds give poor yields of THF’s or cleavage products
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
Ni2+ enhances Fe2+/peroxide-induced oxidation of arachidonic acid and formation of geno/cytotoxic 4-hydroxynonenal: a possible contributory mechanism in nickel toxicity and allergenicity
No full textNi2+, a toxic, carcinogenic and allergenic agent, affected both the kinetic and chemical courses of the Fe2+-induced oxidation of arachidonic acid (AA) in 0.05 M phosphate buffer (pH 7.4) and at 37 degreesC. At 10 muM concentration, Ni2+ decreased the rate of oxidation of peroxide-free AA (200 muM) promoted by 50 muM Fe2+, as determined by measurement of thiobarbituric acid reactive species (TBARS) and H-1 NMR analysis. However, in the presence of low levels of peroxides (e.g. 2%), Ni2+ exerted a significant stimulatory effect on Fe2+- induced AA oxidation and TBARS formation. H-1 NMR analysis showed that Ni2+ (10 muM) enhanced formation of genotoxic alkenals including 4-hydroxy-2-nonenal (4-HNE, GC/MS evidence) by Fe2+-promoted degradation of both AA and 15-hydroperoxy-5,8,11,13-eicosatetracnoic acid (15-HPETE) methyl esters. The observed stimulatory effects of Ni2+ on peroxide breakdown and cytotoxic aldehyde formation provide an attractive explanation to the enhanced sensitization capacity of nickel in inflammatory states compared to normal states
RuO4-mediated oxidative polycyclization of linear polyenes. A new approach to the synthesis of the bis-THF diol core of antitumour cis–cis adjacent bis-THF annonaceous acetogenins
No full textThe RuO4-catalyzed oxidative polycyclization of some selected linear polyenes, possessing a repetitive 1,5-diene structural motif, has been investigated. The all-trans triene (E,E,E)-acetic acid henicosa-2,6,10-trienyl ester gave the expected bis-tetrahydrofuranyl diol product possessing a threo-cis-threo-cis-threo relative configuration, along with a mixture of the corresponding bis-THF ketols. These compounds can be seen as useful intermediates in the synthesis of the bis-THF diol core of adjacent bis-THF antitumour acetogenins possessing a threocis-threo-cis-erythro relative configuration, such as rolliniastatiin-1, membranacin, rollimembrin and membrarollin. Oxidation of the related all-trans tetraene (E,E,E,E)-acetic acid pentacosa-2,6,10,14-tetraenyl ester stops at the second cyclization step giving a mixture of a threo-cisthreo- cis-threo bis-THF diol and the corresponding ketol products. Oxidation of the triene (E,Z,E)-acetic acid 12-acetoxy-dodeca-2,6,10-trienyl ester stops at the monocyclization level failing to give bis-cyclized products, as previously observed for the related isoprenoid triene (E,Z)-farnesyl acetate. This result confirms the difficulty of closing a second THF ring when the central double bond of the triene possesses a cis configuration. Based on the collected results, a plausible model is proposed that both explains the observed cis/trans stereoselectivity for each ring-closing step in these processes, and rationalize the stereochemical course of the previously studied polycyclization of the isoprenoid polyenes (E,E)-farnesyl acetate, geranylgeranyl acetate and squalene
RuO4-catalyzed oxidative polycyclization of squalene. Determination of the configuration of the penta-tetrahydrofuranyl diol product
No full textThe configuration of the penta-tetrahydrofuranyl diol (penta-THF) product obtained by the single-step, RuO4-catalyzed oxidative polycyclisation of squalene has been determined as cis-threocis- threo-trans-threo-trans-threo-trans. The cis-cis-trans-trans-trans sequence for the five contiguous THF rings has been established through extensive 2D-NMR studies carried out both on the intact molecule and on some of its derivatives, including the oxidative cleavage products obtained by degradation of the penta-THF with PCC/AcOH. Four different chemical approaches were devised to determine each of the four threo relationships within each carbon pair connecting adjacent TFH rings in the molecule. To this aim, use has been made of the mono-, bis- and tris-THF diol intermediates of the oxidative process, obtained from the partial oxidation of squalene, as well as, inter alia, of a mono-THF bis-lactone product obtained from the penta-THF through a bidirectional double PCC degradation
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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