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THE PAECILIN PUZZLE - ENANTIOSELECTIVE SYNTHESES OF THE PROPOSED STRUCTURES OF PAECILIN A AND B
No full textFor the synthesis of the two diastereomers 3c and 3d of the proposed structure of paecilin B (3) phenol 19, containing an alkenyl moiety, was treated with Pd(II) in the presence of the chiral BOXAX ligand 9b to give 20 with 96% ee. A subsequent Sharpless dihydroxylation afforded two isomeric diols, which were further transfoimed into 31 and 32. The final steps included removal of the silyl protecting group with simultaneous lactone formation, oxidation and cleavage of the methyl ether. For the preparation of the dimeric paecilin A (1) brominated intermediate 38 was treated with (Bpin)(2), S-Phos and Pd(OAc)(2). The spectroscopic data of the new compounds did not match those of the isolated natural products
A Domino Approach to the Enantioselective Total Syntheses of Blennolide C and Gonytolide C
No full textThe first enantioselective total syntheses of the tetrahydroxanthenone (-)-blennolide C (ent-4) and related gamma-lactonyl chromanone (-)-gonytolide C (ent-3) are reported. Key to the syntheses is an enantioselective domino-Wacker/carbonylation/methoxylation reaction to set up the stereocentre at C-4a. Various chiral BOXAX ligands were investigated, including novel (S,S)-iBu-BOXAX, and allowed access to chromane 8 in an excellent enantioselectivity of 99%. The second stereocentre at C-4 was established employing a diastereoselective Sharpless dihydroxylation. An extensive survey of (DHQ)- and (DHQD)-based ligands enabled the preparation of both the anti-isomer 14a and the syn-isomer 14b in very good to reasonable selectivities of 13.7:1 and 1:3.7, respectively. While 14a was further converted to ent-3 and ent-4, 14b was elaborated to syn-acid 25 and 2'-epi-gonytolide C 28
Selective Glycosylation with the Amino Sugar D-Forosamine for the Synthesis of Spinosyns and Its Analogues
No full textFor the synthesis of novel spinosyn analogues, the selective introduction of the 2-deoxyamino sugar, D-forosamine, necessary for their bioactivity, is described. The reactions of different alcohol acceptors with the D-forosamine donor in the presence of iodine and Et3SiH gave the corresponding glycosides in yields of up to 96% and with an / selectivity of 2:3
ChemInform Abstract: A Domino Approach to the Enantioselective Total Syntheses of Blennolide C and Gonytolide C.
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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