1,720,966 research outputs found
Helical primary structures of 1,3-spiroannelated five-membered rings: (+/-)-trispiro[4.1.1.4.2.2]heptadecane and (+/-)-tetraspiro[4.1.1.1.4.2.2.2]heneicosane
No full textA combination of three spiroannelation methods forms the basis for a synthesis of the first two helical hydrocarbons of 1,3-spiroannelated five-mernbered rings. (C) 2008 Elsevier Ltd. All rights reserved
Synthesis, conformation and dynamics of 4,4,5,5,6,6,7,7,8,8-decamethyl-1-oxa-spiro[2.5]octane and 1,2,3,3,4,4,5,5,6,6-decamethyl-cyclohexene
No full textThe fragmentation of a suitable methylated 1-oxa-spiro[2.5]octane yields permethyl-cyclohexene. The activation parameters for the chair to chair interconversion of the 1-oxa-spiro[2.5]octane and the half-chair to half-chair interconversion of permethyl-cyclohexene were determined via bandshape analyses of exchange-broadened (1)H and (13)C NMR spectra, respectively. (C) 2010 Elsevier Ltd. All rights reserved
Percycloalkylated cyclohexanes: attempted synthesis of a trispropellane
No full textThe acid catalyzed rearrangement of two cyclohexanols of spiroannelated four-membered rings has been studied. In accordance with molecular mechanics calculations, far-reaching reorganizations with formation of unsaturated hexacyclic systems, including a fully cycloalkylated cyclohexene with a bispropellane Partial structure, were observed. Attempts to convert this bispropellane to a trispropellane failed. (C) 2009 Elsevier Ltd. All rights reserved
Helical primary structures of 1,2-spiroannelated five-membered rings: attempted synthesis of (+/-)-tetraspiro[4.0.0.0.4.3.3.3]heneicosane
No full textA beta-hydroxy ketone with a helical carbon skeleton of five 1,2-spiroannelated cyclopentane rings is the main product of a Lewis acid catalyzed rearrangement of suitable sized alpha-hydroxy epoxides followed by an in situ equilibration via retro aldol reactions. Various attempts of a conversion to the title hydrocarbon failed. (c) 2008 Elsevier Ltd. All rights reserved
Protoilludane sesquiterpenes: synthesis of (+/-)-cerapicol, formal synthesis of (+/-)-sterpurene, and synthesis and absolute configuration of (+)-cerapicol
No full textNew approaches to the protoilludane sesquiterpenes (+/-)-cerapicol and (+/-)-sterpurene via rearrangement routes are described. The absolute configuration of (+)-cerapicol has been determined and found in accord with a biosynthesis of the natural product via cyclization of humulene to the so-called protoilludyl cation and a subsequent 1,2-alkyl shift. (C) 2008 Elsevier Ltd. All rights reserved
Pseudohelical and helical primary structures of 1,2-spiroannelated four- and five-membered rings: Syntheses and chiroptical properties
No full text[Graphics] The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved
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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