1,721,133 research outputs found
Alternate Alkylation of P-tert-butylcalix[8]arene In the Presence of Weak Bases
No full textAlkylation of p-tert-butylcalix[8]arene in the presence of weak bases has been investigated using p-methylbenzyl bromide as an electrophile and CsF as a weak base in THF/DMF. Initially, a sequence of monoalkylation steps on progressively more alkylated compounds has been carried out, leading to the isolation and characterization of 13 p-methylbenzyl ethers ranging from mono- to octasubstituted. Successively, the time course of the composition of the reaction mixture has been determined at regular time intervals by chromatographic analysis and products isolation, in order to have a semiquantitative dynamic picture of the whole process. The reaction path goes mainly through the formation of mono-, 1,3-di-, 1,3,5-tri-, 1,3,5,7-tetra-, 1,2,3,5,7-penta-, 1,2,3,4,5,7- and 1,2,3,5,6,7-hexa-, and heptasubstituted derivatives. This result agrees with an ''alternate alkylation'' mechanism based on the preferential formation of monoanions stabilized by two flanking hydrogen bonds. However, the isolation of appreciable amounts of 1,2,4-tris(p-methylbenzyl) ether indicates that less stable monoanions can also be formed, possibly due to either temporary conformational preferences or the conformational mobility of the calix[8]arene macrocycle, resulting in a less effective hydrogen-bonding stabilization of anionic intermediates
Preorganization of Calix[8]arenes - Synthesis of Basket-shaped Doubly-crowned Calix[8]arenes
No full textThe first calix[8]crowns, 3 and 4, frozen in a basket-shaped conformation, have been obtained by reaction of 1,3,5,7-tetrakis(p-tert-butylbenzyl) ether of p-tert-butylcalix[8]arene with tetraethylenglycol ditosylate. Their structure was deduced from mass (FAB) and NMR spectra and the conformational features have been studied by VT NMR. The 1,5-3,7-bis-crown 4, in which one of the ether chains fills the cavity, yields the more mobile and highly symmetrical 4a upon debenzylation
Shaping Calix[8]arene Framework By Intramolecular Bridging - Synthesis of Conformationally Blocked Calix[8]arene Derivatives
No full textThe first examples of 1,5-intrabridged calix[8]arene derivatives are described, which are conformationally frozen in a double-conical shape suitable for ditopical recognition
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
Study On the Alkylation of P-tert-butylcalix[8]arene - Partially O-alkylated Calix[8]arenes
No full textReactions of p-tert-butylcalix[8]arene (1) with various alkylating agents, including para-substituted benzyl bromides, tert-butyl bromoacetate, and 2-chloro-N,N-dimethylacetamide, were carried out in THF-DMF in the presence of a base. With strong bases (Nail or BaO/Ba(OH)(2)) octasubstituted derivatives were obtained, usually in good yield, while in the presence of weak bases (K2CO3 or CsF) more or less complicated reaction mixtures were obtained, wherefrom 1,3,5,7-tetraethers with C-4 symmetry were isolated in yields up to 49%. The eight equivalent ArCH2Ar groups of these compounds give rise to a singlet in the H-1-NMR spectrum, indicating a conformational freedom of the macrocycle, as confirmed by VT-NMR studies. The origin of the 1,3,5,7-regioselectivity can be explained, assuming that alkylation proceeds via a sequence of alternating monodeprotonation and alkylation steps. In each individual monodeprotonation step, those phenoxide monoanions are formed preferentially which are stabilized by two flanking hydrogen bonds. However, this cannot be the whole explanation. In fact, when methyl iodide was used as the electrophile in the reaction with 1, the main products were the 1,2,4-trimethoxy and 1,2,3,4-tetramethoxy derivatives, whereas the 1,3,5,7-tetramethyl ether was not detected in the reaction mixture. This finding leads us to believe that in the reaction with Mel some factor other than stability of oxyanions prevails, possibly the molecular dimension of the electrophile. This seems to be confirmed by the observation that n-butyl iodide, of intermediate dimension between MeI and arylmethyl bromides, gives alkylation products typical of both reaction courses
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