1,720,991 research outputs found
Regioselective Synthesis of 4,5-Diaryl-1-methyl-1H-imidazoles Including Highly Cytotoxic Derivatives by Pd-Catalyzed Direct C-5 Arylation of 1-Methyl-1H-imidazole with Aryl Bromides
No full textA general and efficient three-step procedure for the highly regioselective synthesis of 1-methyl-1H-imidazoles possessing electron-rich, electron-neutral and/or electron-deficient aryl moieties at their 4- and 5-position is described. The first step involves the Pd-catalyzed direct C-5 arylation of commercially available 1-methyl-1H-imidazole with aryl bromides and the second and third step of the sequence involve the selective C-4 bromination of the resulting 5-aryl-1-methyl-1H-imidazoles with NBS followed by a PdCl2(dppf)-catalyzed Suzuki-type reaction between 5-aryl-4-bromo-1-methyl-1H-imidazoles and arylboronic acids under phase-transfer conditions. Two so prepared 4,5-diaryl-1-methyl-1H-imidazoles, which can be regarded as Z-restricted analogues of naturally-occurring combretastatin A-4 (CA-4), have proven to be highly cytotoxic against a variety of human tumor cell lines and one of these derivatives has been shown to be more cytotoxic than CA-4 and all of the imidazole derivatives investigated in the literature so far
Regioselective direct C-5 arylation of imidazoles: synthetic protocols and application s
No full textChiral bis(benzo[1,2-b:4,3-b′]dithiophene) atropisomers: experimental and theoretical investigations of the stereochemical and chiroptical properties
No full textConformational chirality is a feature that may arise from the presence of a hindered rotation around a single bond that corresponds to a stereogenic axis. Atropisomers that feature such an element show interesting characteristics suitable for developing chiral systems with applicability in asymmetric synthesis, catalysis and materials chemistry. Here, we report the investigation of the stereochemical and chiroptical properties of a series of axially chiral intermediates useful in the synthesis of helicenes. Racemates of the four studied atropisomeric bisbenzodithiophenes were resolved by enantioselective HPLC and the single enantiomers were investigated by HPLC, ECD, VCD and CPL. Configurational stability was successfully established by dynamic HPLC or by monitoring thermal racemization, allowing the measurement of the free activation energy values for the stereomutation process. Moreover, the absolute configuration was assigned, thanks to a combination of TD-DFT and DFT calculations and experimental data derived by ECD and VCD, and the emissive chiral properties were measured by recording the CPL spectrum. Attention was paid to the importance of considering also aliphatic pendant groups in order to obtain a good calculation-experiment correspondence for both the ECD and VCD spectra. Considering the significant interest for axially chiral biaryl derivatives, this novel class of bisbenzodithiophenes are expected to have broad applications, not only in asymmetric synthesis, but also as potential chiral ligands or CPL emitters
A process for the preparation of Telmisartan
No full textThe present invention relates to a process for the preparation of Telmisartan by hydroxycarbonylation of compound
of formula (V), wherein X is a substituent which can be transformed into a carboxy group by hydroxycarbonylation. The invention also relates to intermediates for the synthesis of Telmisartan
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
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
- …
