1,720,961 research outputs found
Photoexcitation of Aldehyde derivatives to access chiral added value products
Full text linkA renewed interest is recently experienced in synthetic photochemical processes thanks to the availability of different light sources and, above all, for they extraordinary potential for the sustainable preparation of chiral molecules. The short-lived electronically excited states generated by photostimulation of organic compounds, indeed, can resort to a manifold variety of mechanism reactions that are not available to conventional thermal pathways. In this thesis work, photostimulation of aldehyde derivatives are used for diastereoselective synthesis of strain cyclobutane rings and for enantioselective remote perfluoroalkylation. The cyclobutanol motif is of interest in organic synthesis as a prochiral unit, it is present in natural products and has been used as an intermediate in the total synthesis of some sesterpenoids. In medicinal chemistry, the established benefit of fluorine-containing fragments relies on their ability to alter the physicochemical and pharmacokinetic properties of organic compounds. These aspects explain the importance of developing novel methods for the stereoselective incorporation of trifluoromethyl (CF3) and perfluoroalkyl (RF) units within organic molecules. However, only a few strategies are available for the installation of perfluoroalkyl-containing stereogenic centres. In the main part of the work, an efficient diastereoselective synthesis of cyclobutanol oximes (CBOs) has been achieved via Norrish-Yang photoisomerization of several 2-(hydroxyimino)-aldehydes (R1R2CHC(=NOH)CHO, HIAs) under LED UV light irradiation: The Norrish-Yang reaction is a photo-activated transformation of carbonyl compounds that leads to the formation of cyclobutanols, the only requirement being the presence of a -H in the substrate. The n-π* transition of a carbonyl group can lead, via intramolecular hydrogen abstraction, to the generation of an excited 1,4-biradical triplet state which then undergoes either a Norrish-type II fragmentation reaction or a Norrish-Yang cyclization with the formation of cyclobutanols. The synthetic methodology is generally applicable to a series of HIAs, affording the corresponding cyclobutanol oximes (CBOs) chemoselectively (i.e. without sizable fragmentation side-reactions), diastereoselectively (up to >99:1) and in good to excellent yields (up to 95%). CBO oxime ether derivatives can be purified and diastereomers isolated by standard column chromatography. Being the CBO moiety a confined small ring with many functional groups in proximal vicinity, the spectroscopic determination of the relative configuration of the obtained compounds was a key challenge. The assignment of cis/trans ring signals was determined by combination of spin-decoupled 1H, 13C, HSQC, and NOESY NMR spectroscopy. A detailed investigation combining experimental results, molecular dynamics calculations, spectroscopic data and DFT calculations was undertaken to complete the mechanistic and stereochemical picture of this photocyclization reaction. Moreover, molecular dynamics and NMR analyses were combined to study the observed post-cyclization E/Z isomerization of the CBO oxime double bond. In the second part of the work, carried out at Institut Català d'Investigació Química -ICIQ (Tarragona, Spain), a photochemical procedure for the asymmetric introduction of CF3 and perfluoroalkyl groups at the remote γ position of α-branched enals has been developed: Mechanistic investigations suggest that the stereocontrolled radical perfluoroalkylation of enals at the γ-carbon relies on the excitation of an electron donor acceptor EDA complex between the catalytic dienamine and the perfluoroalkyl iodide. Irradiation of the EDA complex by a blue light LED generates via a single electron transfer, the perfluoroalkyl radical. This radical is then intercepted by the chiral dienamine, in a regio- and stereoselective fashion affording the desired product. All perfluoroalkylated enals are obtained with complete γ-regioselectivity, good enantiomeric ratio (up to 93:7), and in good yield (up to 70%), depending on the structure of catalyst, enals and perfluoroalkyl iodides
Enantioselective catalytic remote perfluoroalkylation of α-branched enals driven by light
No full textHerein, we report a photochemical organocatalytic method for the asymmetric introduction of perfluoroalkyl fragments (including the valuable trifluoromethyl moiety) at the remote gamma-position of alpha-branched enals. The chemistry exploits the ability of extended enamines (dienamines) to form photoactive electron donor-acceptor (EDA) complexes with perfluoroalkyl iodides, which under blue light irradiation generate radicals through an electron transfer mechanism. The use of a chiral organocatalyst, derived from cis-4-hydroxy-l-proline, secures a consistently high stereocontrol while inferring complete site selectivity for the more distal gamma position of the dienamines
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
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