1,721,070 research outputs found
(R)-Trifluoro- and Difluoropyruvaldehyde N,S-Ketals: Chiral Synthetic Equivalents of alpha-Trifluoro and alpha-Difluoro alpha-Amino Aldehydes
No full textA new, efficient, and stereoselective two-step approach to stereochemically defined chiral nonracemic
ç-tri- and ç-difluoro â-amino alcohols (70% to >95% ee) is described, using tri- and difluoropyruvaldehyde
N,S-ketals (R)-1a,b as starting materials. Addition of Grignard reagents to (R)-1
occurs with moderate to excellent anti-stereocontrol, depending on the nature of the organomagnesium
halides, providing the â-p-tolylthio â-benzyloxycarbonylamino secondary carbinols 5.
The stereochemical outcome of these reactions can be rationalized by means of a chelated Cram’s
cyclic model, where the NCbz group is the chelating ligand and the p-tolylthio residue acts as the
stereocontrolling “large” group. Reductive displacement of the 2-p-tolylthio substituent of 5
efficiently takes places by means of the NaBH4/pyridine system, probably via the corresponding
intermediate transient imines 13, providing sulfur-free ç-tri- and ç-difluorinated â-amino alcohols
7 with high levels of anti-stereoselectivity. A considerable shift toward syn-stereoselectivity was
obtained performing the reaction on the corresponding phenylacetates 8. Cleavage and reduction
of the NHCbz moiety of 7 provided tri- and difluoro analogues of, respectively, norephedrine (11)
and ephedrine (12)
Stereoselective Total Synthesis of Enantiomerically Pure 1-Trifluoromethyl Tetrahydroisoquinoline Alkaloids
No full textEnantiomerically pure 1-trifluoromethyl-tetrahydroisoquino- fluoro-â-iminosulfoxide (R)-3, and subsequent elaborations
line alkaloid analogues, in which C-1 is a quaternary stereo- of the sulfinyl auxiliary. The absolute stereochemistry of the
genic centre, have been synthesized by stereoselective intra- stereogenic centre was determined by X-ray diffraction on
molecular Pictet-Spengler reaction of the N-arylethyl ã-tri- the á-phenylpropionic ester (1R,29S)-1
Synthesis of N-Cbz-fluoropyruvaldehyde N,S-ketals: construction of highly stereoselective and high yielding synthetic reactions using multivariate modelling and design
No full textThe synthesis of (R)-fluoropyruvaldehyde-N,S-ketals via a tandem
self-immolative Pummerer-type rearrangement of enantiopure (R)-ot-fluoroalkyl-13-
sulfinylenamines has been studied using statistical experimental design and multivariate
modelling. By this study a procedure has been established which simultaneously gives
excellent enantioselectivity and synthetically useful yields. The improvement of the ee of
the trifluoro derivative was from 69% to 82%. The optimised procedure has been scaled
up (10 fold) and extended to the corresponding difluoro and chlorodifluoro derivatives,
with similarly good results. The chemometric analysis, along with a crossover experiment,
strongly supports the hypothesis of a strictly intramolecular process, according to the
previously proposed mechanism
Stereoselective Synthesis of Enantiopure Amino Compounds, via Mitsunobu Azidation of (2S,RS)-1-(p-Tolylsulfinyl)butan-2-ol
No full textAzidation of (2S,RS)-1±(p-tolysulfinyl)butan-2-ol under Mitsunobu conditions is the key step for a highly stereoselective
preparation of enantiomerically pure amino compounds via chiral sulfoxide chemistr
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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