1,720,979 research outputs found
Synthesis and Conformational Investigation of Methyl 4a-Carba-d-arabinofuranosides
No full textThe synthesis of carbasugar analogues of methyl α-d-arabinofuranoside and methyl β-d-arabinofuranoside (3 and 4) is reported. The route developed involves the conversion of d-mannose into a
suitably protected diene (13), which is then cyclized via olefin metathesis. The resulting cyclopentene
(14) is stereoselectively hydrogenated to provide an intermediate that can be used for the synthesis
of both targets. Through the use of NMR spectroscopy, we have probed the ring conformation of 3
and 4, as well as the rotamer populations about the C4−C5 and C1−O1 bonds. These studies have
demonstrated that there are differences in ring conformation between these carbasugars and their
glycoside parents (1 and 2). However, only minor differences are seen in the rotameric equilibrium
about the C4−C5 bond in 3 and 4 relative to 1 and 2. In regard to the C1−O1 bond, NOE data from
3 and 4 suggest that the favored position about this bond is similar to that in the glycosides; that
is, the methyl group is anti to C2. However, confirmation of this preference through measurement
of 3JC,C between the methyl group and C2 or C4a was not successful
Total Synthesis of Both Methyl 4a-Carba-d-arabinofuranosides
No full textThe total synthesis of methyl 4a-carba-α-d-arabinofuranoside (1) and methyl 4a-carba-β-d-arabinofuranoside (2) has been achieved starting
from d-mannose (5). Key transformations included a ring-closing metathesis of diene 11 employing Schrock's catalyst followed by a stereoselective
hydrogenation with Wilkinson's catalyst
Synthesis and Conformational Investigation of Methyl 4a-Carba-d-arabinofuranosides
No full textThe synthesis of carbasugar analogues of methyl α-d-arabinofuranoside and methyl β-d-arabinofuranoside (3 and 4) is reported. The route developed involves the conversion of d-mannose into a
suitably protected diene (13), which is then cyclized via olefin metathesis. The resulting cyclopentene
(14) is stereoselectively hydrogenated to provide an intermediate that can be used for the synthesis
of both targets. Through the use of NMR spectroscopy, we have probed the ring conformation of 3
and 4, as well as the rotamer populations about the C4−C5 and C1−O1 bonds. These studies have
demonstrated that there are differences in ring conformation between these carbasugars and their
glycoside parents (1 and 2). However, only minor differences are seen in the rotameric equilibrium
about the C4−C5 bond in 3 and 4 relative to 1 and 2. In regard to the C1−O1 bond, NOE data from
3 and 4 suggest that the favored position about this bond is similar to that in the glycosides; that
is, the methyl group is anti to C2. However, confirmation of this preference through measurement
of 3JC,C between the methyl group and C2 or C4a was not successful
Total Synthesis of Carba-d-fructofuranose via a Novel Metathesis Reaction
No full textCarba-d-fructofuranose 2b was synthesized in 11 steps (45%), from 2,3,5-tri-O-benzyl-d-arabinofuranose 5 using a ring closing metathesis.
Schrock's catalyst was employed on the unique substituted diene synthon 4 to furnish the pentahydroxlated cyclopentene 3. Hydrogenation
afforded 2b
Carba-D,L-allal- and -D,L-galactal-derived vinyl N-nosyl aziridines as useful tools for the synthesis of 4-deoxy-4-(N-nosylamino)-2,3-unsaturated-5a-carbasugars
Get PDFThe novel carba-D,L-allal- and carba-D,L-galactal-derived vinyl N-nosyl aziridines were prepared and the regio- and stereoselective behavior in opening reactions with O- and N-nucleophiles examined. The carbaglycosylating ability of the novel aziridines, as deduced by the amount of 1,4-addition products (1,4-regioselectivity) obtained in the acid-catalyzed methanolysis taken as a model reaction, is similar or superior to that observed with the corresponding carba-D,L-allal- and -D,L-galactal-derived vinyl epoxides, respectively. In all 1,2- and 1,4-addition products obtained, a –(N-nosylamino) group is regio- and stereoselectively introduced at the C(4) carbon of a 1,2- or 2,3-unsaturated carbasugar, susceptible to further elaborations toward aminocyclitol derivatives. The stereoselective synthesis of the corresponding, enantiomerically pure carba-D,L-allal- and -D,L-galactal-derived vinyl N-acetyl aziridines is also described
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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