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A facile and general entry to C-glycosyl (R)- and (S)-beta-amino acid pairs from glycosyl cyanides through enamino ester intermediates
No full textFour O-perbenzylated glycosyl cyanides were converted by treatment with BrCH2CO2Et/Zn in THF at reflux (Blaise-Kishi reaction) into the corresponding C-glycosyl beta-enamino esters which in turn were reduced by NaBH(OAc)3 to give four pairs of C-glycosyl (R)- and (S)-beta-amino esters
Two- and three-component Hantzsch reaction using C-glycosylated reagents. Approach to the asymmetric synthesis of 1,4-diyhydropyridines
No full textTwo- and three-component Hantzsch-type cyclocondensation reactions were carried out under thermal conditions using C-glycosylated reagents, namely sugar aldehydes, ketoesters, and enamines. Two different series of 1,4-dihydropyridines bearing C-glycosyl moieties at C4 and C6 were obtained in good to excellent overall yields (61-90%). The asymmetric induction due to carbohydrate residues was also investigated
Model studies toward the synthesis of dihydropyrimidinyl and pyridyl alpha-amino acids via three-component biginelli and hantzsch cyclocondensations
No full textA novel and versatile strategy for the synthesis of heterocyclic α-amino acids has been described. The use of components (aldehyde or β-ketoester) bearing a masked glycinyl moiety in Biginelli and Hantzsch cyclocondensations allowed access to the 4-dihydropyrimidinyl-α-glycines, 4-dihydropyrimidinyl-α-alanines, 4-pyridyl-α-alanines, and 2-pyridyl-α-alanines classes. Dihydropyrimidinylamino acids were obtained as a mixture of diastereoisomers due to the formation of the stereocenter at C4 of the dihydropyrimidinone ring. Individual stereoisomers were isolated as pure compounds and their structures were assigned with the aid of X-ray crystallography and chiroptical properties. The enantiomeric purity of a representative selection of the above amino acids was greater than 96% as verified by derivatization to the corresponding Mosher's amides and subsequent 1H and 19F NMR spectroscopy. Incorporation of the 4-pyridyl-α-alanine derivative into a peptide chain is also described
Model studies toward the Synthesis of Dihydropyrimidinyl and Pyridyl α-Amino Acids via Three-component Biginelli and Hantzsch Cyclocondensations.
No full textA novel and versatile strategy for the synthesis of heterocyclic R-amino acids has been described.
The use of components (aldehyde or â-ketoester) bearing a masked glycinyl moiety in Biginelli and
Hantzsch cyclocondensations allowed access to the 4-dihydropyrimidinyl-R-glycines, 4-dihydropyrimidinyl-
R-alanines, 4-pyridyl-R-alanines, and 2-pyridyl-R-alanines classes. Dihydropyrimidinylamino
acids were obtained as a mixture of diastereoisomers due to the formation of the stereocenter
at C4 of the dihydropyrimidinone ring. Individual stereoisomers were isolated as pure compounds
and their structures were assigned with the aid of X-ray crystallography and chiroptical properties.
The enantiomeric purity of a representative selection of the above amino acids was greater than
96% as verified by derivatization to the corresponding Mosher’s amides and subsequent 1H and
19F NMR spectroscopy. Incorporation of the 4-pyridyl-R-alanine derivative into a peptide chain is
also described
Multicomponent Hantzsch cyclocondensation as a route to highly functionalized 2- and 4-dihydropyridylalanines, 2- and 4-pyridylalanines, and their N-oxides: preparation via a polymer-assisted solution-phase approach
No full textAn improved and efficient entry to highly functionalized β-(2-pyridyl)- and β-(4-pyridyl)alanines and the corresponding 1,4-dihydro and N-oxide derivatives has been developed by one-pot thermal Hantzsch-type cyclocondensation of aldehyde-ketoester-enamine systems in which one of the reagents (aldehyde or ketoester) was carrying the unmasked but protected chiral glycinyl moiety. Thus coupling N-Boc-O-benzyl aspartate β-aldehyde, acetoacetate and aminocrotonate esters afforded tetrasubstituted β-(4-dihydropyridyl)alanines (75% yield). One of these products was almost quantitatively transformed into the β-(4-pyridyl) alanine derivative which in turn was oxidized to the corresponding N-oxide. Each of these enantiomerically pure (Mosher's amide analysis) heterocyclic α-amino acids was incorporated into a tripeptide by coupling with (S)-phenylalanine. In a similar way tetrasubstituted β-(2-dihydropyridyl) alanine, β-(2-pyridyl)alanine and β-(1-oxido-2-pyridyl)alanine were prepared via Hantzsch cyclocondensation reaction using benzaldehyde, aminocrotonate, and acetoacetate carrying the N-Boc-O-benzyl glycinate moiety. It was shown that the work up of the reaction mixtures derived from the cyclocondensation and oxidation reactions can be carried out by the use of polymer supported reagents and sequestrants thus allowing the isolation of the products in high purity without any chromatography. © 2004 Elsevier Ltd. All rights reserved
Dihydropyridine C‐Glyconjugates by Hantzsch Cyclocondensation. Synthesis of a C(6)‐Glycosylated Nifedipine Analogue.
No full textThe use of glycosylated reagents in Hantzsch-type cyclocondensation reactions leading to C-glycosylated dihydropyridines (DHPs) has been investigated. A three-component approach with an anomeric sugar aldehyde (galacto, manno, and ribo derivatives), a β-keto ester, and an aminocrotonate afforded C(4)-glycosylated DHPs in high yield (70-90%). A two-component cyclocondensation approach based on different glycosylated β-amino acrylates (sugar enamines) and an enone derived from the Knoevenagel condensation between benzaldehyde and ethyl acetoacetate was followed for the preparation of C(6)-glycosylated 4-phenyl-substituted DHPs in fair yields (60-70%). The latter compounds were obtained as mixtures of diastereoisomers owing to the asymmetric induction of the sugar moiety in the formation of the C(4)-stereogenic center of the DHP ring. The diastereomer excess of the major products varied from 30 to 60%. The structure of selected compounds was determined by X-ray crystallography and by chiroptical measurements. The two-component cyclocondensation method was also employed for the preparation of a C(6)-ribofuranosyl-containing analogue of the well-known hypotensive agent nifedipine
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
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