1,720,966 research outputs found
New synthesis and new derivatives of benzo[c] rhenanthridine alakaloyds
No full textBetween alkaloids extracted from Chelidonium majus, we focused our researches on chelidonine , a hexahydrophenanthridine
derivative functionalized with a hydroxyl group at C-11 and that is characterized by the presence of two methylenedioxy functions on the aromatic rings. A basic nitrogen and
three stereocentres.
Studies on the biological activity of this molecule were reported but the mechanism of its action as anticancer agent is not clearly understood.
A part Ukrain, a semi-synthetic derivative deriving from chelidonine and tiotepa a lack of extensive SAR studies in the oncogenic field is evinced. This is probably due to the
scarce reactivity of this alkaloid, as will be shown in the next chapter that does not
allow preparing a large number of compounds.
This make chelidonine a “privileged structure from nature” particularly
attractive for a chemist also considering the possibility of a total synthesis of analogues of chelidonine not available starting from this alkaloid by semi-synthesis way.
For these reasons during my researches I planned
- the preparation of a series of derivatives starting from chelidonine
- their evaluation as anticancer agents
- the total synthesis of hexahydrobenzophenanthridine scaffold aiming to obtain analogues of chelidonine characterized by a different substitution pattern and stereoselectivity.
A new synthesis of alfa-amino-beta-hydroxy-isoserine derivatives was also developed
A process for the preparation of isoserine derivatives
No full textThis invention relates to a "one pot" process for the preparation of isoserine derivatives in high diastereoselective way. The process according to the invention includes the steps of reacting a protected glycidic acid with imines to yield isoserines protected both at the -OH and at the -COOH groups, deprotection of the obtained intermediates to isoserines or isoserine 1-4C- alkyl esters. Pure threo derivatives as the main isomer are obtained
Exploring new methods to control the stereoselectivity of glycosylation reactions
No full textThe synthesis of complex carbohydrate remains a challenge for synthetic chemists, especially in terms of stereocontrol (anomeric ratio) of glycosylation reactions. The stereoselective formation of glycosidic bonds depends indeed on numerous factors such as the reaction temperature, the solvent, the reagents concentration, the promoter and the coupling partners. In addition, the monosaccharide residues required for the assembly of larger saccharide structures are precious synthetic intermediates themselves as they require multistep synthesis. Large amount of starting material are usually consumed for the identification of the best reaction conditions using traditional batch procedures, and their optimization and subsequent scale-up of the optimized reaction conditions pose an additional hurdle. Continous-flow microfluidic devices offer a well-engineered approach to meet some of these challenges, especially in terms of better control of reaction parameters.
Quite surprisingly, glycosylation reactions have been still little explored under continous-flow conditions, and only few example of this chemistry are reported in the literature.1
The aim of the present research project is to investigate the use of microfluidic reactor in carbohydrate chemistry area, with particular emphasis on the glycosylation reactions (especially in terms of stereoselection control). Initially, a panel of different glycosyl acceptors (primary and secondary) and different glycosyl donors (with or without neighbouring participating group at C-2 and with different leaving group at the anomeric carbon) were synthesized and employed in glycosylation reactions under batch conditions.
Then, some glycosylation reactions were carried out under microfluidic conditions in order to compare their relative efficiencies.
Stereochemical control of glycosylation reaction can be also achieved by using chiral promoter. Inspired by Fairbanks and co-workers who recently described glycosylation reactions promoted by a chiral Brønsted acid catalyst,2 we decided to explore the use of a chiral Lewis acid catalyst to induce stereoselective glycosylations. In particular, the stereochemical course of a model glycosylation reaction has been investigated using the trimethylsilyl esters derived from (R/S)-BINOL phosphoric acid
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
Excimer laser micropatterning of freestanding thermo-responsive hydrogel layers for cells-on-chip applications
No full textWe report a novel reliable and repeatable technologic manufacturing protocol for the realization of micro-patterned freestanding hydrogel layers based on thermo-responsive poly-(N-isopropyl)acrylamide (PNIPAAm), which have potential to be employed as temperature-triggered smart surfaces for cells-on-chip applications. PNIPAAm-based films with controlled mechanical properties and different thicknesses (100-300m thickness) were prepared by injection compression moulding at room temperature. A 9×9 array of 20m diameter through-holes is machined by means of the KrF excimer laser on dry PNIPAAm films which are physically attached to flat polyvinyl chloride (PVC) substrates. Machining parameters, such as fluence and number of shots, are optimized in order to achieve highly resolved features. Micro-structured freestanding films are then easily obtained after hydrogels are detached from PVC by gradually promoting the film swelling in ethanol. In the PNIPAAm water-swollen state, the machined holes diameter approaches a slight larger value (30m) according to the measured hydrogel swelling ratio. Thermo-responsive behaviour and through-hole tapering characterization are carried out by metrology measurements using an optical inverted and confocal microscope setup, respectively. After the temperature of freestanding films is raised above 32°C, we observe that the shrinkage of the whole through-hole array occurs, thus reducing the holes diameter to less than a half its original size (about 15m) as a consequence of the film dehydration. Different holes diameters (10 and 30m) are also obtained on dry hydrogel employing suitable projection masks, showing similar shrinking behaviour when hydrated and undergone thermo-response tests. Thermo-responsive PNIPAAm-based freestanding layers could then be integrated with other suitable micro-fabricated thermoplastic components in order to preliminary test their feasibility in operating as temperature- activated components in the frame of the development of microfluidic devices in multilayer hybrid materials
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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