1,720,965 research outputs found
Glucose and Glucuronate 2, 2, 2-Trichloroethyl Sulfates: Precursors for Multiply Sulfated Oligosaccharides
No full textSolvent-Free Glycosylation from per-O-Acylated Donors Catalyzed by Methanesulfonic Acid
No full textThe huge importance of carbohydrates and their derivatives in biomedical and industrial applications call for the development of streamlined and sustainable procedures for their synthetic elaboration. Here reported a novel glycosylation method based on direct activation of readily available per-O-acylated (acetylated or benzoylated) donors, promoted under air by methanesulfonic acid as a cheap and green catalyst in the absence of any solvent. Besides the beneficial avoidance of toxic and polluting organic solvents, these conditions were found critical for activating such poorly reactive donors with a very small catalyst loading (only 5 mol %), instead of stoichiometric Lewis acid promoters typically employed. Desired glycosides were quickly obtained, in most cases with high 1,2-trans stereoselectivity. Other main advantages over reported glycosylations with similar donors are the limited stoichiometric excess of the acceptor (or the donor), the easy applicability and low cost of the procedure and the wide target scope, also covering the synthesis of disaccharides and other non-trivial glycosides with applicable potential
(Semi)-synthetic fucosylated chondroitin sulfate oligo-and polysaccharides
No full textFucosylated chondroitin sulfate (fCS) is a glycosaminoglycan (GAG) polysaccharide with a unique structure, displaying a backbone composed of alternating N-acetyl-d-galactosamine (GalNAc) and d-glucuronic acid (GlcA) units on which l-fucose (Fuc) branches are installed. fCS shows several potential biomedical applications, with the anticoagulant activity standing as the most promising and widely investigated one. Natural fCS polysaccharides extracted from marine organisms (Echinoidea, Holothuroidea) present some advantages over a largely employed antithrombotic drug such as heparin, but some adverse effects as well as a frequently found structural heterogeneity hamper its development as a new drug. To circumvent these drawbacks, several efforts have been made in the last decade to obtain synthetic and semi-synthetic fCS oligosaccharides and low molecular weight polysaccharides. In this Review we have for the first time collected these reports together, dividing them in two topics: (i) total syntheses of fCS oligosaccharides and (ii) semi-synthetic approaches to fCS oligosaccharides and low molecular weight polysaccharides as well as glycoclusters displaying multiple copies of fCS species
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
Catalytic Cleavage of the 9-Fluorenylmethoxycarbonyl (Fmoc) Protecting Group under Neat Conditions
No full textThis work reports the first solvent-free catalytic approach for the cleavage of the fluorenylmethoxycarbonyl (Fmoc) protecting group from amine and alcohol functionalities. Various saccharide, peptide, and glyco-amino acid substrates were efficiently deprotected by simple treatment with 20 mol % neat 4-dimethylaminopyridine (DMAP) (one of the effective base catalysts found), without any solvent or stoichiometric additives. Small model structures were finally assembled through one-pot, base-catalyzed, solvent-free multistep sequences combining the Fmoc cleavage with esterification, amidation, and/or glycosylation steps
Switchable synthesis of glycosyl selenides or diselenides with direct use of selenium as the selenating agent
No full textSymmetrical diglycosyl-selenides or diselenides can be readily prepared with high chemoselectivity by direct use of elementary selenium as a cheap selenating agent (reduced in situ by sodium borohydride), and glycosyl iodides (or other halides) as reactive glycosyl donors. The chemoselectivity of the synthesis is critically affected by both the presence or not of triethyl phosphite, and the sodium borohydride to selenium stoichiometric ratio. Taking advantage of indications from mechanistic studies, the scope of the strategy was further extended to the synthesis of unsymmetrical diglycosyl selenides. Preliminary biological experiments evidenced an interesting antiproliferative effect of galactosyl diselenide against some tumor cell lines, otherwise negligible with the corresponding selenide
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
A study for the access to a semi-synthetic regioisomer of natural fucosylated chondroitin sulfate with fucosyl branches on N-acetyl-galactosamine units
No full textFucosylated chondroitin sulfate (fCS) is a glycosaminoglycan found up to now exclusively in the body wall of sea cucumbers. It shows several interesting activities, with the anticoagulant and antithrombotic as the most attractive ones. Its different mechanism of action on the blood coagulation cascade with respect to heparin and the retention of its activity by oral administration make fCS a very promising anticoagulant drug candidate for heparin replacement. Nonetheless, its typically heterogeneous structure, the detection of some adverse effects and the preference for new drugs not sourced from animal tissues, explain how mandatory is to open an access to safer and less heterogeneous non-natural fCS species. Here we contribute to this aim by investigating a suitable chemical strategy to obtain a regioisomer of the natural fCS polysaccharide, with sulfated L-fucosyl branches placed at position O-6 of N-acetyl-D-galactosamine (GalNAc) units instead of O-3 of D-glucuronic acid (GlcA) ones, as in natural fCSs. This strategy is based on the structural modification of a microbial sourced chondroitin polysaccharide by regioselective insertion of fucosyl branches and sulfate groups on its polymeric structure. A preliminary in vitro evaluation of the anticoagulant activity of three of such semi-synthetic fCS analogues is also reported
- …
