1,720,963 research outputs found
Photocatalytic transformations of the anomeric carbon of monosaccharides
No full textPhotocatalysis has proved to be a valid instrument to overcome the classic problems related to functionalization of carbohydrates, enabling new pathways of reactivity and leading to high-value intermediates. In this context, we are focusing our attention on different structural modifications of monosaccharides that can be performed photochemically, such as arylation and reduction of OH groups.
Inspired by a paper by MacMillan and co-workers on the metallaphotoredox deoxygenative arylation of alcohols using an Ir photocatalyst, we here report a cross-coupling process for the synthesis of aryl glycosides, whose applications are widespread in drug development, material science and biochemistry. These derivatives can be synthesized from monosaccharides unprotected at the anomeric position by employing organic dyes as photocatalysts and a Ni complex to promote the C-C bond formation between an aryl halide and the anomeric carbon of the substrate (in-situ activated by a N-heterocyclic carbene). A scope on carbohydrate derivatives and aryl halides will be shown in this contribution.
A synthetic method for the reduction of lactols to pyranes has been also developed, exploiting hydrogen atom transfer (HAT) and specifically combining an Ir-based photocatalytic cycle with a thiol (R-SH)-mediated H atom donation. This approach represents a faster and greener alternative compared to tedious, multi-step and (often) heavy metal-based procedures to get important carbohydrate derivatives. The resulting products are 1-deoxy sugars, an important class of synthetic intermediates for the preparation of natural synthetic analogues, such as Aeruginosin 298-A core and Ophiocerin D
Mannose-based glycomimetics acting as selective ligands for L-SIGN
Full text linkDC-SIGN and L-SIGN – also referred to as Dendritic Cell-Specific Intracellular Adhesion molecules (ICAM)-3 Grabbing Non-integrin (CD209) and Liver/Lymphnode-Specific ICAM-3 Grabbing Non-integrin (CD209L), respectively – are two C-type lectin receptors (CLRs), sharing 77% of their amino acidic sequences.1 They are able to bind to glycan motifs directly expressed at the surface of different pathogens thanks to a Ca2+ ion in the binding site – the so-called Carbohydrate Receptor Domain (CRD) – suddenly inducing the activation of the initial stages of adaptive immune response.
Nevertheless, several deadly viruses – such as HIV, Ebola, hepatitis C viruses, Dengue and West Nile virus – have developed strategies to subvert the function of CLRs to escape antiviral immunity and promote infection.2 Beyond the cited microorganisms, DC-SIGN and L-SIGN have been recently found to be entry co-factors for SARS-CoV-2, promoting trans-infection of ACE2-expressing cells.3
Interestingly, while fundamental studies have been performed in last years to design and characterize molecules acting as inhibitors of DC-SIGN-mediated infections, the first set of mannose-based ligands for L-SIGN was reported by our group only in 2022.4 According to this paper, the tested glycomimetics could inhibit the interaction of both lectins with the SARS-CoV-2 spike glycoprotein in a Surface Plasmon Resonance (SPR) assay and are more potent than mannose by up to 36-fold (DC-SIGN) and 10-fold (L-SIGN).
In this context, we present a new set of mannose-based glycomimetics which could selectively inhibit L-SIGN against DC-SIGN. All the molecules are based on a mono-mannose scaffold, modified at position 2 with a triazole moiety
Glycomimetic ligands acting as selective antagonists of L-SIGN
Full text linkDC-SIGN and L-SIGN – also referred to as Dendritic Cell-Specific Intracellular Adhesion molecules (ICAM)-3 Grabbing Non-integrin (CD209) and Liver/Lymphnode-Specific ICAM-3 Grabbing Non-integrin (CD209L), respectively – are two C-type lectin receptors (CLRs), sharing 77% of their amino acidic sequences.1 They are able to bind to glycan motifs directly expressed at the surface of different pathogens thanks to a Ca2+ ion in the binding site – the so-called Carbohydrate Receptor Domain (CRD) – suddenly inducing the activation of the initial stages of adaptive immune response.
Nevertheless, several deadly viruses – such as HIV, Ebola, hepatitis C viruses, Dengue and West Nile virus – have developed strategies to subvert the function of CLRs to escape antiviral immunity and promote infection.2 Beyond the cited microorganisms, DC-SIGN and L-SIGN have been recently found to be entry co-factors for SARS-CoV-2, promoting trans-infection of ACE2-expressing cells.3,4
Interestingly, while fundamental studies have been performed in last years to design and characterize molecules acting as inhibitors of DC-SIGN-mediated infections, the first set of mannose-based ligands for L-SIGN was reported by our group only in 2022.5 According to this paper, the tested glycomimetics could inhibit the interaction of both lectins with the SARS-CoV-2 spike glycoprotein in a Surface Plasmon Resonance (SPR) assay and are more potent than mannose by up to 36-fold (DC-SIGN) and 10-fold (L-SIGN).
In this context, we present a new set of mannose-based glycomimetics which could selectively inhibit L-SIGN against DC-SIGN. All the molecules are based on a mono-mannose scaffold, modified at position 2 with a triazole moiety
Photocatalytic deoxygenative functionalization of the anomeric position of carbohydrates
No full textIn recent decades, photocatalysis has gained significant traction, as light-driven activation of substrates enables unconventional pathways, in particular radical chemistry, under mild conditions. This approach has proven especially powerful in carbohydrate chemistry, where it has addressed longstanding challenges in functionalization, unlocking novel reactivities and streamlining the synthesis of high-value intermediates. Inspired by MacMillan’s work on metallaphotoredox deoxygenative arylation of alcohols, we developed an α-aryl glycoside synthesis via in situ generation of glycosyl radicals. This is achieved through N-heterocyclic carbene activation and subsequent homolytic cleavage of the hydroxy group and a cross-coupling with aryl bromides of the newly formed radical, using a dual catalytic system involving a nickel species and a donor-acceptor cyanoarene photocatalyst, replacing the iridium catalyst of the original protocol.
Additionally, inspired by Wendlandt and co-workers’ approach to sugar epimerization, we further adapted the method by incorporating a synergistic HAT-photocatalytic step. This enabled hydroxy-to-hydrogen substitution, a formal deoxygenation which may potentially represent an alternative to the stannane-based Barton-McCombie reaction. The reaction proved effective on both sugar and general alcohol substrates
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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