1,720,964 research outputs found
Importance of metals in organic synthesis: development of new methodologies for bond formation mediated by Lewis acid
No full textChemical catalysis affects our lives in innumerable ways. Catalysis provides a means of changing the rates at which chemical bonds are formed and broken and of controlling the yields of chemical reactions to increase the amounts of desirable products from these reactions and reduce the amounts of undesirable ones. Thus, it lies at the heart of our quality of life: the reduced emissions of cars, the abundance of fresh food at our stores, and the new pharmaceuticals that improve our health are made possible by chemical reactions controlled by catalysts. The petrochemical, chemical, and pharmaceutical industries depend on catalysts to produce everything from fuels to drugs, to paints and cosmetics. Today, we, as humans and chemists, face a variety of challenges in developing alternative energy sources, reducing dangerous by-products in manufacturing, cleaning up the environment and preventing future pollution, protecting citizens from the release of toxic substances and infectious agents, and creating safe pharmaceuticals. Catalysts are needed to meet these challenges, but their complexity and diversity demand a revolution in the way catalysts are designed and used. This revolution can become reality through the application of new methods for synthesizing and characterizing molecular and material systems. Organic chemistry can make a significant contribution in this change. In fact, in the last years green chemical processes started playing a crucial role in sustainable development, and efficient environmental catalysts, that can be conveniently applied in various chemical reactions, are the key elements for the improvement of sustainable synthetic processes. Many organic transformations rely on Lewis acid catalysts, and such compounds were widely studied in organic synthesis. The rare earth metal compounds come to play a significant role in these studies, because the majority of rare earth metal catalyst show characteristic properties of sustainable catalysts, such as low cost, aqueous/air/thermal stability, recyclability, and last but not the least high catalytic efficiency. With this idea in mind we studied new bond forming reactions using cerium trichloride in combination with sodium iodide. As reported in the first chapters of this work, we described new synthetic methods promoted by cerium(III) such as hydroiodination reaction of alkynes, synthesis of polysubstituted furans and oxazole derivatives. Furthermore we showed an example of the excellent catalytic activity of cerium trichloride in synergistic catalysis with other metal cocatalyst such as copper iodide, in the synthesis of 2-substituted benzimidazoles. We also demonstrated that these procedures are useful tools for the construction of more complex molecules having biological activity. In addition, we should recall that during the second half of the 20th century, transition metals began playing an important role in organic chemistry and this led to the development of a large number of transition metal-catalyzed reactions for creating organic molecules. In fact, transition metals have a unique ability to activate various organic compounds and through this activation they can catalyze the formation of new bonds. Recent years have witnessed tremendous growth in the number of gold-catalyzed highly selective chemical reactions. Gold was considered as an inert metal for a long time, and its ability to behave as Lewis-acid was recently recognized as a source of inspiration for organic chemists. Gold catalysts can activate unsaturated functionalities and therefore allow the creation of carbon-carbon and carbon-heteroatom bonds by addition of various nucleophiles, under mild conditions. As part of our ongoing studies on metal use in organic chemistry, in the last chapter we reported progress made on Cu-Au cocatalyzed one-pot synthesis of 2,5-dihydrofurans derivates from terminal alkynes, which could be very attractive and sustainable method, due to the reduction of work-up and purification steps and reduction of costs, materials and time
DFT calculations as a powerful tool for ESR spin trapping experiments
No full textA combined electron spin resonance (ESR) and density functional theory study has been carried out to obtain deeper insight in the
reaction mechanism of the photorearrangement of 3‐hydroxyindolic nitrones. Also, in this case, the combination of these
techniques constitutes a powerful tool when discriminating between different reaction pathways. In particular, density functional
theory calculations played an important role for unraveling the overlapping of signals coming from different species in the ESR
spin trapping experiments. In the present study, the computed energies and the corresponding ESR parameters of each possible
isomer hypothesized have been considered, taking also into account their different possibilities of intramolecular H‐bond
formation. However, the results obtained indicate that the possibility of intramolecular H‐bonding did not play a determinant role
in this case
Amidinoquinoxaline N-oxides: spin trapping of O- and C-centered radicals
Full text linkAmidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. The stability of the adducts was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxide spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. The N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of the OH adducts under the employed reaction conditions could be ascribed to their possible stabilization by IHBs with two different acceptors: the N–O˙ moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N–O˙ group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as an IHB acceptor.Fil: Gruber, Nadia. Università Politecnica delle Marche; Italia. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Orelli, Liliana Raquel. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Departamento de Química Orgánica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cipolletti, Roberto. Università Politecnica delle Marche; ItaliaFil: Stipa, Pierluigi. Università Politecnica delle Marche; Itali
An Efficient Catalytic Method for Regioselective Sulfenylation of Electron-rich Aza-aromatics at RoomTtemperature
Full text linkElectron-rich aza-aromatic compounds such as indoles and pyrroles represent systems of particular interest and importance in organic chemistry. An useful methodology for regioselective direct introduction of sulfenyl group on electron-rich aza-aromatics through S-alkyl and S-arylthiophthalimides as sulfenylating agents is described. Catalytic amounts of CeCl3.7H2O-NaI system are crucial to the promotion of this regioselective carbon-sulfur bond-forming Friedel-Crafts reaction. The reaction occurred in mild conditions and the products were obtained in good to excellent yields. Due to the large importance of functionalized indoles among natural compounds and pharmaceutical products, the methodology represents an efficient preparation of sulfenyl aza-aromatics, which are useful intermediates for important organic transformations
Novel and efficient CeCl3.7H2O/NaI-catalyzed sulfenylation of indoles and pyrroles
No full textThe preparation and manipulation of functionalized low molecular weight heteroaromatic compounds is of increasing importance for obtaining new more potent pharmaceuticals. In particular, 3-sulfenyl indoles and 2-sulfenyl pyrrole are an important class of compounds due their activity towards the treatment of several deseases, and as inhibitors in medicinal chemistry. Most of methods for their preparation are based on electrophilic aromatic sulfenylation, but a considerable number of them have been unsatisfactory because the procedures provide low yields of the desired products, and/or require harsh conditions that are incompatible with sensitive functional groups [4]. In the framework of our ongoing program in the development of Lewis acid-catalyzed reactions by CeCl3 [5], we have developed a new general and efficient methodology for the sulfenylation of indoles and pyrroles by N-(alkylthio)- and N-(arylthio)phthalimides in the presence of our combination CeCl3.7H2O/NaI
Isolation, characterization, and synthesis of degradation products of methylprednisolone derivatives
No full textCorticosteroids are important synthetically produced glucocorticoids used as anti-inflammatory drugs. Although the active pharmaceutical substances such as methylprednisolone derivatives have been on the market for many years, not much information is available in the literature regarding their degradation behaviors. During the course of our studies on polyfunctional molecules with biological activity, we have been interested in exploring the determination of possible degradation products of methylprednisolone hemisuccinate, and also their identification. These degradation products are interesting molecules from the pharmaceutical point of view, and are currently under biological evaluation. On the basis of fragmentation pathway and molecular weight obtained by LC-MS results, possible chemical structures can be proposed. Most of them are not sufficiently stable to be isolated, then only by synthesis it is possible to confirm the structures proposed by LC-MS results
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
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