1,720,980 research outputs found
Regiocontrolled 1,3-Dipolar cycloadditions of nitrile imines with acetylenes and α,β-unsaturated systems: Synthesis of polysubstituted and ring fused pyrazoles with pharmaceutical activity
No full text1,3-Dipolar cycloadditions of nitrile imines with functionalised acetylenes and α,β-unsaturated systems of various sizes have been studied. After cycloaddition, mono- and ring-fused pyrazoles have been obtained. Computational studies allowed a theoretical description of the observed reactivity, in agreement with the experimentally observed regio-chemistry. Selected targeted structures for medicinal applications have been synthetized and in vitro cytotoxicity showed that these pyrazoles can be considered powerful lead compounds for further medicinally relevant targets development
Nucleophilic Dearomatization of Activated Pyridines
No full textAmongst nitrogen heterocycles of different ring sizes and oxidation statuses, dihydropyridines (DHP) occupy a prominent role due to their synthetic versatility and occurrence in medicinally relevant compounds. One of the most straightforward synthetic approaches to polysubstituted DHP derivatives is provided by nucleophilic dearomatization of readily assembled pyridines. In this article, we collect and summarize nucleophilic dearomatization reactions of pyridines reported in the literature between 2010 and mid-2018, complementing and updating previous reviews published in the early 2010s dedicated to various aspects of pyridine chemistry. Since functionalization of the pyridine nitrogen, rendering a (transient) pyridinium ion, is usually required to render the pyridine nucleus sufficiently electrophilic to suffer the attack of a nucleophile, the material is organized according to the type of N-functionalization. A variety of nucleophilic species (organometallic reagents, enolates, heteroaromatics, umpoled aldehydes) can be productively engaged in pyridine dearomatization reactions, including catalytic asymmetric implementations, providing useful and efficient synthetic platforms to (enantioenriched) DHPs. Conversely, pyridine nitrogen functionalization can also lead to pyridinium ylides. These dipolar species can undergo a variety of dipolar cycloaddition reactions with electron-poor dipolarophiles, affording polycyclic frameworks and embedding a DHP moiety in their structures
An organocatalytic enantioselective direct alpha-heteroarylation of aldehydes with isoquinoline N-oxides
No full textA new protocol for the enantioselective direct alpha-heteroarylation of aldehydes with isoquinoline N-oxides, via chiral enamine catalysis, has been successfully developed. High enantiomeric excesses and moderate to good yields were achieved for a variety of alpha-heteroarylated aldehydes
Tandem Functionalization‐Carboxylation Reactions of π‐Systems with CO 2
No full textThe tandem catalytic functionalization/carboxylation of double as well as triple carbon-carbon bonds with CO2 represent an emerging research area in synthetic organic methodology. In particular, the combination of mild reaction conditions, stoichiometric acceptor/donorless conditions (visible light photoredox catalysis) and chiral catalysts contributed to a rapid development of this intriguing research area capable of creating chemical diversity/ complexity from readily available unsaturated hydrocarbons and CO2 as a C1-buinding block. The most recent developments in the field have been collected in the present review article and organized, based on the different sets of π-systems/intermediates/reactive partners employed (i.e., nickelalactones, organo-halides) as well as synthetic strategies (i.e., visible-light photo redox catalysis)
Aminocatalytic [8+2] Cycloaddition Reactions toward Chiral Cyclazines
No full textAn efficient and exceptionally stereoselective synthesis of chiral cycl[3.2.2]azines has been realized by means of the rational design and utilization of novel (E)-3-benzylidene-3H-pyrrolizines in iminium-ion-catalyzed [8+2] cycloaddition reactions. The presented protocol allows for the incorporation of diverse enals, including cinnamaldehydes, enolizable aldehydes, and substrates of extended conjugation. The obtained products contain both an electron-rich alkenyl pyrrole moiety and an electron-deficient carbaldehyde substituent, and both moieties can undergo selective transformations with retention of the stereochemical information established in the [8+2] cycloaddition
Catalytic Enantioselective Povarov Reactions of Ferrocenecarbaldehyde-Derived Imines - Bronsted Acid Catalysis at Parts-Per-Million Level Loading
Full text linkDespite the broad interest in ferrocene containing compounds, ferrocenyl substrates have been employed in catalytic asymmetric settings only sporadically. Herein, catalytic asymmetric Povarov reactions with ferrocenecarbaldehyde-derived N-aryl imines are presented. This study demonstrates that the stereoelectronic properties of ferrocenyl imines do not preclude their engagement in enantioselective phosphoric acid catalysis: cycloadducts derived from benzyl N-vinylcarbamate were obtained in good yields and nearly enantiopure form using 0.1mol% of a standard Bronsted acid catalyst. Furthermore, it is shown that specific optimisation with some substrates allowed to lower the catalyst loading up to 10-20 parts-per-million, an unprecedented value for phosphoric acid catalysts. Such low loading protocol could be applied to a preparative scale reaction, and to imines derived from arylaldehydes
Organocatalytic Enantioselective Construction of Conformationally Stable C(sp2)–C(sp3) Atropisomers
Full text linkNonbiaryl atropisomers are molecules defined by a stereogenic axis featuring at least one nonarene moiety. Among these, scaffolds bearing a conformationally stable C(sp2)–C(sp3) stereogenic axis have been observed in natural compounds; however, their enantioselective synthesis remains almost completely unexplored. Herein we disclose a new class of chiral C(sp2)–C(sp3) atropisomers obtained with high levels of stereoselectivity (up to 99% ee) by means of an organocatalytic asymmetric methodology. Multiple molecular motifs could be embedded in this class of C(sp2)–C(sp3) atropisomers, showing a broad and general protocol. Experimental data provide strong evidence of the conformational stability of the C(sp2)–C(sp3) stereogenic axis (up to t1/225 °C >1000 y) in the obtained compounds and show kinetic control over this rare stereogenic element. This, coupled with density functional theory calculations, suggests that the observed stereoselectivity arises from a Curtin–Hammett scenario establishing an equilibrium of intermediates. Furthermore, the experimental investigation led to evidence of the operating principle of central-to-axial chirality conversions
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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