1,721,043 research outputs found
Green chemistry in undergraduate education: organocatalysis in practical laboratory classes
No full textStereoselective synthesis of spirocyclic compounds: Organocatalyzed methodologies
No full textThe synthesis of complex molecules is one of the most challenging goals for synthetic chemists. The high complexity of natural products has fascinated organic chemists over the decades. One of the most important challenges for synthetic organic chemists is the regiocontrolled and stereocontrolled synthesis of quaternary carbon centers, in particular, the enantioselective synthesis of spirocyclic compounds. The spirocycles demonstrate high complexity because of the presence of central chirality or even axial chirality. Spiroatoms can be chiral even though they do not possess four different substituents normally observed in stereocenters. The high complexity of spiro compounds has attracted synthetic chemists since the nineteenth century. This chapter draws the readers' attention to the enantioselective organocascade C–C bond-forming reactions that have a huge role in the rapid synthesis of highly complex spirocyclic compounds. The goal is to mimic the natural conditions by highly enantioselective sequential syntheses.<br/
Synthesis of Lactams by tandem reactions
No full textTandem reactions form two or more new bonds, sequentially and in only one step, avoiding the purification of each step and the use of protecting groups, thus making them greener procedures. Lactams are cyclic amides present in numerous natural products and pharmaceuticals, therefore the development of new methodologies for their synthesis attracted the attention of synthetic chemists. The aim of this review is to summarize the last advances, since 2012, in the synthesis of lactams through tandem reactions
Synergistic catalysis: enantioselective ring expansion of vinyl cyclopropanes combining 4 catalytic cycles. Synthesis of spiro-cyclopentanes bearing up to 4 stereocenters
No full textA double synergistic cascade reaction is reported merging transition metal and amine catalysis. The reaction between vinyl cyclopropanes and enals render the final cyclopentane derivatives in excellent yields and stereoselectivities.<br/
Synergistic catalysis: Highly enantioselective acetyl aza-arene addition to enals
No full textA novel catalytic enantioselective methodology based on synergistic catalysis for the synthesis of chiral 2‐acyl pyridines and pyrazines is reported. The strategy involves the metal–Lewis acid activation of acetyl aza‐arenes and the secondary‐amine activation of enals. The proposed mechanism is supported by DFT calculations
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
RLAN impact on Sentinel-1 C-band SAR performance
No full textSentinel-1 C&D are expected to be launched in 2021, intended as a recurrent implementation of the previous -1A and -1B satellites. However, the C-band is under consideration for a potential allocation of the 5th Generation (5G) devices, including Wi-Fi and mobile devices that could interfere with the sensor operations and impact on Sentinel-1 SAR performance. In this paper, we present an assessment of the various aspects of the problem: the interference characterization, their adverse effects on S-1 SAR acquisition and a preliminary investigation about on-board possible solutions to reduce the effects of band occupation
- …
