Search results for "Selectivity"
showing 10 items of 1148 documents
Unusual Regioselectivity in the Opening of Epoxides by Carboxylic Acid Enediolates
2008
Addition of carboxylic acid dianions appears to be a potential alternative to the use of aluminium enolates for nucleophilic ring opening of epoxides. These conditions require the use of a sub-stoichiometric amount of amine (10% mol) for dianion generation and the previous activation of the epoxide with LiCl. Yields are good, with high regioselectivity, but the use of styrene oxide led, unexpectedly, to a mixture resulting from the attack on both the primary and secondary carbon atoms. Generally, a low diastereoselectivity is seen on attack at the primary center, however only one diastereoisomer was obtained from attack to the secondary carbon of the styrene oxide.
Adolescents’ and mothers’ temperament types and their roles in early adolescents’ socioemotional functioning
2018
The present study examined adolescents’ and mothers’ temperament types and their roles in the socioemotional functioning of early adolescents. A total of 869 sixth-grade students and 668 mothers participated in the study. The students rated their temperament and socioemotional functioning and the mothers rated their own temperament. Latent profile analyses identified four temperament types among the adolescents (resilient, reserved, average, and mixed) and three types among the mothers (resilient, average, and mixed). The results showed that the adolescents with resilient or reserved temperaments reported significantly fewer conduct problems and emotional symptoms, less hyperactivity, and …
Bienestar Docente durante la Pandemia COVID-19 : La Comparación entre Ecuador y Chile
2021
El quehacer docente se ha visto afectado a causa de la crisis sanitaria a nivel mundial, los docentes han debido transformar la forma de hacer su trabajo y enfrentar nuevos desafíos profesionales. El objetivo de este estudio es identificar y comparar entre docentes chilenos y ecuatorianos percepciones de bienestar emocional, social y laboral durante la pandemia; también identificar diferencias por género y la relación entre el nivel de bienestar y diferentes dimensiones socioemocionales. Participaron 881 docentes de preescolar, primaria y secundaria de ambos países, quienes respondieron un cuestionario en línea sobre carga laboral/doméstica, horario laboral, apoyo institucional, afectos …
Understanding the stereo- and regioselectivities of the polar Diels-Alder reactions between 2-acetyl-1,4-benzoquinone and methyl substituted 1,3-buta…
2009
The polar Diels–Alder (DA) reactions of 2-acetyl-1,4-benzoquinone (acBQ) with methyl substituted 1,3-butadienes have been studied using DFT methods at the B3LYP/6-31G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3-dienes to the β conjugated position of the acBQ followed by ring-closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acB…
Selective chiral inhibition of Ca2+ entry promoted by bisbenzyltetrahydroisoquinolines in rat uterus.
1992
Abstract The effects of diltiazem and six bisbenzyltetrahydroisoquinoline alkaloids (antioquine, 7-O-methylantioquine, dimethylantioquine, monterine, granjine and cordobimine) were studied in rat isolated uterus in order to clarify the mechanisms of their relaxant actions. All the compounds tested completely relaxed KCl-induced contractions and totally or partially inhibited oxytocin-induced rhythmic contractions. Only alkaloids with absolute configurations (1R,1′S or 1R,1′R) acted intracellularly, promoting relaxation of contractile responses induced by oxytocin in a Ca2+-free medium, as does papaverine. Alkaloids of the antioquine series (1S,1′R) selectively inhibited Ca2+ entry. The grea…
Unexplored nucleophilic ring opening of aziridines.
2010
The reactivity of dianions of carboxylic acids towards aziridines has been studied. Although, a similar reactivity to that of enolates from ketones, esters or amides has been observed, the method directly yields g-aminoacids in one step. The method is complementary of previous results of enenediolate reactivity with other electrophiles. A comparative study with the reactivity of this enediolates with epoxides is included.
Influence of Surface-Related Phenomena on Mechanism, Selectivity, and Conversion of TiO2 -Induced Photocatalytic Reactions
2018
Heterogeneous photocatalysis is the result of an inextricable connection of several factors differently contributing to the overall process. Photon absorption is the “sine qua non” condition for the reaction to occur. In fact, photons can be considered as immaterial reactants, and all of the phenomena related to the interaction of light–matter play a prominent role. However, other factors contribute in a concerted way to address the reaction, so that the relative contribution of each of them is often difficult to evaluate. In this framework, the present paper highlights some aspects of the interaction of TiO 2 surface-adsorbate species that could be underestimated and their influence on the…
Highly Syndiotactic or Isotactic Polyhydroxyalkanoates by Ligand-Controlled Yttrium-Catalyzed Stereoselective Ring-Opening Polymerization of Function…
2017
International audience; Reported herein is the first stereoselective controlled ROP of a specific family of racemic functional β-lactones, namely 4-alkoxymethylene-β-propiolactones (BPL(OR) s). This process is catalyzed by an yttrium complex stabilized by a nonchiral tetradentate amino alkoxy bisphenolate ligand _ONOO(R'2) ₍2-) , which features both a good activity and a high degree of control over the molar masses of the resulting functional poly(3-hydroxyalkanoate)s. A simple modification of the R' substituents in ortho and para position on the ligand platform allows for a complete reversal from virtually pure syndioselectivity (Ps up to 0.91 with R'=cumyl) to very high isoselectivity (Pi…
Le plaisir à manger en EHPAD : point de vue du senior consommateur
2016
Revue; Le plaisir à manger en EHPAD : point de vue du senior consommateur. journée d'études ALIMS
Goût et préférences du consommateur âgé
2017
Revue; Goût et préférences du consommateur âgé. Sixième journée mondiale de la déglutition :