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141.
The behaviour of the regioselectively generated carbocation centers at C(2) and C(6) in 1,2-trimethylenenorbornanes was investigated in order to study the occurrence or absence of a degenerate rearrangement E⇄M in the adamantane rearrangement of both 1,2-endo- ( 1 ) and 1,2-exo-trimethylenenorbornane ( 2 ) to 2-endo,6-endo-trimethylenenorbornane ( 3 ). A degenerate rearrangement E⇄M is inevitably involved inasmuch as a 1,2-trimethylenenorborn-2-yl cation E not only is formed directly as manifested by the conversions of the reactants 4 (C(2), C(3)-olefin) and 6 (C(2), C(3′)-olefin), but also indirectly (via F→E ) if the leaving group at C(6) to be ionized occupies the endo-position (6-endo-alcohol 8 ). No degenerate rearrangement E⇄M is operative starting from reactants that lead directly to a 2,6-trimethylenenorborn-2-yl cation G ; this is the case with both the ionization of the 6-exo-alcohol 10 having the leaving OH-group in a stereoelectronically favoured configuration to undergo simultaneous C(1), C(2)-bond migration (→ G ) as well as the protonation of the olefin 13 which is followed by same reaction pathway. 相似文献
142.
Dr. Douriya Z. Khedaioui Camille Tribout Julie Bratasanu Dr. Franck D'Agosto Dr. Christophe Boisson Dr. Damien Montarnal 《Angewandte Chemie (International ed. in English)》2023,62(12):e202300225
The activity of various additives promoting siloxane equilibration reactions is examined and quantified on model compounds. We found in particular that the “superbase” phosphazene derivative P4-tBu can promote very fast exchanges (a few seconds at 90 °C) even at low concentration (<0.1 wt %). We demonstrate that permanent silicone networks can be transformed into reprocessable and recyclable dynamic networks by mere introduction of such additives. Annealing at high temperature degrades the additives and deactivates the dynamic features of the silicone networks, reverting them back into permanent networks. A simple rheological experiment and the corresponding model allow to extract the critical kinetic parameters to predict and control such deactivations. 相似文献
143.
Le Corre L Kizirian JC Levraud C Boucher JL Bonnet V Dhimane H 《Organic & biomolecular chemistry》2008,6(18):3388-3398
Various substituents could be diastereoselectively introduced into the 5-position of pipecolic acid via electrophilic or free-radical-initiated addition to the carbon-carbon double bond of endocyclic enecarbamates derived from pipecolic acid. This study allowed the diastereoselective synthesis of both cis- and trans-5-guanidino pipecolates, which were designed as constrained arginine mimetics and whose potential inhibition of nitric oxide synthase (NOS) was evaluated with three NOS isoforms. 相似文献
144.
Let K be a Lie group and P be a K-principal bundle on a manifold M. Suppose given furthermore a central extension of K. It is a classical question whether there exists a -principal bundle on M such that . Neeb (Commun. Algebra 34:991–1041, 2006) defines in this context a crossed module of topological Lie algebras whose cohomology
class is an obstruction to the existence of . In the present article, we show that is up to torsion a full obstruction for this problem, and we clarify its relation to crossed modules of Lie algebroids and
Lie groupoids, and finally to gerbes.
相似文献
145.
We have performed measurements of boundary hysteresis loops, reversal curves, and subloops in p+-type porous silicon, a porous material composed of straight non-interconnected pores. These data show that a strong interaction mechanism exists between the pores. The pores of porous silicon are non-independent, whereas they are not interconnected. This hysteretic behavior is very similar to that observed in porous glass, which consists of cavities connected to each other by constrictions. This questions the so-called pore blocking/percolation model developed to explain the behavior of fluid in porous glass. More generally, if we disregard the shape of the boundary hysteresis loops which depends on the porous material (H1 for MCM-41 and SBA-15, H2 for porous glass and p+-type porous silicon), the hysteretic features inside the main loop are qualitatively the same for all these porous systems. This shows that none of these systems are composed of independent pores. A coupling between the pores is always present whether they are interconnected or not and whatever the shape of the main loop is. 相似文献
146.
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148.
Chloride anions associated with various soft cations (like tetraalkyl ammoniums, alkyl imidazoliums or pyridiniums) were shown to be able to promote the alkylation of carbonyl derivatives with acidic compounds, as exemplified on Knoevenagel and aldol condensations under relatively mild conditions. This activity was attributed to an enhanced nucleophilicity of the chlorine anion, originating from a softness/hardness mismatch between the anion and the cation. 相似文献
149.
Camille S. Bowen‐Forbes Donna A. Minott 《Magnetic resonance in chemistry : MRC》2009,47(11):1004-1006
The dipeptide hypoglycin B, one of two toxins of the ackee fruit (Blighia sapida Koenig), was characterized for the first time by NMR spectral data, which led to the discovery that it exists naturally as a pair of diastereomers. No distinction was observed in the 1H NMR signals of the diastereomers; however, complete and distinct 13C NMR assignments for the individual diastereomers were made. The 13C NMR spectrum of hypoglycin B compared very well with that of the corresponding signals in the spectrum for hypoglycin A, which is one of its constituent amino acids. The 1H and 13C NMR assignments were further supported by DEPT, gCOSY, gHSQC and gHMBC experiments. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
150.
Tereza Pavlíčková Dr. Valérie Bultel-Poncé Dr. Alexandre Guy Amandine Rocher Guillaume Reversat Dr. Claire Vigor Dr. Thierry Durand Dr. Jean-Marie Galano Dr. Ullrich Jahn Dr. Camille Oger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(44):10090-10098
Oxidative stress (OS) is an in vivo process leading to free radical overproduction, which triggers polyunsaturated fatty acid (PUFA) peroxidation resulting in the formation of racemic non-enzymatic oxygenated metabolites. As potential biomarkers of OS, their in vivo quantification is of great interest. However, since a large number of isomeric metabolites is formed in parallel, their quantification remains difficult without primary standards. Three new PUFA-metabolites, namely 18-F3t-isoprostane (IsoP) from eicosapentaenoic acid (EPA), 20-F4t-neuroprostane (NeuroP) from docosahexaenoic acid (DHA) and 20-F3t-NeuroP from docosapentaenoic acid (DPAn-3) were synthesized by two complementary synthetic strategies. The first one relied on a racemic approach to 18(RS)-18-F3t-IsoP using an oxidative radical anion cyclization as a key step, whereas the second used an enzymatic deracemization of a bicyclo[3.3.0]octene intermediate obtained from cyclooctadiene to pursue an asymmetric synthesis. The synthesized metabolites were applied in targeted lipidomics to prove lipid peroxidation in edible oils of commercial nutraceuticals. 相似文献