Capture électronique | EPFL Graph Search

vignette|Différents modes de désintégration radioactive : radioactivités α, β et β, capture électronique ε, émission de neutron n et . La capture électronique (plus précisément capture électronique orbitale, section « Notations »), ou désintégration ε, ou parfois désintégration bêta inverse, est un processus de physique nucléaire au cours duquel un noyau atomique déficient en neutrons absorbe un électron situé sur une couche électronique de l’atome. Variante de la désintégration β, sa description théorique est formulée par la théorie publiée par Enrico Fermi en 1933. La conséquence de la capture, selon la loi de conservation de la charge électrique, est qu’il y a une transmutation de l’atome puisqu’un proton, en absorbant l’électron intrus devient un neutron, et émission d’un neutrino électronique pour conserver le nombre leptonique ; l’atome qui avait Z protons et N neutrons devient un atome avec (Z-1) protons et (N+1) neutrons : : + ⟶ + , où X et Y désignent respectivement le noya

Dissymmetrical Double-Chain Elongation Approach to the Asymmetric Synthesis of Long-Chain Polypropionates

Claudia Jana Exner

Polyketides show interesting biological activities and therefore represent an important class of natural products. The necessity of their diastereo- and enantioselective synthesis led to the development of numerous methodologies. A brief overview of this topic is presented. The research carried out in our group has led to the development of a C-C-bond forming reaction involving Umpolung of a chiral, electron rich 1,3-dioxy-1,3-diene due to hetero-Diels-Alder reaction with sulphur dioxide. The so-formed six-membered sultines (6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides) can be ionized into a zwitterion in the presence of a Lewis acid promoter, which can be trapped by nucleophiles, e.g. enoxy- or allylsilanes, to generate the corresponding alkenylsulfinates. Protolysis of the latter to sulfinic acids initiates retro-ene desulfitation to produce polyketide- or polypropionate fragments. Studies towards the influence of electronic and steric effects on the diastereoselectivity of the SO2-reaction cascade between 1,3-dioxy-1,3-dienes and enoxysilanes have been performed. A novel series of chiral, p-substituted 1-arylethoxy-3-acyloxy-1,3-dienes has been synthesised and their reactivity and diastereoselectivity in the SO2-cascade has been studied. Enoxysilanes and silyl-based Lewis acid promoters of different steric bulk have been employed in the SO2-cascade with the aim to improve the diastereoselectivity of the reaction. The utility of the SO2-methodology is demonstrated by the short and selective synthesis of several natural and unnatural δ-lactones with various relative configurations from the same starting materials (stereodiversity). The stereoselectivity of the reaction cascade has been verified to be governed exclusively by the chiral auxiliary, even in the presence of additional stereocentres. The methodology has been extended to the synthesis of pseudo-C2- or -CS-symmetrical, long-chain polyketide precursors via bidirectional elongation strategy, either through multistep or one-pot operations employing 1,3-dienes of the novel series and allylsilanes in the first case or allylbissilanes in the second. The so-obtained products are readily desymmetrised and have been shown to undergo selective chain elongations in both directions, which allow for the selective formation of advanced polyketide fragments.

EPFL2010

Studying the effect of non-adiabatic passing electron dynamics on microturbulence self-interaction in fusion plasmas using gyrokinetic simulations

Ajay Chandrarajan Jayalekshmi

Microturbulence driven by plasma instabilities is in most cases the dominant cause of heat and particle loss from the core of magnetic confinement fusion devices and therefore presents a major challenge in achieving burning plasma conditions. The role of passing electron dynamics in turbulent transport driven by ion-scale microinstabilities, in particular Ion Temperature Gradient (ITG) and Trapped Electron Mode (TEM) instabilities, has been given relatively little attention. In first approximation, these particles, which are highly mobile along the confining magnetic field, are assumed to respond adiabatically to the low frequency ion-scale modes. However, near mode rational surfaces (MRSs), the non-adiabatic response of passing electrons becomes important and can no longer be neglected. This non-adiabatic electron response actually has a destabilising effect and leads to generation of fine-structures located at the MRSs of each eigenmode. This thesis focuses on the effects of non-adiabatic response of passing electrons in tokamak core turbulence. One such effect of non-adiabatic passing electrons that is of particular interest to this work is the self-interaction mechanism. It is essentially a process by which a microinstability eigenmode that is extended along the direction parallel to the magnetic field interacts non-linearly with itself, in turn generating E x B zonal flows. Unlike the usual picture of zonal flow drive in which microinstability eigenmodes coherently amplify the flow via modulational instabilities, the self-interaction drive of zonal flows from these eigenmodes are uncorrelated with each other. In the case of ITG driven turbulence, using novel statistical diagnostic methods, it is shown that the associated shearing rate of the fluctuating zonal flows therefore reduces as more toroidal modes are resolved in the simulation. In simulations accounting for the full toroidal domain, such an increase in the density of toroidal modes corresponds in fact to an increase in the system size, leading to a finite system size effect that is distinct from the other better known system size effects such as profile shearing or finite radial extend of the unstable region. The study of non-adiabatic passing electron dynamics is pursued further to include more reactor relevant conditions such as collisions and background shear flow. It is found that, with increasing collisionality, electrons behave more adiabatic-like, especially the trapped electrons away from MRSs, thereby leading to a decrease in the growth rate of ITG eigenmodes. Furthermore, the shortened electron mean free path in presence of collisions leads to a radial broadening of the fine-structures at the MRS of corresponding eigenmodes. In nonlinear simulations, the turbulent flux levels decrease with increasing collisionality, as a result of the reduced drive from the less unstable ITG eigenmodes. The radial width of the fine structures at MRSs is found to reduce with increasing collisionality as a result of reduced nonlinear modification of the eigenmodes in turbulence simulations. A study of the effect of collisions on the self-interaction mechanism reveals that for physically relevant values of collisionality, the effect of self-interaction is still significant. A preliminary study of the effect of background E x B flow shear shows that the fine-structures associated with the non-adiabatic passing electron response persist even with finite background flow.

EPFL2020

New access to long-chain polyketides and spiroketals by umpolung of electron rich dienes with sulfur dioxide

Freddy Fonquerne

Natural polyketides show important biological activity. The inherent stereochemical complexity present in this class of molecules has captured the imagination of organic chemists. Their stereocontrolled, asymmetric total synthesis is a permanent and stimulating area of research. Previous efforts in our group demonstrated that the Umpolung of electron rich 1,3-dienes with SO2 could represent an efficient synthetic strategy to polyketides. Under appropriate conditions sulfur dioxide reacts with 1,3-dienes in the hetero-Diels-Alder fashion. In the case of 1-alkoxy-1,3-dienes, the 6-alkoxy-3,6-dihydro-1,2-oxatiin-2-oxides (sultines) so-obtained can be ionized in the presence of a Lewis or protic acid. The resulting zwitterionic intermediates can be trapped with enoxysilanes (oxyallylation) to generate the corresponding substituted alkenylsulfinates. Desilylation of the latter under acidic conditions leads to allylsulfinic acid intermediates, which, after retro-ene desulfitation, produce 4-alkoxyhepta-6-en-2-one core, a valuable fragment in polyketide natural product synthesis. This new SO2 cascade reaction, hetero-Diels-Alder addition/oxyallylation/retro-ene desulfitation (Vogel's cascade) has been extended to 1-alkoxy-3-acyloxy-1,3-pentadienes. Their use in this cascade reaction allowed the generation of 1,3-polyols array containing keto and enol ester functions, suitable for further chain elongation. In addition, this work highlights the extension of this methodology to other types of p-nucleophiles. The use of carefully elaborated allylsilanes allowed the preparation in a single operation (one pot double oxyallylation reaction) of long chain polyketides of 13 carbons units. Finally, a new route to the spiroketals A/B and C/D and of Spongistatins is presented in order to demonstrate the utility and the versatility of this approach.

EPFL2005