Tension de cycle

Résumé

En chimie organique, la tension de cycle ou contrainte cyclique désigne la déstabilisation d'une molécule cyclique, telle un cycloalcane, causée par l'orientation spatiale des atomes qui la composent. Cette tension provient d'une combinaison (1) de contrainte d'angle, (2) de contrainte de torsion (ou tension de Pitzer) et (3) de la tension trans-annulaire (ou contrainte de van der Waals). Exemples Les molécules possédant une grande tension de cycle sont composées de 3, 4 et parfois , dont : cyclopropanes, cyclopropène, cyclobutanes, cyclobutènes, [1,1,1]propellanes, [2,2,2]propellanes, époxydes, aziridines, cyclopentènes et norbornènes. Ces molécules présentent des angles de liaison entre les cycles qui sont plus aigus que l'angle optimal tétraédrique (109,5°) et des angles de liaison trigonale plane (120°) sont requis par leurs liaisons respectives sp3 et sp2. À cause de plus petits angles de liaison, les liaisons possèdent plus d'énergie et adoptent en partie les caracté

Source officielle

https://fr.wikipedia.org/wiki/Tension_de_cycle

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Macrocyclization of omega-Isocyanoaldehydes and Towards the Total Synthesis of Jamaicensamide A

Teerawat Songsichan

In the first chapter of this thesis, the macrocyclization of a new type of bifunctional substrates, omega-isocyanoaldehyde derivatives, is described. Ten different omega-isocyanoaldehydes in terms of different ring sizes and functional groups were prepared. They were smoothly proceeded under the simple multicomponent reactions including Passerini reaction, Ugi reaction and modified Ugi reaction. A wide range of 9- to 22-membered macrocyclic lactams were obtained in low to excellent yields with up to four bonds were formed in a single step. In the second chapter, synthetic studies towards the total synthesis of cyclic peptide jamaicensamide A are presented. We applied our strategy to cyclize the advanced intermediate containing omega-isocyanoaldehyde into the cyclic peptide in one-pot procedure. The precursors for the preparation of omega-isocyanoaldehydes with different functional groups and protecting groups were prepared. The derived 24-membered cyclic peptides were formed in good yields. The study on the deprotection of protecting groups is still on-going to complete the total synthesis of jamaicensamide A.

EPFL2021

Chargement

Developing Reactions of Strained Rings

Daniele Perrotta

Carbo- and heterocyclic structures containing nitrogen-substituted stereocenters are recurrent structural motifs in natural and bioactive compounds, therefore constituting a privileged class of synthetic targets. In this regard, substituted three- and four-membered carbocycles are powerful building blocks. Activation by a catalyst allows for an efficient exploitation of their ring strain, enabling a large variety of transformations. In particular, activation via oxidative addition by transition metals has been widely applied in annulations of these rings. Another common mode of activation consists in the use of Lewis acids to promote heterolytic ring opening of electron-poor strained rings, among which donor-acceptor cyclopropanes and cyclobutanes stand out for their well-established and broad chemistry. In this thesis, intermolecular annulations of three-membered carbocycles via oxidative addition by transition metals were first investigated. We mainly focused on the use of aminocyclopropanes as well as more strained aminomethylidenecyclopropanes. Despite a broad screening of transition metal complexes and modifications of the cyclopropane structure, no desired product could be formed. The use of aminomethylidene cyclopropanes in Lewis acid catalyzed intermolecular annulations was also studied. Employing aldehydes as reaction partners delivered highly functionalized 2-aminotetrahydrofurans. Instead, the use of all-carbon dipolarophiles for the synthesis of carbocycles was not successful. Subsequently, we explored enantioselective transformations of donor-acceptor cyclopropanes. We first investigated a new Lewis acid catalyzed DYKAT of donor-acceptor aminocyclopropanes with 2-siloxyfurans as dipolarophiles. After a thorough optimization, the desired product could be obtained in high er. However, the yield was not satisfactory, and any attempt to improve it by modifying the structure of the aminocyclopropane or by screening additives and reaction conditions was unsuccessful. On the other hand, we succeeded in the development of the first Lewis Acid catalyzed enantioselective desymmetrization of donor-acceptor meso-diaminocyclopropanes. The methodology consists in a copper(II) catalyzed Friedel-Crafts alkylation of indoles and a pyrrole, and delivers enantioenriched urea derivatives, which are recurrent structures in natural and bioactive compounds. The reaction tolerates electronically and sterically diverse substituents on the benzene ring of the indole. The choice of appropriate protecting groups on the cyclopropane was required to guarantee its solubility and to maintain good levels of enantioselectivity. The modification of an underexplored subclass of bisoxazoline ligands, bearing bulky diarylmethanol groups in alpha to the nitrogen atoms, was essential for obtaining high enantiomeric ratios, and led to the synthesis of several novel ligands. Finally, we developed Lewis acid catalyzed [4+2] annulations of donor-acceptor aminocyclobutanes with aromatic aldehydes, which afforded useful 2-aminotetrahydropyrans. A scandium catalyst was employed for the annulation of unsubstituted aminocyclobutanes, which delivered products in yields up to 95% and diastereoselectivities up to 17:1. On the other hand, one equivalent of a cheap and non-toxic iron Lewis acid was required for substituted aminocyclobutanes, which afforded densely functionalized six-membered heterocycles bearing three stereocenters in diastereoselectivities up to 5:1.

EPFL2018

Chargement

Body-centered cubic metals are of high technological interest: for example tungsten as potential plasma facing component in future fusion reactors, molybdenum employed in aircraft parts, niobium as superconducting magnets, etc. The characteristics of their mechanical behavior are therefore of prime importance from scientific, technical and economic points of view. The specific plastic deformation of bcc metals below the critical temperature raised numerous questions on the fundamentals of dislocations dynamic. Screw dislocations are anchored in the lattice because of a three dimensional core structure and they are consequently controlling the deformation. They have been observed to glide on both {110} and {112} planes, and the type of plane where the dislocations glide by elementary steps is still a matter of debate. Furthermore, deformation has been reported to occur sometimes with slip systems for which the resolved shear stress is very low, often called anomalous slip, and this is still not understood properly. Several models or theories have been developed to explain anomalous slip: some refer to the effect of stress components other than the shear stress in the slip direction, others to the role of dislocation-dislocation interactions. Most of our experimental knowledge comes from slip traces analysis or post-mortem transmission electron microscopy. Both methods give valuable insights, however incomplete what concerns the slip activity and dislocation dynamics. Here, in-situ Laue diffraction has been employed in order to follow non-destructively the dislocations dynamics during deformation of single crystal micro-pillars. Micro-compression experiments are performed during Laue diffraction at the microXAS beamline of the Swiss Light Source Synchrotron and allow following the sequence of activated slip systems in bcc pillars with diameters of 1-2 micron. Diffraction patterns are recorded continuously during the deformation in transmission geometry with a 5-23keV x-ray beam presenting a full-width at half maximum of approximately 1μm2. Complementary Laue scans are performed before and after the load to observe the local crystallographic orientations, activated dislocation slip systems and spatial distribution of strain gradients in the pillars. Complementary slip trace analysis is carried out in a scanning electron microscope. This type of experiments provides complementary insight into the deformation of bcc crystals. It has been applied on three different bcc metals having different critical temperatures (W, Mo and Nb). Various crystallographic orientations of the compression axes were probed to determine the influence of crystal orientation on the activation of slip systems. It is shown that when the highest resolved shear stress is on a {112} plane, slip can be described with elementary steps on {110} planes occurring by frequent alternation or by collective cross-slip, depending on the local stresses in the pillar and the mobility of the screw dislocations. It is furthermore shown that anomalous slip occurs in tungsten and molybdenum micro-pillars at high strains, suggesting the role of dislocation-dislocation interactions or at least the local stress variations induced by these interactions. The experiments also demonstrate the importance of the sample geometry when testing materials at the micron scale: stress concentrations and confinements provoked by specific geometries can lead to differences in slip system activation and/or early plastification.

EPFL2013

Chargement

EPFL2013

Developing Reactions of Strained Rings

Daniele Perrotta

EPFL2018