PtCl₂-Catalyzed Rearrangement of Methylenecyclopropanes

Alkylidenecyclopropanes readily convert into cyclobutene derivatives on treatment with catalytic amounts of PtCl2. The reaction is strongly accelerated when performed under an atmosphere of CO (1 atm). The resulting cyclobutenes are isolated in good to excellent yields for substrates bearing aliphatic as well as aromatic substituents R on their olefinic site. If the substituent R, however, is a very electron-rich arene, the cyclobutenes initially formed react further to give dimeric products with a previously unknown 1,2,2a,7a-tetrahydrospiro[cyclobuta[a]indene-7,1‘-cyclobutane skeleton. A mechanism accounting for these experimental observations as well as for a deuterium-labeling experiment is proposed which implies reactive intermediates at the nonclassical cation/carbene interface. Furthermore it is shown that the PtCl2-catalyzed cyclobutene formation can be geared with subsequent ring-opening/ring-closing metathesis (ROM/RCM) events. Finally, a convenient “one pot” method for the preparation of the alkylidenecyclopropane substrates used in this study is presented, which is based on a modified Julia−Kocienski olefination of aldehydes with readily available 1-tert-butyl-1H-tetrazol-5-yl-cyclopropyl sulfone under Barbier conditions

A Rhodium-Catalyzed C−H Activation/Cycloisomerization Tandem

A reaction cascade comprising a rhodium-catalyzed C−H activation, a subsequent hydrometalation of an alkylidene cyclopropane in vicinity, regioselective C−C bond activation of the flanking cyclopropane ring, followed by reductive elimination of the resulting metallacycle, opens a new entry into functionalized cycloheptene derivatives. This crossover of C−H activation and higher order cycloaddition has been performed in two different formats, either using alkylidenecyclopropanes with a lateral vinylpyridine moiety or with a pending aldehyde group as the trigger. The reaction tolerates various functional groups, leaves chiral centers α to the reacting sites unaffected, and proceeds with excellent stereoselectivity. Labeling experiments support the proposed mechanism explaining the observed net cycloisomerization process

Utilisation de la mesure thermique du flux de sève pour l'évaluation de la transpiration d'un palmier dattier

International audienceL'amélioration de la gestion de l'eau en système oasien constitue l'un des principaux défis pour la préservation de son agrosystème. Des approches complémentaires doivent être mises en oeuvre pour y contribuer par la prise en compte du comportement des agriculteurs mais aussi par la compréhension du système biophysique. En particulier, la quantification des différents termes du bilan hydrique, et notamment celui de la transpiration du palmier dattier, a été relativement peu étudiée jusqu'à présent. Le terme de consommation hydrique du palmier est un indicateur indispensable en regard des contraintes de gestion de l'eau et du bilan hydrique global. Cette problématique a été abordée au sein de l'oasis de Fatnassa Nord à Kébili (33°,8 N ; 8°,7 E) dans un contexte caractérisé à la fois par de faibles fréquences de tour d'eau et une salinité plutôt élevée. L'agrosystème bi-étagée palmier/fourrage du système oasien de cette région nécessite de bien cerner la composante palmier de la strate fourrage. Une technique de mesure thermique du flux de sève a donc été mise en oeuvre pour évaluer la transpiration spécifique du palmier dattier au sein d'une parcelle dont les termes du bilan hydriques sont bien mesurés en continu. L'avantage de cette méthode est qu'elle permet aussi une mesure continue et qu'elle intègre, grâce au suivi simultané de quatre arbres, une certaine hétérogénéité du système palmier à la fois due au système radiculaire et au développement de la canopée, tous deux liés au stipe, voie unique de passage de la sève entre ces deux compartiments de l'arbre. Dans cet article, les principaux résultats des mesures réalisées depuis plus d'un an sont analysés et présentent une bonne cohérence avec la dynamique nycthémérale de la nappe superficielle donnée par un suivi piézométrique fin. Les flux de transpiration cumulés sur la journée ont varié globalement de 0.5 à 3,5 mm/j avec une moyenne de l'ordre de 2 mm/jour reflétant une consommation hydrique modeste en regard de la demande climatique. Les valeurs montrent également que ces flux journaliers, malgré leur variation inter-arbres importantes, varient en lien étroit avec le cycle des apports d'eau par irrigation. Le palmier étant malgré sa résistance aux conditions climatiques, dont en particulier une demande forte, une espèce qui régule ses pertes par régulation stomatique

Activation of the hsp70 promoter by environmental inorganic and organic chemicals: relationships with cytotoxicity and lipophilicity.

International audienceStress proteins (heat shock proteins, HSPs) have been proposed as general markers of cellular aggression and their use for environmental monitoring is often suggested. The aim of this work was to study the potency of various environmentally relevant organic and inorganic chemicals to induce the expression of the HSP70 marker. For this purpose, we used an established HeLa cell line containing the chloramphenicol acetyl transferase (CAT) gene under the control of the hsp70 promoter. The screening of three metallic and 15 organic chemicals revealed differences in their capacities to induce the hsp70 promoter. The three metals tested (cadmium, zinc and mercury) were able to induce a stress response. Some organochlorine compounds (chlorophenol derivatives, tetrachlorohydroquinone, 3, 4-dichloroaniline, ethyl parathion and 1-chloro-2,4-dinitrobenzene) induced a response, whereas other common halogenated pesticides or aromatic hydrocarbons (e.g. benzo(a)pyrene, 2, 4-dichlorophenoxyacetic acid, endosulfan, diuron, 4-nonylphenol) did not. The potency to induce hsp70 was significantly correlated to the octanol-water partition coefficient (log K(ow)) of the inducing chemicals, except for 1-chloro-2,4-dinitrobenzene and ethyl parathion. Cytotoxicity assays run in parallel to the induction measurements revealed that the three metals were effective at non cytotoxic doses whereas all organic compounds, except tetrachlorohydroquinone and 1-chloro-2,4-dinitrobenzene, induced the promoter at cytotoxic doses. These results suggest that hsp70 is induced by different mechanisms of toxicity. We propose that this model can be used in mechanistic studies for the detection of toxic effects of certain pollutants

A stable fish reporter cell line to study estrogen receptor transactivation by environmental (xeno)estrogens.

International audienceCross-species differences between human and fish estrogen receptor (ER) binding by environmental chemicals have been reported. To study ER transactivation in a fish cellular context, we stably co-transfected the PLHC-1 fish hepatoma cell line with a rainbow trout estrogen receptor (rtER) and the luciferase reporter gene driven by an estrogen response element (ERE). This new cell model, called PELN-rtER (for PLHC-1-ERE-Luciferase-Neomycin), responded to 17beta-estradiol (E2) in a both concentration- and temperature-dependent manner, as well as to environmental ER ligands from different chemical classes: natural and synthetic estrogens, zearalenone metabolites, genistein, alkyphenoles and benzophenone derivatives. The comparison with other in vitro models, i.e. human reporter cell lines (HELN-rtER, MELN) and vitellogenin induction in primary cultures of rainbow trout hepatocytes, showed an overall higher sensitivity of the human cells for a majority of ligands, except for benzophenone derivatives which were active at similar or lower concentrations in fish cells, suggesting species-specificity for these substances. Correlation analyses suggest that the fish cell line is closer to the trout hepatocyte than to the human cell context, and could serve as a relevant mechanistic tool to study ER activation in fish hepatic cellular context

Integration of Dirac-Jacobi structures

We study precontact groupoids whose infinitesimal counterparts are Dirac-Jacobi structures. These geometric objects generalize contact groupoids. We also explain the relationship between precontact groupoids and homogeneous presymplectic groupoids. Finally, we present some examples of precontact groupoids.Comment: 10 pages. Brief changes in the introduction. References update

Total Synthesis of Iejimalide A-D and Assessment of the Remarkable Actin-Depolymerizing Capacity of These Polyene Macrolides

A concise and convergent total synthesis of the highly cytotoxic marine natural products iejimalide A−D (1−4) is reported, which relies on an effective ring-closing metathesis (RCM) reaction of a cyclization precursor containing no less than 10 double bonds. Because of the exceptional sensitivity of this polyunsaturated intermediate and its immediate precursors toward acid, base, and even gentle warming, the assembly process hinged upon the judicious choice of protecting groups and the careful optimization of all individual transformations. As a consequence, particularly mild protocols for Stille as well as Suzuki reactions of elaborate coupling partners have been developed that hold considerable promise for applications in other complex settings. Moreover, a series of non-natural “iejimalide-like” compounds has been prepared, differing from the natural lead in the polar head groups linked to the macrolide's N-terminus. With the aid of these compounds it was possible to uncover the hitherto unknown effect of iejimalide and analogues on the actin cytoskeleton. Their capacity to depolymerize this microfilament network rivals that of the latrunculins which constitute the standard in the field. Structural modifications of the peptidic terminus in 2 are thereby well accommodated, without compromising the biological effects. The iejimalides hence constitute an important new class of probe molecules for chemical biology in addition to their role as promising lead structures for the development of novel anticancer agents