Stereoselective chromium- and molybdenum-mediated transformations of arenes

Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [alpha] values ranging from −174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser-Claisen rearrangement and a regio- and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO)3 group have been realized. The reactions show strong analogies to the Cr(CO)3-mediated reactions, but exhibit also marked differences: the arene-Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analog

Stereoselective chromium-and molybdenum- mediated transformations of arenes*

Abstract: Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [α] values ranging from -174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser-Claisen rearrangement and a regio-and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO) 3 group have been realized. The reactions show strong analogies to the Cr(CO) 3 -mediated reactions, but exhibit also marked differences: the arene-Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs. The search for ever more efficient chemo-, regio-, and stereoselective routes to complex molecules from simple starting materials is an important task in organic synthesis. Progress in this area continues to advance at a fast pace. Arenes, the subject of this article, are widely available, highly stable, and readily derivatized through reactions such as electrophilic and nucleophilic aromatic substitution [1], ortholithiation followed by reaction with electrophiles [2], or metal-catalyzed substitution and coupling reactions This notwithstanding, the synthesis of complex organic molecules via elegant dearomatization chemistry has undergone intensive investigation. Examples involve the Birch reduction, which achieves *Plenary and invited lectures presented at th

Habitat continuity and stepping-stone oceanographic distances explain population genetic connectivity of the brown alga Cystoseira amentacea

Effective predictive and management approaches for species occurring in a metapopulation structure require good understanding of interpopulation connectivity. In this study, we ask whether population genetic structure of marine species with fragmented distributions can be predicted by stepping-stone oceanographic transport and habitat continuity, using as model an ecosystem-structuring brown alga, Cystoseira amentacea var. stricta. To answer this question, we analysed the genetic structure and estimated the connectivity of populations along discontinuous rocky habitat patches in southern Italy, using microsatellite markers at multiple scales. In addition, we modelled the effect of rocky habitat continuity and ocean circulation on gene flow by simulating Lagrangian particle dispersal based on ocean surface currents allowing multigenerational stepping-stone dynamics. Populations were highly differentiated, at scales from few metres up to thousands of kilometres. The best possible model fit to explain the genetic results combined current direction, rocky habitat extension and distance along the coast among rocky sites. We conclude that a combination of variable suitable habitat and oceanographic transport is a useful predictor of genetic structure. This relationship provides insight into the mechanisms of dispersal and the role of life-history traits. Our results highlight the importance of spatially explicit modelling of stepping-stone dynamics and oceanographic directional transport coupled with habitat suitability, to better describe and predict marine population structure and differentiation. This study also suggests the appropriate spatial scales for the conservation, restoration and management of species that are increasingly affected by habitat modifications.MARES Grant (Doctoral Programme in Marine Ecosystem Health and Conservation) [EU-512002-1-2010-1-BE-EMJD]; Ghent University [FPA 2011-0016]; FCT (Portugal); project TETRIS; [SFRH/BPD/63703/2009]; [SFRH/BPD/107878/2015]; [SFRH/BPD/111003/2015]info:eu-repo/semantics/publishedVersio

Highlights from the 1st Firmenich Innovation Through Synthesis (FITS) Symposium, Geneva, Switzerland, 26–27th June 2008: Conference Report

This conference report highlights the different lectures that took place for Geneva for the first organic synthesis symposium organized by Firmenich

Conservation des populations de Cystoseira en régions Provence-Alpes-Côte-d'Azur et Corse

In the Mediterranean Sea, rocky shores are mainly inhabited by Cystoseira species. These canopy forming algae are engineers of the ecosystem. All species except for C. compressa, are protected by two international conventions, Berne (1979) and Barcelona (1992). Despite their protected status, no management plans have been developed yet. This work focused on 4 species: C. amentacea var. stricta, C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. The aim of the thesis was to i) gather information about the distribution of these species and highlight the main threats and source of their disappearance along PACA and Corsican coasts, ii) Understand the resilience capacities of C. amentacea after a partial destruction of its habitat and iii) understand the diversity and the genetic structure of populations of C. amentacea along PACA and Corsican shores. The distribution of C. amentacea appears to be still important. On the contrary, the situation is more critical for species of shallow waters like C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. C. amentacea showed good capacity of resilience after the perturbation. The recolonization follows the facilitation model. Finally the genetic study highlights a strong genetic structure within and among populations of C. amentacea. The within populations structure might be due to high inbreeding. No IBD was revealed among the all dataset which means that long distance dispersal might be possible for C. amentacea. The results obtained here were not sufficient to elaborate a management plan for Cystoseira species. But they completed the ecological knowledge we have on them, they brought the first large scale genetic results and they identified on what future studies must focused on.This thesis confirms the necessity to adopt a multidisciplinary approach in conservation biology.En Méditerranée, les zones rocheuses photophiles sont principalement peuplées par des espèces du genre Cystoseira C. Agardh. Ces espèces, ingénieures d’écosystèmes, forment des forêts qui structurent l’habitat benthique des étages infralittoral et circalittoral. Ces espèces (sauf C. compressa) sont protégées par les conventions de Berne depuis 1979 et de Barcelone (1992). Pourtant aucun plan de conservation n’a encore été mis en place. L’objectif de cette thèse était de i) connaître la répartition de ces espèces le long des littoraux PACA et Corse, ii) comprendre les capacités de résilience de C. amentacea var. stricta, iii) comprendre la diversité et la structure génétique des populations de C. amentacea. L’étude de la répartition des espèces a mis en évidence l’abondance de C. amentacea et la forte régression des espèces de petits fonds i.e. C. barbata, C. crinita et C. foeniculacea f. tenuiramosa. L’étude de résilience a montré que C. amentacea est capable de recoloniser son milieu après une perturbation aiguë. Cette recolonisation suit le modèle de facilitation Enfin l’étude de génétique des populations a mis en évidence une forte structuration inter et intra-populations ainsi qu’une forte consanguinité au sein de ces populations. L’absence d’IBD sur l’ensemble de l’échantillonnage révèle la possibilité de dispersion sur de longues distances. Ces résultats n’ont pas permis de développer un plan de gestion pour ces espèces. Cependant, ils ont apporté un certain nombre de connaissances supplémentaires sur l’écologie des espèces, ils ont amorcé le travail de génétique des populations pour le genre Cystoseira et surtout ils ont permis d’identifier les axes de recherches à développer en priorité dans les futures études. Cette thèse confirme la nécessité d’une approche pluridisciplinaire en biologie de la conservation

Preservation of the populations of Cystoseira in regions Provence-Alpes-Côte-d'Azur and Corsica

En Méditerranée, les zones rocheuses photophiles sont principalement peuplées par des espèces du genre Cystoseira C. Agardh. Ces espèces, ingénieures d’écosystèmes, forment des forêts qui structurent l’habitat benthique des étages infralittoral et circalittoral. Ces espèces (sauf C. compressa) sont protégées par les conventions de Berne depuis 1979 et de Barcelone (1992). Pourtant aucun plan de conservation n’a encore été mis en place. L’objectif de cette thèse était de i) connaître la répartition de ces espèces le long des littoraux PACA et Corse, ii) comprendre les capacités de résilience de C. amentacea var. stricta, iii) comprendre la diversité et la structure génétique des populations de C. amentacea. L’étude de la répartition des espèces a mis en évidence l’abondance de C. amentacea et la forte régression des espèces de petits fonds i.e. C. barbata, C. crinita et C. foeniculacea f. tenuiramosa. L’étude de résilience a montré que C. amentacea est capable de recoloniser son milieu après une perturbation aiguë. Cette recolonisation suit le modèle de facilitation Enfin l’étude de génétique des populations a mis en évidence une forte structuration inter et intra-populations ainsi qu’une forte consanguinité au sein de ces populations. L’absence d’IBD sur l’ensemble de l’échantillonnage révèle la possibilité de dispersion sur de longues distances. Ces résultats n’ont pas permis de développer un plan de gestion pour ces espèces. Cependant, ils ont apporté un certain nombre de connaissances supplémentaires sur l’écologie des espèces, ils ont amorcé le travail de génétique des populations pour le genre Cystoseira et surtout ils ont permis d’identifier les axes de recherches à développer en priorité dans les futures études. Cette thèse confirme la nécessité d’une approche pluridisciplinaire en biologie de la conservation.In the Mediterranean Sea, rocky shores are mainly inhabited by Cystoseira species. These canopy forming algae are engineers of the ecosystem. All species except for C. compressa, are protected by two international conventions, Berne (1979) and Barcelona (1992). Despite their protected status, no management plans have been developed yet. This work focused on 4 species: C. amentacea var. stricta, C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. The aim of the thesis was to i) gather information about the distribution of these species and highlight the main threats and source of their disappearance along PACA and Corsican coasts, ii) Understand the resilience capacities of C. amentacea after a partial destruction of its habitat and iii) understand the diversity and the genetic structure of populations of C. amentacea along PACA and Corsican shores. The distribution of C. amentacea appears to be still important. On the contrary, the situation is more critical for species of shallow waters like C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. C. amentacea showed good capacity of resilience after the perturbation. The recolonization follows the facilitation model. Finally the genetic study highlights a strong genetic structure within and among populations of C. amentacea. The within populations structure might be due to high inbreeding. No IBD was revealed among the all dataset which means that long distance dispersal might be possible for C. amentacea. The results obtained here were not sufficient to elaborate a management plan for Cystoseira species. But they completed the ecological knowledge we have on them, they brought the first large scale genetic results and they identified on what future studies must focused on.This thesis confirms the necessity to adopt a multidisciplinary approach in conservation biology

New Chromium Carbonyl Catalysts for [6+2] Cycloaddition Reactions

The complexes, (benzene)chromiumdicarbonyl(methyl acrylate) and chromiumdicarbonylbis(cyclohexadiene), are precursors for the highly coordinatively unsaturated chromium dicarbonyl fragment 3, a catalyst for the cycloaddition of activated olefins to cycloheptatriene

Conservation des populations de Cystoseira en régions Provence-Alpes-Côte-d'Azur et Corse

En Méditerranée, les zones rocheuses photophiles sont principalement peuplées par des espèces du genre Cystoseira C. Agardh. Ces espèces, ingénieures d écosystèmes, forment des forêts qui structurent l habitat benthique des étages infralittoral et circalittoral. Ces espèces (sauf C. compressa) sont protégées par les conventions de Berne depuis 1979 et de Barcelone (1992). Pourtant aucun plan de conservation n a encore été mis en place. L objectif de cette thèse était de i) connaître la répartition de ces espèces le long des littoraux PACA et Corse, ii) comprendre les capacités de résilience de C. amentacea var. stricta, iii) comprendre la diversité et la structure génétique des populations de C. amentacea. L étude de la répartition des espèces a mis en évidence l abondance de C. amentacea et la forte régression des espèces de petits fonds i.e. C. barbata, C. crinita et C. foeniculacea f. tenuiramosa. L étude de résilience a montré que C. amentacea est capable de recoloniser son milieu après une perturbation aiguë. Cette recolonisation suit le modèle de facilitation Enfin l étude de génétique des populations a mis en évidence une forte structuration inter et intra-populations ainsi qu une forte consanguinité au sein de ces populations. L absence d IBD sur l ensemble de l échantillonnage révèle la possibilité de dispersion sur de longues distances. Ces résultats n ont pas permis de développer un plan de gestion pour ces espèces. Cependant, ils ont apporté un certain nombre de connaissances supplémentaires sur l écologie des espèces, ils ont amorcé le travail de génétique des populations pour le genre Cystoseira et surtout ils ont permis d identifier les axes de recherches à développer en priorité dans les futures études. Cette thèse confirme la nécessité d une approche pluridisciplinaire en biologie de la conservation.In the Mediterranean Sea, rocky shores are mainly inhabited by Cystoseira species. These canopy forming algae are engineers of the ecosystem. All species except for C. compressa, are protected by two international conventions, Berne (1979) and Barcelona (1992). Despite their protected status, no management plans have been developed yet. This work focused on 4 species: C. amentacea var. stricta, C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. The aim of the thesis was to i) gather information about the distribution of these species and highlight the main threats and source of their disappearance along PACA and Corsican coasts, ii) Understand the resilience capacities of C. amentacea after a partial destruction of its habitat and iii) understand the diversity and the genetic structure of populations of C. amentacea along PACA and Corsican shores. The distribution of C. amentacea appears to be still important. On the contrary, the situation is more critical for species of shallow waters like C. barbata, C. crinita and C. foeniculacea f. tenuiramosa. C. amentacea showed good capacity of resilience after the perturbation. The recolonization follows the facilitation model. Finally the genetic study highlights a strong genetic structure within and among populations of C. amentacea. The within populations structure might be due to high inbreeding. No IBD was revealed among the all dataset which means that long distance dispersal might be possible for C. amentacea. The results obtained here were not sufficient to elaborate a management plan for Cystoseira species. But they completed the ecological knowledge we have on them, they brought the first large scale genetic results and they identified on what future studies must focused on.This thesis confirms the necessity to adopt a multidisciplinary approach in conservation biology.NICE-Bibliotheque electronique (060889901) / SudocSudocFranceF

New Chromium Enolates

Three new (?2-acrylate)(?6-arene)dicarbonylchromium complexes are reported. They were obtained either by CO/acrylate exchange in [Cr(?6-benzene)(CO)3] (1) via the photolytically generated ?2-cyclooctene intermediate or by arene exchange in [Cr(?2-methyl acrylate)(?6-benzene)(CO)2] (3) (Scheme 1). On crystallization, [Cr(?2-methyl acrylate)(?6-o-xylene)(CO)2] (5) underwent partial resolution. The degree of this resolution was analyzed via X-ray crystal-structure determination (Fig. 1) and was correlated to the CD spectra in solution (Fig. 6), thus allowing the assignment of the absolute configuration. The reaction of [Cr(?2-acrylate)(?6-benzene)(CO)2] complexes with cyclopentad-1,3-diene or 1H-indene afforded new (?6-cyclopenta-2,4-dien-1-yl)- or (?6-1H-inden-1-yl)(?3-oxaallyl)chromium complexes (Scheme 2). A mechanism is proposed that involves arene-ligand substitution by the diene ligand, initiated by haptotropic rearrangement of the acrylate from a ?2- to a ?4-coordination mode, followed by hydride migration from the diene to the acrylate (Scheme 3). An X-ray crystal-structure determination of [Cr(CO)2(?5-1H-inden-1-yl){?3-CH(CH2CF3)C(O)OEt}] (8) reveals a metal enolate that is best described as ?3-oxaallyl species (Fig. 2). A shorter, more-efficient route to the [Cr(?5-1H-inden-1-yl)(?3-oxaallyl)] complexes was devised via the reaction of [Cr(CO)2(?2-cyclooctene)(?6-1H-indene)] with methyl acrylate (Scheme 4)