Reações orgânicas em meio aquoso

The use of water as solvent in organic reactions has been uncommon for several reasons, among them the low solubility of the reactants, the incompatibility of the intermediates with water, and the competition between the desired reaction and hydrolysis. Breslow in 1980, demonstrated that the hydrophobic effect accelerates Diels-Alder reactions and gives a high endo/exo selectivity. Since then, many other reactions were studied in this medium, and below we show the principal results. Besides the academic interest, human and economic aspects are included in this study. Water as a solvent minimizes environmental impact, costs and increases operational safety.

How are riparian plants distributed along the riverbank topographic gradient in Mediterranean rivers? Application to minimally altered river stretches in Southern Spain.

Species structure and composition in Mediterranean riparian forests are determined by hydrological features, longitudinal zonation, and riverbank topography. This study assesses the distribution of four native riparian plants along the riverbank topographic gradient in three river stretches in southern Spain, with special emphasis on the occupation of adult and young feet of each species. The studied stretches suffered minimal human disturbances, displayed semi-arid conditions, and had wide riparian areas to allow the development of the target species: black alder (Alnus glutinosa), salvia leaf willow (Salix salviifolia), narrow-leafed ash (Fraxinus angustifolia), and oleander (Nerium oleander). Thalweg height was used to define the riverbank topographic gradient. The results showed a preferential zone for black alder and salvia leaf willow in the range of 0-150 cm from the channel thalweg, with adult alders and willows being more common between 51 and 150 cm and young alders being more common under 50 cm. Conversely, narrow-leafed ash and oleander were much more frequent, and showed greater development, in the ranges of 151-200 cm and 201-250 cm, respectively, whereas the young feet of both species covered the entire topographic range. Adult feet of the four species were spatially segregated along the riverbank topographic gradient, indicating their differential ability to cope with water stress from the non-tolerant alders and willows to more tolerant narrow-leafed ash trees and oleanders. Young feet, however, showed a strategy more closely linked to the initial availability of colonisation sites within riparian areas to the dispersion strategy of each species and to the distribution of adult feet. In Mediterranean areas, where riparian management has traditionally faced great challenges, the incorporation of species preferences along riverbank gradients could improve the performance of restoration projects.

The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes

The encapsulation of metal clusters in endohedral metallofullerenes (EMFs) takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. There exist several possible explanations for the choice of the hosting cages in EMFs, although the final reasons are actually not totally well understood. Moreover, the reactivity and regioselectivity of (endohedral metallo)fullerenes have in the past decade been shown to be generally dependent on a number of factors, such as the size of the fullerene cage, the type of cluster that is being encapsulated, and the number of electrons that are transferred formally from the cluster to the fullerene cage. Different rationalizations of the observed trends had been proposed, based on bond lengths, pyramidalization angles, shape and energies of (un)occupied orbitals, deformation energies of the cages, or separation distances between the pentagon rings. Recently, in our group we proposed that the quest for the maximum aromaticity (maximum aromaticity criterion) determines the most suitable hosting carbon cage for a given metallic cluster (i.e. EMF stabilization), including those cases where the IPR rule is not fulfilled. Moreover, we suggested that local aromaticity plays a determining role in the reactivity of EMFs, which can be used as a criterion for understanding and predicting the regioselectivity of different reactions such as Diels-Alder cycloadditions or Bingel-Hirsch reactions. This review highlights different aspects of the aromaticity of fullerenes and EMFs, starting from how this can be measured and ending by how it can be used to rationalize and predict their molecular structure and reactivity

Efeitos da subsolagem e da adubação verde nas propriedades físicas do solo em pomar de citros

O presente trabalho teve por objetivo avaliar o efeito da subsolagem e da adubação verde nas propriedades físicas de um Latossolo Vermelho distrófico típico, cultivado com citros (laranjeira Valência). Os tratamentos empregados foram: manejo mecânico das plantas invasoras com roçadora (Testemunha); subsolagem em um lado da planta na entrecopa; subsolagem nos dois lados da planta na entrecopa, e semeadura direta de Crotalaria spectabilis na entrecopa. As propriedades físicas do solo avaliadas foram: porosidade total, macroporosidade, microporosidade, densidade, resistência mecânica à penetração vertical e condutividade hidráulica. Na avaliação efetuada na safra 2001-2002, obtiveram-se os maiores valores de densidade do solo e resistência do solo à penetração vertical, e os menores valores de macroporos e condutividade hidráulica do solo nas entrecopas com e sem tráfego no pomar, em relação à região da copa, principalmente na camada de 0,0-0,2 m de profundidade. Os tratamentos aplicados não surtiram efeitos positivos significativos nas propriedades físicas do solo.

Synthetic approaches to syn-diol containing biologically active compounds /

The deoxy derivative of pancratistatin 1.10 was prepared in good yield through the use of a [4+2] Diels-Alder cycloaddition and Bischler-Napieralski cyclization approach. The Bischler-Napieralski cyclization was shown to yield two additional side products 2.9, 2.10, however, under slightly modified hydrolysis conditions, the tetracyclic product 2.11 was obtained exclusively in greater than 84% yield. Initial screening of the di-hydroxylatgd derivative, and the other complementary pair analogue 1.10' previously prepared in our laboratories gave interesting results. Both of these compounds were shown to exhibit cytostatic activity; the mono-alcohol was marginally active while the di-hydroxylated analogue proved to be more potent although one to two magnitudes less potent than pancratistatin itself Human tumour cell line assay results indicated that the di-hydroxylated derivative exhibited selective cytotoxic inhibition in the following cell lines: non-small cell lung cancer line NCI-H226 (ED50 - 0.65 ^g/mL), leukemia cell lines CCRF-CEM (ED30 = 0.55 Hg/mL) and HL-60(TB) (ED50 = 0.89^ig/mL). Our results demonstrated that the pharmacophore is not a mono-alcohol, and that the minimum pharmacophore contains the hydroxyl group at the C4 position in addition to either, or both, of the hydroxyl groups present at C2 and C3.' The minimum pharmacophore has been narrowed to only three possibilities which are current synthetic targets in several research groups. The controlled Grignard addition to the tartaric acid derived bis-Weinreb amide 1.25 afforded a direct entry to a host of 1,4-diflferentiated tartaric acid derived intermediates (2.12-2.18). This potentially usefiil methodology was demonstrated through the efficient synthesis of the naturally occurring lactone 2.23, which bears the inherent syn-dio\ subunit. Based on this result, a similar approach to the synthesis of syn-dio\ bearing natural products looks very promising? A direct 2,3-diol desymmetrization method using TIPS-triflate was shown to be effective on the selective differentiation of Z,-methyl tartrate (and diisopropyl tartrate). The mono-silyl-protected intermediates 2.31 also proved to be useful when they were selectively differentiated at the 1,4-carboxyl position (2.35, 2.36) through the use of a borohydride reducing agent. Furthermore, the mono-silyl-protected derivative underwent periodate cleavage affording two synthetically useful a,P-unsaturated esters 2.43, 2.44, with one of esters being obtained via a silyl-migration method.''

Towards the enantioselective synthesis of lycoricidine alkaloids /

Two efficient, regio- and stereo controlled synthetic approaches to the synthesis of racemic analogs of pancratistatin have been accomplished and they serve as the model systems for the total synthesis of optically active 7-deoxy-pancratistatin. In the Diels-Alder approach, an efficient [4+2] cycloaddition of 3,4-methylenedioxyco- nitrostyrene with Danishefsky's diene to selectively form an exo-nitro adduct has been developed as the key step in the construction of the C-ring of the target molecule. In the Michael addition approach, the key step was a conjugate addition of an organic zinc-cuprate to the 3,4-methylenedioxy-(B-nitrostyrene, followed by a diastereocontroUed closure to form the cyclohexane C-ring of the target molecule via an intramolecular nitro-aldol cyclization on a neutral alumina surface. A chair-like transition state for such a cyclization has been established and such a chelation controlled transition state can be useful in the prediction of diastereoselectivity in other related 6-exo-trig nitroaldol reactions. Cyclization of the above products fi^om both approaches by using a Bischler-Napieralski type reaction afforded two lycoricidine derivatives 38 and 50 in good yields. The initial results from the above modeling studies as well as the analysis of the synthetic strategy were directed to a chiral pool approach to the total synthesis of optically active 7-deoxy-pancratistatin. Selective monsilylation and iodination of Ltartaric acid provided a chiral precursor for the proposed key Michael transformation. The outlook for the total synthesis of 7-deoxy-pancratistatin by this approach is very promising.A concise synthesis of novel designed, optically pure, Cz-symmetrical disulfonylamide chiral ligands starting from L-tartaric acid has also been achieved. This sequence employs the metallation of indole followed by Sfj2 replacement of a dimesylate as the key step. The activity for this Cz-symmetric chiral disulfonamide ligand in the catalytic enantioselective reaction has been confirmed by nucleophilic addition to benzaldehyde in the disulfonamide-Ti (0-i-Pr)4-diethylzinc system with a 48% yield and a 33% e.e. value. Such a ligand tethered with a suitable metal complex should be also applicable towards the total synthesis of 7-deoxy-pancratistatin.

Applications of dihydroarenediols to chemoenzymatic synthesis : approaches to total synthesis of morphine alkaloids

The present studies describe, as a primary goal, our recent progess toward the synthesis of morphine alkaloids from aromatic precursors. Model substrates were synthesized which allowed investigation into Diels-Alder, radical cascade, and palladium-catalyzed bond-forming reactions as possible routes to the morphine alkaloid skeleton. As a secondary objective, three separate series of aromatic substrates were subjected to whole-cell oxidation with Escherichia coli JM 109 (pDTG601), a recombinant organism over-expressing the enzyme toluene dioxygenase. Included in this study were bromothioanisoles, dibromobenzenes, and cyclopropylbenzene derivatives. The products of oxidation were characterized by chemical conversion to known intermediates. The synthetic utility of one of these bacterial metabolites, derived from oxidation of o-dibromobenezene, was demonstrated by chemical conversion to (-)conduritol E.

Model studies toward the total synthesis of thebaine by an intramolecular cycloaddition strategy

The present studies describe recent progress toward the synthesis of the thebaine. Model substrates were synthesized using pyridazine derivatives as a starting material, which allowed to assess the key Diels-Alder reaction as a route to construct the thebaine core.

The Use of Ipso-dihydrodiols Enzymatically Derived from Benzoic Acid in Enantioselective Synthesis. Appoaches to Total Synthesis of Vinca Alkaloids

The present studies describe our recent progress in target oriented synthesis of complex organic molecules from aromatic precursors. The latest synthetic approaches toward vinca alkaloids are described and include the construction of model substrates for the investigation into Diels-Alder, radical cascade, and tandem Michael addition reactions as possible routes to the family of alkaloids. Also described are the chemoenzymatic syntheses of the natural product (-)-idesolide and unnatural polyhydroxylated pyrrolidines generated from the biotransformation of benzoic acid with Ralstonia eutropha B9.

Synthèse de 4-désoxy hexopyrannoses, C-disaccharides et C-glycosides biologiquement actifs

Les glucides constituent la classe de molécules organiques la plus abondante et ceux-ci jouent des rôles cruciaux dans divers processus biologiques. De part leur importance médicinale, la préparation des désoxy-sucres, des C-glycosides et des C-disaccharides est devenue un sujet de pointe en synthèse organique. De façon générale, cette thèse décrit une nouvelle synthèse de novo des 4-désoxy hexopyrannoses en plus de la préparation de C-glycosides biologiquement actifs. De plus, une attention particulière a été portée à la préparation de novo de 4-désoxy-C-disaccharides. Dans un premier temps, le catalyseur de Cr(III) de Jacobsen et un complexe binaphtol/titane ont été utilisés pour réaliser des hétéro-Diels-Alder énantiosélectives. Les dihydropyrannes ainsi générés ont été transformés en 4-désoxy hexopyrannoses présents dans la nature. De cette façon, un dérivé de l’acide ézoaminuroïque, un précurseur de la désosamine et de la néosidomycine, a été préparé suivant cette approche de novo. De plus, à titre comparatif, la néosidomycine a également été fabriquée selon une approche chiron, à partir du méthyl alpha-D-mannopyrannoside. Finalement, une évaluation biologique préliminaire de la néosidomycine a été effectuée sur une la concanavaline-A (Chapitre 2). Dans un deuxième temps, une allylation stéréosélective sur un aldéhyde lié via des liens C-C à une unité mannoside a permis de générer un alcool homoallylique. Cette dernière fonctionnalité a été transformée en 4-désoxy hexopyrannose de configuration D ou L. De cette façon, la préparation de pseudo 4-désoxy-C-disaccharides, de 4-désoxy-C-disaccharides et de pseudo 4-désoxy aza-C-disaccharides a facilement été réalisée. Les rapports diastéréoisomériques de la réaction d’allylation ont été déterminés en plus de la configuration absolue des nouveaux centres stéréogéniques formés. La transformation des alcools homoallyliques en pyrannes poly hydroxylés ou en lactames poly hydroxylés a été réalisée, en plus de la déprotection de certains membres de cette famille pour une évaluation biologique préliminaire sur la concanavaline-A (Chapitre 3). Finalement, la synthèse de C-glycosides biologiquement actifs a été réalisée selon deux volets: i) préparation de 3-C-mannopyrannosyl coumarines et ii) synthèse de C-galactosides, inhibiteurs de la lectine PA-IL. Pour ce faire, le couplage de Heck a été utilisé à partir d’un ester alpha,bêta-insaturé, attaché à une unité glycosidique via des liens C-C, pour générer un dérivé glycosyl cinnamate de méthyle. Cependant, lorsque le 2-iodophénol est utilisé comme partenaire de Heck, la coumarine correspondante a été isolée. Les dérivés C-galactopyrannosyl cinnamates de méthyle représentent de bons inhibiteurs monovalents de la PA-IL avec un Kd aussi bas que 37 micro M (Chapitre 4).

Synthèse totale de la (+)-Désoxygaliéllalactone et les N-tosyloxycarbamates comme source de nitrènes pour l'aziridination énantiosélective des styrènes

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal

Études vers la synthèse totale de l'indolizidine 223A

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Reconstitution paléo-écologique et contexte magnéto-stratigraphique de la forêt fossile de l'île Bylot (Nunavut)

Ce projet porte sur la reconstitution paléo-écologique d'un environnement forestier fossile retrouvé sur un plateau au sud-ouest de l'île Bylot. Il a comme objectifs 1) de préciser la chrono-stratigraphie du site; 2) d’établir une liste et une succession des différents taxons polliniques retrouvés dans les différentes unités stratigraphiques du site; 3) d’estimer leur âge et 4) d’en inférer des conditions climatiques (température et précipitations). Plusieurs coupes stratigraphiques ont été excavées puis échantillonnées afin de réaliser des analyses stratigraphiques, paléomagnétiques et polliniques. Un GPS différentiel fut également utilisé afin de caractériser à petite et grande échelle les unités stratigraphiques associées aux unités organiques fossiles. Les résultats des analyses granulométriques indiquent la séquence de dépôt suivante au sein d’une dépression dans la roche en place (schiste tertiaire) : 1) un diamicton glaciaire local ; 2) un sédiment limoneux d’origine glacio-lacustre ; 3) une unité organique tourbeuse; 4) une unité de type alluvial ; 5) un sédiment fluvio-glaciaire et 6) un diamicton glaciaire d’origine allochtone. Les analyses polliniques suggèrent une végétation similaire à celle présente près de la limite des arbres actuelle, environ 2000 km plus au sud. Les conditions climatiques plus humides et plus chaudes permettaient notamment la croissance du pin (Pinus type strobus et banksiana), de l’épinette (Picea cf. mariana), de l’aulne (type crispa et incana) et du mélèze (Larix, indifférencié). Enfin, les études paléomagnétiques et la présence d’espèces éteintes suggèrent un âge pour les dépôts organiques fossiles situé entre 2,14 et 2,15 Ma ou entre 2,581 et 3,040 Ma.

Studies on the Synthesis and Transformations of a few Bicyclic and Dispiro Compounds

In this thesis, we report our endeavours in the synthesis of a few polycyclic compounds. We were interested in the synthesis of a few bicyclic compounds designed to undergo interesting photochemical transformations including tripletmediated di-π-methane rearrangement and/or competing singlet-mediated electrocyclic reactions. Our target molecules have "inbuilt" structural features which will potentially alter the photochemistry of the substrate under consideration.The present investigation was undertaken to test our hypothesis on selective intramolecular quenching of singlet or triplet excited states of molecules.We adopted Dies-Alder reaction for the synthesis of several of the bicyclic compounds we were interested in. Some of the precursor dienes synthesised by us are capable of undergoing intramolecular cycloaddition reactions as well. So, it was important to delineate the conditions and structural features that will enable a particular molecule to undergo intermolecular and intramolecular Dies-Alder reaction when treated with a suitable dienophile.Though, the main focus of this thesis is on the synthesis of bicyclic and tricyclic systems capable of undergoing di-π-methane rearrangement, in the last chapter of this thesis, we describe our findings on the synthesis of a few dispirocompounds. These systems were encountered as unexpected products in the attempted synthesis of novel dibenzoylalkene-type systems. Consequently, a brief survey on the synthesis and transformations of dibenzoylalkenes is also included as an integral part of this thesis.

Synthesis and Photochemistry of a few Olefin appended Dibenzobarrelenes and Bisdibenzobarrelenes

The thesis entitled: ‘Synthesis and Photochemistry of a few Olefin appended Dibenzobarrelenes and Bisdibenzobarrelenes’ is divided into 5 chapters.In Chapter 1, the fundamental concepts of Diels-Alder reaction, di-r:methane rearrangement and energy transfer process in organic photochemistry is discussed.Chapter 2 presents the synthesis of 9-olefin appended anthracenes and bisanthracenes. The target of synthesising various bridgehead olefin appended dibenzobarrelenes and some novel bisdibenzobarrelenes, led us to the synthesis of the appropriate alkenylanthracenes and bisanthracenes as precursor molecules. Diels-Alder reaction was the synthetic tool for the preparation of the target olefin appended dibenzobarrelenes and bisdibenzobarrelenes. This chapter attempts to throw light on our endeavours in synthesising the various 9-alkenylanthracenes and bisanthracenes.Chapter 3 deals with the synthesis of olefin appended dibenzobarrelenes and bisdibenzobarrelenes. Ever since the discovery of di-It-methane rearrangement dibenzobarrelenes, tailored with dijferent substituents at various positions have always been a tool to photochemists in unravelling the mechanisms of light induced reactions. Our intention of analysing the role of a It-moiety at the bridgehead position of the dibenzobarrelene, was synthetically envisaged via the Diels-Alder reaction. Bisdibenzobarrelenes were synthesised through tandem Diels-Alder reaction. Various alkenylanthracenes and bisanthracenes were employed as dienes and the dienophiles used were dimethyl acetylenedicarboxylate and dibenzoylacetylene. In this chapter, we report our venture in synthesising the various olefin appended dibenzobarrelenes and bisdibenzobarrelenes. Chapter 4 describes the preliminary time-resolved fluorescence studies of some olefin appended dibenzobarrelenes and bisdibenzobarrelenes.To understand the primary and secondary physicochemical processes in a photochemical reaction it is necessary to characterise the excited states and the transient intermediates during their short lifetime. A number of methods developed on the basis of the physical properties of the transient species are available for their detection. Time-correlated single-photon counting technique has been utilised in the present study of the excited states of olefin appended dibenzobarrelenes and bisdibenzobarrelenes. To understand the primary and secondary physicochemical processes in a photochemical reaction it is necessary to characterise the excited states and the transient intermediates during their short lifetime. A number of methods developed on the basis of the physical properties of the transient species are available for their detection. Time-correlated single-photon counting technique has been utilised in the present study of the excited states of olefin appended dibenzobarrelenes and bisdibenzobarrelenes.Chapter 5 portrays the photochemistry of olefin appended dibenzobarrelenes and bisdibenzobarrelenes. Dibenzocyclooctatetraene and dibenzosemibullvalene are the photoproducts obtained respectively through the singlet excited state and the triplet excited state of dibenzobarrelenes. Chemical literature shows evidences of the photoreactivity of dibenzobarrelenes generating both the singlet and triplet mediated photoproducts, in a single photoreaction. Our research target in synthesising various bridgehead olefin appended dibenzobarrelenes and bisdibenzobarrelenes, was based on the perception that olefins are eflicient triplet quenchers, thereby quenching intramolecularly the triplet excited state of the barrelenes. A It-moiety at the bridgehead position of the dibenzobarrelene, creates a tetra tr-methane system, which similar to a 6li—7l' or tri-tr-methane systems, could be the fertile ground for interesting photochemical rearrangements. Our attempts in deciphering the photochemistry of the olefin appended dibenzobarrelenes and bisdibenzobarrelenes is the substance of this chapter.