Dissertations / Theses on the topic 'Diels-Alder Reaction'

Die Aufgabe des synthetischen Chemikers besteht darin neue Moleküle, Bauelemente oder Medikamente aus einfachen Edukten herzustellen. Die wachsende Komplexität dieser synthetischen Produkte bedingt jedoch, dass die Entwicklung der dazu benötigten Werkzeuge ebenso schritthalten muss. Licht ist mit seiner herausragenden räumlichen, zeitlichen und energetischen Auflösung sowie seinem nicht-invasiven Charakter den traditionell verwendeten Stimuli überlegen. In dieser Arbeit wurde durch die Kombination eines molekularen Photoschalters mit der Diels-Alder-Reaktion die Photokontrolle über eine dynamisch-kovalente Reaktion erlangt. Das Ergebnis der Reaktion eines furyl-substituierten Diarylethens mit Maleinimid konnte ausschließlich durch die Bestrahlung mit Licht unterschiedlicher Farbe bestimmt werden. Dieses prototypische System wurde dann unter Erhalt seiner photochemischen Eigenschaften für die Anwendung unter physiologischen Bedingungen weiterentwickelt. Es konnte die photoreversible Freisetzung therapeutisch wirksamer Konzentrationen verschiedener Anti-Tumor-Medikamente in gepufferter, wässriger Lösung bei Körpertemperatur durch bioorthogonales sichtbares Licht ausgelöst werden. Weiterhin wurden die Feinabstimmung der Freisetzung dieser photoschaltbaren Diels-Alder-Addukte und eine ausführliche Untersuchung der Freisetzungs-Dynamiken gezeigt. Hierdurch wurde das System auf eine Stufe gehoben, an der in vivo Anwendungen erwägt werden können. Des Weiteren wurden die Grundlagen zur Implementierung der photokontrollierbaren Diels-Alder-Reaktion in der reversiblen kovalenten Funktionalisierung von Kohlenstoffnanoröhren gelegt. Trotzdem noch keine erfolgreiche Funktionalisierung nachgewiesen werden konnte, wurde durch die Untersuchung der zugrundeliegenden Prinzipien der Weg für die Anwendung fortgeschrittener Techniken zur Modulation physikochemischer Eigenschaften von sp2-Kohlenstoff Allotropen geebnet.
The synthetic chemist’s profession is devoted to the formation of chemical bonds to create new molecules, building blocks, or drugs from basic starting materials. However, the growing complexity of these synthetic products necessitates that the development of the tools to fulfill this task must also keep pace. Light as a stimulus with its superior spatial, temporal, as well as energetic resolution in combination with its non-invasive character outperforms the traditional means to control a chemical reaction, such as heat, pressure, or pH, clearly. In this work, the unprecedented reversible photocontrol over a dynamic covalent reaction was established by the combination of a molecular photoswitch and the well-known Diels-Alder reaction. The outcome of the reaction of a furyl-substituted diarylethene and maleimide could be determined exclusively by illumination with differently colored light. This prototypical system was then advanced for the application under physiological conditions while retaining its superior photochemical properties. The release of therapeutically effective concentrations of different antitumor agents could be photoreversibly triggered in buffered, aqueous solution at body temperature employing bioorthogonal visible light. It is shown how the release properties of these photoswitchable prodrugs can be fine-tuned and a thorough investigation of the release dynamics is presented promoting this system to a level where in vivo application can be considered. Furthermore, the foundations for the implementation of the photocontrollable Diels-Alder reaction in the reversible covalent functionalization of carbon nanotubes have been laid. Despite the fact that a successful functionalization has not yet been achieved, the basic principles were investigated and have paved the way for the application of more advanced techniques to effectively modulate the physicochemical properties of sp2-carbon allotropes.

This thesis describes the investigation of the Diels-Alder reaction of the imine (140) with the diene (141) to give four products (142,143,156, 157). At low temperatures the enone (156) is the major product while at ambient temperature the bicyclic compounds (142) and (143) predominate. The reaction is highly solvent dependent, with the best results being obtained in benzene solution. Lewis acids appear to have little effect on the course of the reaction. The reaction of the imine (140) with the TBDMSO diene (159) gives the silyl enol ethers (160) and (161) as well as the above four products. Possible mechanisms are discussed. COOM. OSIM., In order to investigate asymmetric induction the 8- phenylmenthyl derived imine (194) was reacted with the diene (141) under a variety of conditions but no asymmetric induction was seen in the exo adducts, and only a small degree in the endo adducts. The bromination and Baeyer-Villiger oxidation of the ketone (142) were investigated and the resultant lactone (209) was converted into the bromide (258). ISOPROSOPININE A (84) ISOPROSOPININE B (S3) Several methods to prepare a suitable side chain for isoprosopinine A were investigated. The prosopis alkaloids isoprosopinine A (84) and B (85) were prepared by alkylation of the sulphones (252) and (266) with the bromide (258) followed by reductive removal of the N-tosyl and sulphone moieties with sodium amalgam. The imine (274) has been prepared and shown to undergo a Lewis acid catalysed imino-Diels-Alder reaction with the diene (141) to give the adducts (283) and (284). hCH,M=CHCOjM» (2T4) Alternative imines for asymmetric synthesis, such as (299) and (288) have been investigated. Preparation of the sulphinamide imine (299) has proved difficult. Attempts to carry out aqueous Diels-Alder reactions of the silyloxydiene (159) with iminium ions, generated in situ from amines, aldehydes and acids, have proved fruitless.

Described in this thesis is the development of a key Diels-Alder reaction for use in a total synthesis of the potently immunosuppressant marine alkaloid palau'amine. This study focuses on the originally proposed structure of palau'amine. The strategy utilizes a computationally designed thiohydantoin dienophile as the 2pi component and a thermally stable 2-silyloxy-5-(silyloxymethyl)cyclopentadiene as the 4pi component in the critical Diels-Alder reaction. Essential to the design and study of these partners was the use of computational density functional theory (DFT) predictions.
Four families of dienophiles---oxazolones, dehydroalanines, hydantoins and thiohydantoins---were investigated. The hydantoins and thiohydantoins react with cyclopentadiene with up to 13:1 exo-selectivity while the dehydroalanines are modestly selective and the oxazolones are unselective. The exo-selectivity of the hydantoin and thiohydantoin dienophiles is attributed to a novel stabilizing interaction whereby the lone pair of electrons on nitrogen interacts with a developing positive charge on the diene in the transition state. In terms of reactivity, the oxazolones are the superior dienophiles. The reactivity of the thiohydantoins is greater than that of analogous hydantoins and approaches that of the oxazolones. The bolstered reactivity of the thiohydantoins is attributed to the increased donation of the nitrogen lone-pair into the thiocarbonyl bond. This phenomenon was recognized first in DFT predictions and then proven experimentally. Synthetic access to the requisite beta-(chloromethylene)thiohydantoins necessitated the development of a reaction between dilithium diamides and thiophosgene.
2-silyloxy-5-(silyloxymethyl)cyclopentadienes were studied experimentally and computationally and found to be much more stable than conventional 5-substituted cyclopentadienes towards thermal decomposition by 1,5 hydride shift. The new dienes undergo Diels-Alder reaction with oxazolone and thiohydantoin dienophiles with exoselectivities as high as 6:1 for nonchlorinated dienophiles. Fully functionalized chlorinated exo-cycloadducts were prepared with up to 1.5:1 selectivity and subjected to oxidative ring-opening to reveal the persubstituted core of palau'amine.

This thesis explores the use of in situ generated acyclic 1-aminodendralenes in multicomponent diene-transmissive Diels-Alder (DTDA) reaction sequences. Dendralenes have previously been shown to generate polycyclic frameworks in a step-economic manner. The 1-amino substituent is shown to promote very high levels of site selectivity in these processes. Chapter 1 reviews the Diels-Alder reactions of 1-amino-1,3-butadienes and is divided into three sections. The first two sections cover the Diels-Alder reactions of 1-amino-1,3-butadienes and 1-amino-3-siloxy-1,3-butadienes (Rawal’s dienes) generated with a stoichiometric amount of amine. The third section covers enantioselective Diels-Alder reactions involving 1-amino-1,3-butadienes generated in situ with a catalytic amount of a chiral amine. While there have been many reports of Diels-Alder reactions of 1-amino-1,3-butadienes and 1-amino-3-siloxy-1,3-butadienes, there has been only one involving a semi-cyclic 1-amino[3]dendralene. There have been few examples which combine these Diels-Alder reactions with other transformations in multicomponent reactions to generate polycyclic frameworks. Chapter 2 describes the use of acyclic 1-amino[3]dendralenes in multicomponent reactions to generate a diverse range of heterocyclic structures. The condensation/Diels- Alder reaction sequence was tolerant of a variety of amines as well as carbon and hereoatom-based dienophiles. The Diels-Alder reactions of 1-amino[3]dendralenes were highly site-selective, taking place exclusively at the amine substituted 1,3-butadiene unit. The sequence was extended to a one-pot four-component reaction by including an additional dienophile for a Diels-Alder reaction to take place at the newly generated semi-cyclic diene. These condensation/Diels-Alder/Diels-Alder cycloadducts were generated with high diastereoselectivity, the origins of which were investigated and explained with the use of density functional theory calculations (carried out by Prof Paddon-Row). By reversing the order of events, that is performing a Diels-Alder reaction on the skipped dienal precursor before the condensation/Diels-Alder reaction sequence, constitutional isomers were accessed. The second Diels-Alder reaction could be performed intramolecularly when an amine bearing an alkenyl substituent was used. This condensation/Diels-Alder/intramolecular Diels-Alder reaction sequence furnished a variety of tricyclic and tetracyclic heterocycles. Chapter 3 describes the use of acyclic 1-aminodendralenes bearing chiral amines in organocatalytic, enantioselective Diels-Alder reactions to deliver enantioenriched cycloadducts. The enantioselective Diels-Alder reaction between 1-amino[3]dendralenes, the condensation product of skipped dienals and chiral amines, and various dienophiles followed by elimination of the amine generated trienal cycloadducts in good yield and high enantioselectivity. The reaction tolerates substitution on the skipped dienal as well as dienophiles possessing an aldehyde substituent at the α position. Extension of this methodology by performing Wittig and Diels-Alder reactions on the trienal cycloadducts enabled access to enantioenriched polycyclic products. By using a diene-dialdehyde as the starting precursor, it is anticipated that the Horeau principle would operate in the twofold condensation/Diels-Alder/elimination reaction sequence would furnish the cycloadduct in high enantioselectivity. A preliminary attempt successfully generated the desired cycloadduct as the major product.

Thesis (Ph. D.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on February 13, 2008) Vita. Includes bibliographical references.

The Diels-Alder is an important reaction that is one of the primary tools for synthesizing cyclic carbon structures, while simultaneously introducing up to four stereocenters in the resulting product. Not only is it a widely explored reaction in organic chemistry, but a vital tool in industry to construct novel compounds for pharmacological applications. Still, a remaining concern is the fact that upon the introduction of stereogenic carbons, the possibility of stereoselective control is greatly diminished. A common solution to the problem of undesirable stereoisomers is to employ chiral auxiliaries and ligands as means to increase the yield of a certain stereoisomer. However, incorporating these types of compounds in order to obtain an enantiomerically pure product increases the amount of synthetic steps to be regulated, implying that one or more purification steps are necessary to obtain the desired result. An accompanying thought leans toward the environmental aspect, as the principles of green chemistry are of great importance. This thesis presents the attempts to explore the possibility of engineering an enzyme that can catalyze an asymmetric Diels-Alder reaction through the use of molecular modeling. Based on previous work, the catalytically proficient enzyme ketosteroid isomerase had been deemed a probable candidate as a Diels-Alderase. To evaluate the enzyme thoroughly, a set of compounds was scored against the active binding site where the best hits against the wild type were saved and evaluated repeatedly after the introduction of rational mutations. Although no conclusive indication of an optimal design could be obtained at the end of this work, valuable insight was retrieved on plausible design strategies, which eventually could help lead to the first catalytically proficient Diels-Alderase.
Diels-Alder är en viktig reaktion då den är ett redskap för att syntetisera cykliska kolstrukturer, samtidigt som uppemot fyra stereocentra introduceras i den resulterande produkten. Reaktionen används inte enbart inom organisk kemi, utan är även ett viktigt redskap inom industriella sammanhang för att ta fram nya preparat som direkt kan tillämpas inom farmakologi. En återstående problematik är faktumet att introduktionen av nya stereogena kol bidrar till att drastiskt minska möjligheten att bibehålla en stereoselektiv kontroll. En vanlig lösning för att undvika oönskade stereoisomerer är att nyttja kirala hjälpmolekyler och ligander för att öka utbytet av en specifik stereoisomer. Dock innebär införandet av dessa hjälpmolekyler i strävan att erhålla en enantiomeriskt ren produkt ett ökat antal syntes-steg att hantera, vilket antyder att ett eller flera reningssteg är nödvändiga för att uppnå önskat resultat. Ur en miljösynpunkt är detta värt att ha i åtanke, då principerna för grön kemi är viktiga. Detta arbete utforskar möjligheterna att konstruera ett enzym som kan katalysera en asymmetrisk Diels-Alder-reaktion, med hjälp av molekylär modellering. Baserat på tidigare arbeten har enzymet ketosteroid isomeras valts ut som en potential kandidat till ett Diels-Alderase. För att noggrant evaluera enzymet så screenades ett set av substrat mot dess aktiva säte, där de bästa träffarna gentemot vildtypen sparades och återevaluerades allteftersom rationella mutationer kontinuerligt introducerades. Trots avsaknaden av klara indikationer på att en optimal design har kunnat tas fram vid slutet av detta arbete, så erhölls värdefull insikt på möjliga design-strategier, vilket skulle kunna bistå sökandet av det första katalytiskt effektiva Diels-Alderase.

The Diels-Alder reaction has proven to be an invaluable tool in the arsenal of the synthetic organic chemist for the relatively facile construction of cyclic cores. However, the utility of this reaction is not without its shortcomings. Lack of regio and stereoselectivity are two problems that are often encountered with asymmetric dienophiles. Although Lewis acids have been successfully employed to control the selectivity of Diels-Alder reactions involving activated dienophiles, there is little precedent in the literature for their use with dienes containing a tertiary alcohol functionality, due to elimination complications. To this end, a new method has been developed, and reported herein, for the control of both the regio and facial selectivity of the Diels-Alder reaction via a Lewis acid tether to the dienophile directed by the tertiary alcohol on the diene.* *Please refer to dissertation for diagrams.

Reactions de diels alder en solution aqueuse d'alkyls-5 3h-furannones-2 et de cetosulfoxydes ou d'ester-sulfoxydes : obtention de lactones bicycliques utilisables pour la synthese de lactones sesquiterpeniques

Dans le cadre d'un programme visant a realiser une synthese totale des brassinosteroides, la possibilite de generer l'unite centrale, polycyclique de ces hormones vegetales naturelles par cyclisation de type diels-alder intramoleculaire (imda) a ete etudiee. Dans ce but, un derive cyclohexanique prefigurant le cycle c et ortho-disubstitue par un motif methylbutadienyle, d'une part, et un motif butenone, d'autre part, a ete prepare en quelques etapes au depart de pyridine. Il est a souligne que le motif dienique a ete elabore par condensation d'un derive de l'alcool propargylique avec une lactone cyclohexanique, suivie d'une reduction par l'hydrure double de lithium et d'aluminium du diol propargylique ainsi forme. L'introduction sur ce cyclohexane de la partie dienophile, adjacente du substituant dienique a alors ete realisee de facon classique. La cyclisation imda du derive trienique ainsi obtenu se fait dans des conditions douces lorsqu'elle est catalysee par l'etherathe de trifluorure de bore. On obtient ainsi une dodecahydrophenanthrenone fortement ressemblante au systeme a-b-c des brassinosteroides. Ces resultats ouvrent la voie pour une synthese totale de ces steroides

Fluoropolymers have useful properties including high thermal stability, chemical resistance, low dielectric constants, and both hydrophobic and oleophobic character, as compared to non-fluorinated analogues. Meanwhile, Diels-Alder polyphenylenes (DAPPs) are known for thermal stability as well as their rigid structure and glassy physical characteristics, which have led to a variety of film and membrane applications. This dissertation merges these two fields by demonstrating a novel and general synthetic approach to highly fluorinated DAPPs. These polymers are expected to retain the physical characteristics of glassy, non-fluorinated DAPPs while also incorporating the desirable attributes of fluoropolymers. The polymer synthesis described herein is based on the well-established polycondensation of bis(cyclopentadienone) (CPD) monomers and dialkynes. Our first main scientific contribution is a general method for preparing CPDs containing both a fluoroaromatic linker and variable fluoroaromatic head-groups. Our CPD synthesis uses nucleophilic aromatic substitution reactions of cyclopentadienyl anions and perfluoroarenes, as well as a new catalytic method of converting cyclopentadiene methylene (CH₂) groups into the corresponding ketones (C=O) that is the primary dissertation subject of Brian S. Hickory in our laboratory. The overall synthesis is notable for its use of inexpensive starting materials, its efficiency, and its structural versatility. Our second main contribution is the synthesis of novel highly fluorinated Diels-Alder polyphenylenes (DAPPs). Fluorinated DAPPs varied in their molecular weight, in the identity of the lateral fluoroaryl substituent (pentafluoro-phenyl or tetrafluoro-4-pyridyl), and in the structure of the aromatic dialkyne monomer. These polymers are glassy materials with high glass transition temperatures and high thermal stability. Since the polyphenylene structure is intrinsically rigid, the polymers form brittle films even at molecular weights of over 30,000 (M_w). Unlike many fluoropolymers, the fluorinated DAPPs are freely soluble in common organic solvents such as tetrahydrofuran and chloroform. An unknown side reaction competes with the polymer propagation and reduces the highest obtainable molecular weights, which limit the ability to form films. However, a stoichiometric imbalance leads to highly fluorinated polyphenylene oligomers terminated with either alkyne or CPD end groups (M_n = 9000). Because preliminary experiments had shown that the desired Diels-Alder propagation reaction was slower than expected, we also undertook a detailed model study of the reaction conditions needed for Diels-Alder reactions of fluorinated CPDs and aromatic alkynes. These experiments showed that protic polar solvents (e.g., m-cresol) and conventional heating at ca. 150 °C optimize reaction rate while minimizing side-reactions that can contribute to lower molecular weight in corresponding polymerization reactions.
Ph. D.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2002.
Includes bibliographical references (leaves 152-169). Also available in electronic version. Access restricted to campus users.

Thesis (Ph.D.)--Kent State University, 2005.
Title from PDF t.p. (viewed July 28, 2005). Author's first name appears on the abstract page as: Nataliya. Advisor: Paul Sampson. Keywords: b-hydroxysulfones; [beta]-hydroxysulfones; epoxide opening; intramolecular Diels-Alder reaction; vinylsulfones. Includes bibliographical references (p.194- 202).

Nosso grupo de pesquisa já vem há alguns anos estudando métodos para a síntese do núcleo estrutural dos furanoeliangolidos, mais especificamente do goiazensolido. A estrutura básica deste composto corresponde a um sistema 7-oxabiciclo [6.2.1] undecano, e ele possui atividades biológicas como esquistossomicida, citotóxica e antiinflamatória. Em nosso trabalho utilizamos como etapa chave a reação de Diels-Alder, que é geralmente eficiente e rápida para formação de sistemas policíclicos. Na ultima etapa da proposta sintética, propomos a clivagem da ligação interna do sistema policíclico para obtenção do macrociclo desejado.
Our research group has been studying, for some years, new methods for the synthesis of the core structure of the furanoheliangolides, more specifically of goyazensolide. The basic structure of this compound is a 7-oxabicyclo [6.2.1] undecane system, and it shows biological activities such as schistosomicidal, cytotoxic and anti-inflammatory. In this work, we have used the Diels-Alder reaction as a key step, since it is usually fast and efficient for the synthesis of polycyclic structures. As a last step, we proposed that the cleavage of the internal bond of the rings should furnish the desired macrocycle.

The synthesis of methyl 3,6-dimethylpyridine-2-carboxylate (192) by the Diels-Alder addition of 3-butene-2-one dimethylhydrazone (190) and methyl-2-butynoate (16) is described. The structure of 192 was confirmed by the synthesis of methyl 2,6-dimethylpyridine-3-carboxylate (195), the opposite regioisomer.The synthesis of methyl 3,5-dimethylpyridine-2-carboxylate (199) from [4+2] cycloaddition of 2-methyl-2-propenal dimethylhydrazone (171) and 16 is described. In addition to producing a small amount of 199 directly, the reaction produced an intermediate of possible structure 198 which yielded 199 on catalytic dehydrogenation.On prolonged heating, 2-acetyl-5-methylpyridine (200) was produced by the reaction of diene 171 with methyl vinyl ketone (176).The preparation of 2-cyan-5-methylpyridine (201) from the tetrahydropyridine derivative 175 via catalytic dehydrogenation is described.The synthesis of 5,7-dioxo-3-methyl-6-phenyl-1,1.,5,6,7-pentahydropyrroloC3,1.-b] pyridine (202) through the Diels-Alder addition of diene 171 and N-phenylmaleimide (149) is described. Also reported is the preparation of 5,7-dioxo-3-methyl-6-phenyl-3,5,6-trihydropyrrolo [3,1+-b]pyridine (203) via dehydrogenation of 202.
Department of Chemistry

Doutoramento em Química
A futura e inevitável escassez dos recursos fósseis, juntamente com o aumento imprevisível dos seus preços, levou, nas últimas décadas, a um aumento impressionante de iniciativas dedicadas não só à procura de fontes alternativas de fornecedores de energia, mas também de produtos químicos e polímeros a partir de fontes renováveis, em particular da biomassa vegetal. Entre estes, os polímeros derivados de monómeros furânicos constituem uma classe única de materiais cujas estruturas podem, em princípio, simular virtualmente os seus homólogos actualmente derivados de recursos fósseis. O anel furânico é uma estrutura heterocíclica com um carácter diénico pronunciado, o que torna-o um dieno particularmente apropriado para a reacção de Diels-Alder (DA) com dienófilos como a maleimida. Um dos aspectos mais relevantes da reacção de DA é a sua reversibilidade em função da temperatura, a qual permite que os aductos sejam facilmente revertidos nos seus precursores por aumento da temperatura (reacção de retro-DA). No caso específico da combinação furano-maleimida, a formação do aducto predomina até cerca de 60ºC, enquanto a reacção inversa é dominante acima de 100ºC. A combinação desta característica da reacção de DA com a química de compostos furânicos pode abrir um novo caminho para a preparação de materiais macromoleculares funcionais com base em fontes renováveis e com aplicações promissoras como auto-reparação e reciclabilidade. O principal objectivo desta Tese, é a síntese e caracterização de novos materiais poliméricos termo-reversíveis, aplicando a reacção de DA a monómeros complementares com estruturas dos tipos furânico (o dieno, designado por A) e de maleimida (o dienófilo, designado por B). A primeira etapa neste trabalho envolveu a síntese, purificação e caracterização de novos monómeros furânicos e de maleimida do tipo AA, A3, BB, B3, AB, AB2, cada um com diferentes grupos separadores das funções reactivas. Posteriormente, estes monómeros foram polimerizados e despolimerizados por ciclos de DA/retro-DA utilizando diferentes combinações. A formação e dissociação de todos os aductos de DA foram seguidas por ambas espectroscopias de UV e RMN de 1H. O primeiro sistema de DA estudado foi uma combinação modelo entre reagentes mono-funcionais (-A+-B), nomeadamente o acetato furfurílico (FA) e a N-metilmaleimida (MM), ambos comercialmente disponíveis. O objectivo desta abordagem foi estudar a cinética e o equilíbrio da formação/dissociação dos aductos de DA e obter indicações sobre as condições mais adequadas a serem usadas na preparação dos correspondentes novos materiais macromoleculares. Além disso, pretendia-se verificar a presença ou ausência de reacções secundárias que poderiam intervir em ambas as vias directa e inversa das reacções, mesmo após vários ciclos. A espectroscopia de UV forneceu informação quantitativa sobre a cinética de formação do aducto através da diminuição progressiva da absorvência máxima a 293 nm correspondente ao grupo maleimida, a diferentes temperaturas (35, 50, 65 ºC) Reciprocamente, a correspondente reacção de retro-DA foi seguida a 90 ºC através do aumento do mesmo pico. A reversibilidade destes sistemas foi verificada com sucesso após uma sequência de ciclos de DA/retro-DA. Adicionalmente, verificou-se que os espectros originaram um ponto isosbéstico, provando que estes sistemas não envolvem quaisquer reacções secundárias. Uma vez que foi usado um excesso de FA, as reacções de DA modelo apresentaram um comportamento cinético de pseudo-primeira ordem, com a constante de velocidade k mais alta (2.1x10-5 dm3mol-1s-1) para T=65 ºC. A correspondente energia de activação foi de 39.0 kJ.mol-1. A reacção de retro-DA seguiu um comportamento de primeira ordem, com constante de velocidade de 1.6x10-6 s-1. A evolução deste sistema por RMN de 1H a 65ºC deu-nos informações mais detalhadas sobre a sua evolução estrutural, ou seja, à medida que a intensidade dos picos atribuídos à formação do aducto aumentaram progressivamente ao longo do tempo, os pertencentes aos reagentes iniciais diminuiram proporcionalmente. O “rendimento final”, calculado após 20 dias à temperatura ambiente, foi de aproximadamente 70%. A reacção de retro-DA foi depois seguida a 90ºC, observando-se tal como na espectroscopia de UV, o deslocamento da reacção no sentido da regeneração dos reagentes de partida. A viabilidade de múltiplos ciclos de DA/retro-DA estabelecidos pela espectroscopia de UV foi igualmente confirmada por RMN de 1H. O passo seguinte envolveu o estudo de um sistema de policondensação linear baseado no crescimento gradual por reacção de DA entre um monómero bisfurânico A-A e um do tipo bismaleimida B-B, seguindo a mesma abordagem que no sistema modelo. O poliaducto linear foi obtido a partir de soluções equimolares dos monómeros, por reacção de DA a 65ºC. O progresso desta polimerização foi seguido por espectroscopia de UV e RMN de 1H e, mais qualitativamente, pelo aumento da viscosidade do meio. A reacção seguiu um comportamento de segunda ordem, com uma constante de velocidade de 9.4x10-6 dm3mol-1s-1, e observou-se novamente um ponto isosbéstico nos dados de UV. Os espectros de RMN apresentaram o padrão esperado, nomeadamente o aumento progressivo dos sinais associados ao aducto e a correspondente diminuição dos grupos furano e maleimida livres. A despolimerização do poliaducto através da reacção de retro-DA foi seguida a 110ºC usando as mesmas técnicas. Os dados de UV mostraram o retorno progressivo da absorção dos grupos de maleimida, seguindo um comportamento cinético de primeira ordem, com constante de velocidade de 2.5x10-6 s-1, até à completa regeneração de ambos os monómeros. Os espectros de RMN providenciaram mais uma vez informação estrutural sobre o progresso da despolimerização, a qual foi acompanhada por uma diminuição progressiva da viscosidade. Adicionalmente, para seguir a retro- DA, adicionou-se um excesso de composto furânico monofuncional, nomeadamente o 2,5-dimetilfurano (DMFu), ao sistema de modo a bloquear as funções maleimida complementares, evitando assim a repolimerização após arrefecimento. Os productos isolados foram então o monómero bisfurânico AA, DMFu que não reagiu e o bisaducto não-polimerizável de BB com DMFu. Este resultado indicou claramente que o polímero foi de facto revertido nos seus monómeros durante a reacção de retro-DA. O terceiro sistema estudado foi outra polimerização linear, seguindo as mesmas condições experimentais que os anteriores, mas com uma estratégia diferente de modo a contornar o problema clássico de assegurar a estequiometria exacta dos monómeros. As estruturas dos monómeros utilizados incorporam ambos os grupos reactivos, i.e, moléculas do tipo A-B. A polimerização prematura destes monómeros intrinsecamente reactivos foi evitada com a protecção do grupo maleimida na forma de um aducto de DA com furano, até a incorporação do substituinte furânico na outra extremidade. Portanto, a policondensação destes monómeros foi iniciada após a desprotecção in situ deste composto mediante aquecimento, seguido de arrefecimento até à temperatura adequada para polimerizar. Os resultados obtidos por UV e RMN sugerem que de facto o uso de monómeros do tipo A-B oferece um melhor sistema linear. Em seguida, foram estudados sistemas de policondensação não-linear por reacção de DA, entre monómeros (um ou ambos) com funcionalidade superior a dois, nomeadamente sistemas do tipo A3+B-B ou A-A+B3, seguindo mais uma vez as mesmas condições experimentais. Uma vez que utilizam monómeros complementares contendo, em média, mais de duas funcionalidades, estes sistemas conduzem a materiais reticulados. Nestes estudos, foram usadas três razões molares de [maleimida]/[furano], nomeadamente 1.0, 0.75 e 0.5, de modo a estudar ambas as situações de não-gelificação e reticulação. Ambos sistemas apresentaram um comportamento regular e boa reciclabilidade quer para gerar situações que possam conduzir à formação de redes a diferentes graus de conversão, ou que possam parar antes da sua obtenção, conforme previsto pela equação de Flory-Stockmayer. Como esperado, a utilização de grupos complementares em quantidades estequiométricas produziu o espessamento mais rápido e a reticulação quase completa; à medida que a quantidade relativa de monómero trifuncional decresceu, as reacções pararam antes da reticulação, ou seja, originaram meios altamente viscosos contendo polímeros solúveis altamente ramificados. As reacções de retro-DA a 110 ºC conduziram à gradual dissolução das partículas de gel (quando presentes), tendo sido comprovado pelos espectros de UV e de RMN de 1H, evidenciado a regeneração dos monómeros. Tal como no sistema do tipo A-A+B-B, a reacção de retro-DA foi seguida adicionando um excesso de DMFu ao sistema reaccional. Como esperado, os produtos finais foram os monómeros furânicos, o DMFu em excesso e o trisaducto ou o bisaducto maleimida-DMFu, o que confirma a eficiência da despolimerização com regeneração dos monómeros iniciais. O último sistema de policondensação por reacção de DA envolveu um monómero assimetricamente substituído do tipo AB2, capaz de originar estruturas macromoleculares hiper-ramificadas que não reticulam. Este estudo preliminar deste sistema foi seguido nas mesmas condições experimentais que os anteriores, apresentando um comportamento com as características esperadas.
The inevitable future scarcity of fossil resources, together with their skyrocketting prices, has led in the last decades to a dramatic increase of initiatives devoted not only to the search for alternative sources of energy providers, but also to the search for chemical commodities and polymers from renewable resources, namely the vegetable biomass. Among these, polymers from furan monomers constitute a unique class of materials whose structures can in principle simulate virtually all their current fossil-derived counterparts. The furan heterocycle displays a pronounced dienic character, which makes it highly suitable as diene in the Diels-Alder (DA) reaction, with dienophiles like maleimide. One of the most relevant aspects of the DA reaction is its thermal reversibility, which allows the adducts to be readily reverted to their precursors as the temperature is raised (retro-DA reaction). In the specific case of the furan-maleimide combination, the adduct formation dominates up to ca. 60ºC, whereas the reverse reaction is overwhelming above ca. 100ºC. The coupling of this DA feature with the furan chemistry can open a new way to prepare functional macromolecular materials based on renewable resources with novel properties and promising applications like mendability and recyclability. In this Thesis, the main goal is the synthesis and characterisation of novel thermoreversible polymer materials by means of the DA reaction, using complementary monomers bearing furan (the diene, assigned as A) and maleimide (the dienophile, assigned as B) moieties. The first step in this study was the synthesis, purification and characterization of new furan and maleimide monomers, viz. AA, A3, BB, B3, AB, AB2, each bearing different spacing moieties separating the reactive functions. They were then polymerized and depolymerised using different modes of DA/retro-DA cycles. The formation and decoupling of all the DA adducts were followed by both UV and 1H NMR spectroscopy. The first DA system investigated was on a model combination of monofunctional reagents (-A+-B), namely the commercially available furfuryl acetate (FA) and N-methylmaleimide (MM). The purpose of this approach was to study the kinetics and the equilibrium of the formation/decoupling of the DA adducts and to provide clear indications about the most appropriate conditions to be applied in the preparation of the corresponding novel macromolecular materials. Additionally, it aimed at verifying that no detectable side reactions intervened in both forward and backward pathways, even after several cycles. The UV spectroscopy provided quantitative information on the kinetics of the DA adduct formation by monitoring the progressive decrease in the maximum absorbance at 293 nm related to the maleimide moiety, at different temperatures (35, 50, 65ºC). Conversely, the corresponding retro-DA reaction was followed at 90ºC through the increase of the same peak. The reversibility of these systems was successfully checked over a couple of cycles. Additionally, all set of spectra gave rise to an isosbestic point, which proved that these systems did not involve any side reactions.Since an excess of FA was used here, these model DA reactions displayed a pseudo-first order behaviour, with the highest rate constant k (2.1x10-5 M-1.s-1) for T= 65ºC. The corresponding activation energy was found to be 39.0 kJ.mol-1. The retro-DA reaction followed a first order behaviour, with a rate constant of 1.6x10-6s-1.The evolution of the same system followed by 1H NMR spectroscopy at 65ºC gave a detailed insight into its structural evolution, viz. as the intensity of the peaks assigned to the adduct formation progressively increased with time, those belonging to the initial reagents decreased correspondingly. The “final yield” taken after 20 days at room temperature, was approximately 70%. The retro- DA reaction was then carried out at 90ºC, and showed the same features as its UV counterparts, namely the displacement of the reaction towards the regeneration of the initial reagents. The feasibility of multiple DA/retro-DA cycles established by UV spectroscopy was confirmed here. The subsequent stage involved the study of a linear polycondensation system based on the stepwise DA growth between a difuran monomer A-A and a complementary bismaleimide B-B, following the same approach as the model system. The formation of the linear DA polyadduct was achieved by letting equimolar solutions evolve at 65 ºC. The progress of this polymerization was followed by UV and 1H NMR spectroscopy and, more qualitatively, by the increase in the viscosity of the medium. The reaction followed a second order behaviour, with a rate constant of 9.4x10-6 M-1.s-1, and again, a characteristic isosbestic point was observed in the UV data. The NMR spectra gave the expected pattern, namely the progressive increase in the signals associated with the adduct and the corresponding decrease of those attributed to the unreacted furan and maleimide cycles. The depolymerization of the polyadduct through the retro-DA reaction was followed at 110ºC by the same techniques. The UV time scans showed the progressive return of the absorption due to maleimide moieties following a first-order kinetic pattern, with a rate constant of 2.5x10-6 s-1, until both monomers were fully regenerated. The 1H NMR spectra provided once more structural information about the course of this depolymerization, which was accompanied by a progressive decrease in viscosity. Additionally to follow the retro-DA, an excess of a monofunctional furan compound, namely 2,5-dimethylfurfural (DMFu), was added to the system in order to block the complementary maleimide functions, thus avoiding the repolymerisation after cooling. The isolated products were then the difuran monomer AA, the unreacted DMFu and the non-polymerizable bisadduct of BB with DMFu. This result clearly indicated that the polymer had indeed reverted to its monomers during the retro-DA. The third system studied was another linear polymerization, following the same experimental conditions as the previous, but with a different strategy, in order to avoid the classical problem of ensuring the exact monomer stoichiometry. Here, the monomers incorporated both reactive moieties in their structures, i.e. A-B molecules. Here, the premature polymerization of these intrinsically reactive monomers was avoided by protecting the maleimide group in the form of a furan-DA adduct, until the incorporation of the furan moiety. Therefore, the polycondensation of these monomers was initiated after the in situ deprotection of this compound through heating, followed by the cooling to appropriate temperature for polymerization. The results obtained by both UV and NMR spectroscopy suggested that indeed the use of an A-B monomer provides a better linear system. Non-linear DA polycondensations between monomers (one or both) with functionality higher than two, viz. A-A+B3 or A3+B-B systems, were then studied, following once more the same experimental conditions. The use of complementary monomers bearing on average more than two functionalities lead to crosslinked materials. In these systems, three [maleimide]/[furan] molar ratios, viz. 1.0, 0.75 and 0.50, were applied in order to examine both nongelling and crosslinking situations. Both types of systems displayed a clean-cut behaviour and good recyclability to generate situations either leading to the formation of a network at different degrees of conversions, or ending before its attainment, as predicted by the Flory-Stockmayer equation. As expected, the use of complementary moiety stoichiometry produced the most rapid thickening and near-complete crosslinking, whereas as the relative amount of trifunctional monomer was decreased, the reactions stopped short of crosslinking, i.e. gave highly viscous media containing soluble highly branched polymers. The retro-DA reactions at 110ºC revealed the gradual dissolution of the gel particles (when present), while the UV and 1H NMR spectra gave, once more, the details of the depolymerization processes leading the systems steadily back to the monomers. As in A-A+B-B depolymerization, to follow the retro-DA, an excess of DMFu DA trap, was added to the systems under the same conditions. As expected, the final products were the furan monomers, the unreacted DMFu and the non-polymerizable trisadduct or the bisadduct of maleimide-DMFu, confirming the efficiency of the retro-DA depolymerisation in the regeneration of the starting monomers. The last DA polycondensation system involved an assymmetrically substituted monomer of the AB2 type, capable of generating hyperbranched macromolecular structures that do not crosslink. This preliminary study was followed by the experimental approach applied above and the behaviour of the system displayed the expected features.
POCTI; FCT; FSE - SFRH/BD/28271/2006

Heterocyclic compounds represent almost two-thirds of all the known organic compounds: they are widely distributed in nature and play a key role in a huge number of biologically important molecules including some of the most significant for human beings. A powerful tool for the synthesis of such compounds is the hetero Diels-Alder reaction (HDA), that involve a [4+2] cycloaddition reaction between heterodienes and suitable dienophiles. Among heterodienes to be used in such six-membered heterocyclic construction strategy, 3-trialkylsilyloxy-2-aza-1,3-dienes (Fig 1) has been demonstrated particularly attractive. In this thesis work, HDA reactions between 2-azadienes and carbonylic and/or olefinic dienophiles, are described. Moreover, substitution of conventional heating by the corresponding dielectric heating as been explored in the frame of Microwave-Assisted-Organic-Synthesis (MAOS) which constitutes an up-to-grade research field of great interest both from an academic and industrial point of view. Reaction of the azadiene 1 (Fig 1) will be described using as dienophiles carbonyl compounds as aldehyde and ketones. The six-membered adducts thus obtained (Scheme 1) have been elaborated to biologically active compounds like 1,3-aminols which constitutes the scaffold for a wide range of drugs (Prozac®, Duloxetine, Venlafaxine) with large applications in the treatment of severe diseases of nervous central system (NCS). Scheme 1 The reaction provides the formation of three new stereogenic centres (C-2; C-5; C-6). The diastereoselective outcome of these reactions has been deeply investigated by the use of various combination of achiral and chiral azadienes and aliphatic, aromatic or heteroaromatic aldehydes. The same approach, basically, has been used in the synthesis of piperidin-2-one scaffold substituting the carbonyl dienophile with an electron poor olefin. Scheme 2 As a matter of fact, this scaffold is present in a very large number of natural substances and, more interesting, is a required scaffold for an huge variety of biologically active compounds. Activated olefins bearing one or two sulfone groups, were choose as dienophiles both for the intrinsic characteristic flexibility of the “sulfone group” which may be easily removed or elaborated to more complex decorations of the heterocyclic ring, and for the electron poor property of this dienophiles which makes the resulting HDA reaction of the type “normal electron demand”. Synthesis of natural compounds like racemic (±)-Anabasine (alkaloid of Tobacco’s leaves) and (R)- and (S)-Conhydrine (alkaloid of Conium Maculatum’s seeds and leaves) and its congeners, are described (Fig 2).

Heterocyclic compounds represent almost two-thirds of all the known organic compounds: they are widely distributed in nature and play a key role in a huge number of biologically important molecules including some of the most significant for human beings. A powerful tool for the synthesis of such compounds is the hetero Diels-Alder reaction (HDA), that involve a [4+2] cycloaddition reaction between heterodienes and suitable dienophiles. Among heterodienes to be used in such six-membered heterocyclic construction strategy, 3-trialkylsilyloxy-2-aza-1,3-dienes (Fig 1) has been demonstrated particularly attractive. In this thesis work, HDA reactions between 2-azadienes and carbonylic and/or olefinic dienophiles, are described. Moreover, substitution of conventional heating by the corresponding dielectric heating as been explored in the frame of Microwave-Assisted-Organic-Synthesis (MAOS) which constitutes an up-to-grade research field of great interest both from an academic and industrial point of view. Reaction of the azadiene 1 (Fig 1) will be described using as dienophiles carbonyl compounds as aldehyde and ketones. The six-membered adducts thus obtained (Scheme 1) have been elaborated to biologically active compounds like 1,3-aminols which constitutes the scaffold for a wide range of drugs (Prozac®, Duloxetine, Venlafaxine) with large applications in the treatment of severe diseases of nervous central system (NCS). Scheme 1 The reaction provides the formation of three new stereogenic centres (C-2; C-5; C-6). The diastereoselective outcome of these reactions has been deeply investigated by the use of various combination of achiral and chiral azadienes and aliphatic, aromatic or heteroaromatic aldehydes. The same approach, basically, has been used in the synthesis of piperidin-2-one scaffold substituting the carbonyl dienophile with an electron poor olefin. Scheme 2 As a matter of fact, this scaffold is present in a very large number of natural substances and, more interesting, is a required scaffold for an huge variety of biologically active compounds. Activated olefins bearing one or two sulfone groups, were choose as dienophiles both for the intrinsic characteristic flexibility of the “sulfone group” which may be easily removed or elaborated to more complex decorations of the heterocyclic ring, and for the electron poor property of this dienophiles which makes the resulting HDA reaction of the type “normal electron demand”. Synthesis of natural compounds like racemic (±)-Anabasine (alkaloid of Tobacco’s leaves) and (R)- and (S)-Conhydrine (alkaloid of Conium Maculatum’s seeds and leaves) and its congeners, are described (Fig 2).

The structures of Diels-Alder cycloaddition of cyclopentadiene to 2,6-dimethyl-p-benzoquinone and methylcyclopentadiene to 2,6-dimethyl-p-benzoquinone were assigned by analysis of 1-D and 2-D proton and carbon-13 NMR spectra. The structures of the cycloadduct of methylcyclopentadiene to 2,6-dimethyl-p-benzoquinone and that of the corresponding intramolecular [2+2] photocyclization product were also obtained by single crystal X-ray structural analysis. As the second part of the study, a new polycyclic "cage" molecule, a substituted trishomocubane isomer, was synthesized. In this synthesis, reductive bond cleavage followed by Dieckmann condensation was employed. Wolff-Kishner reduction then was used to convert a β-keto ester "cage" molecule to the corresponding carboxylic acid. A compound that possesses twofold symmetry was isolated from reaction product mixture. The structure of this compound has been established by single crystal X-ray crystallography.

Cette thèse présente la réactivité de diènes polyfonctionnalisés incorporant un atome de bore (diènes borylés) en présence de composés de type nitroso. La première partie est une étude de la réactivité des diènes borylés en présence de dérivés arylnitroso. Le produit formé ainsi que l'efficacité de la réaction est dépendant de la substitution sur l'atome de bore ainsi que du solvant utilisé pour la réaction. Suivant les conditions employées, la formation de composés avec différentes structures; pyrroles, oxazines substitués par un atome de bore, ou nitrones a été obtenue. Une stratégie one-pot a ensuite été développée pour la synthèse d'oxazolines utilisant la formation des nitrones en tant qu'intermédiaire. Des investigations théoriques et pratiques concernant le mécanisme de la formation du pyrrole ont été réalisées. La séquence expliquant la synthèse de cette structure est basée sur la cascade suivante; cycloaddition régiosélective (Nitroso Diels-Alder)/réarrangement/élimination du borate. La formation des composés de type nitrone n'a pas été totalement explicité. Des recherches sont actuellement en cours dans le but d'obtenir plus de détails sur le mécanisme mis en jeu. La deuxième partie de l'étude est orientée sur la réactivité des diènes borylés en présence de dérivés carbonylnitroso. Au contraire des dérivés arylnitroso, la nature du composé carbonylnitroso joue un rôle majeur sur la réactivité. Si le même type de réactivité engendrant la formation de pyrroles et d'oxazines substitués par un atome de bore a été observé dans certains cas, l'utilisation d'un composé carbonylnitroso étant assez pauvre en électrons génère la formation du produit issu d'une réaction nitroso-ène. Ce produit, issu de la réaction nitrosoène, a été utilisé comme intermédiaire clé pour la synthèse de différents produits polycycliques. Une séquence one-pot, multicomposants, Diels-Alder/allylboration a été optimisée donnant différentes structures suivant le dienophile et l'aldehyde employé. Dans le but de diversifier la chimie du produit résultant de la réaction nitroso-ène, une autre séquence a été mise au point utilisant la synthèse des pyrroles. Une voie multi-étape a permis la synthèse de nouvelles structures bicycliques incorporant une oxazine et un pyrrole
This thesis presents the reactivity of dienylboronated compounds towards arylnitroso and carbonylnitroso derivatives and its use in the synthesis of heterocyclic and polycyclics compounds. The first study focused on the reactivity of dienylboronate compounds with arylnitroso derivatives resulting in pyrrole or furan products. The outcome and efficiency of the reaction is related to the boron on the dienyl moiety and the solvent used. Using the certain conditions, pyrroles, boronated MIDA ester oxazines or nitrones could be obtained. A one-pot strategy was then applied to synthesise oxazoline derivatives using nitrones as an intermediate. Theoretical, as well as experimental, work has supported that the formation of the pyrrole was obtained by a regioselective nitroso Diels-Alder reaction/rearrangement/borate elimination cascade process. Details on the nitrone formation have not been clarified, but further investigations are on-going. Secondly, attention was focused on the reactivity of dienylboronate compounds with carbonylnitroso derivatives. Contrary to the arylnitroso species, the nature of the carbonylnitroso had a dramatic impact on reactivity. On the one hand, similar reactivity towards the formation of pyrroles and boronated MIDA ester oxazines was observed, however, by employing a higher electron-deficient carbonylnitroso species, the product resulting from a nitroso-ene reaction was obtained. Ene-product was used as the key intermediate for the synthesis of different polycyclic compounds. A multicomponent, one-pot, Diels-Alder/allylboration procedure was optimised to yield various structures depending on the nature of the dienophile and the aldehyde. To diversify the chemistry of the ene-product another sequence was designed using the pyrrole synthesis. A multi-step pathway was optimised to afford novel fused bicylic oxazine-pyrrole products

The preparation of bicyclic dienes of the general structures (72), (82), (83) and (162) is described. These materials have been prepared via a novel annulation sequence involving (a) the alkylation of various carbonyl containing substances with the donor acceptor reagents (43), (44), (108)-(114) and (154), (b) the conversion of the alkylation products into the enol triflates, and (c) the palladium(O) catalyzed intramolecular coupling of the enol triflate-vinylstannane moieties via either a "one" or "two pot" process. The Diels-Alder reactions of the "parent" bicyclic diene (75), the cisoid cis diene (145) and the cisoid trans diene (146) are described. Four basic questions regarding the face selectivity, regioselectivity, stereoselectivity and comparative reactivity of the dienes in the formation of the Diels-Alder adducts of general structure (174) are addressed. The annulation sequences described above have been applied to the first total syntheses of the dolastane diterpenoids (±)-(14S)-dolasta-1(15),7,9-trien-14-ol (239) and (±)-amijitrienol (242). Thus, the substituted cycloheptanone (262), prepared in seven steps from the commercially available material (261), was converted via a series of transformations, including the newly developed annulation process, into the bicyclic diene (264). Introduction of the two appendages to (264) proceeded stereoselectively to provide the keto vinylstannane (265). Ring closure was effected with the desired stereochemistry to yield (±)-(239). A reduction, deprotection sequence afforded the ketone (249) from the diene ketal (263). A series of three steps involving an aldol condensation, Swern oxidation and stereoselective methylation provided the diketone (290). Chemo- and stereoselective reduction of (290) followed by protection of the alcohol moiety yielded the silyl ether (303). Cyclization, according to the methodology described herein, and deprotection of the silyl ether moiety produced (±)-(242). [Formula Omitted]
Science, Faculty of
Chemistry, Department of
Graduate