RESUMO
This review highlights important developments in the pursuit of chiral conformational order in dendritic structures. To be able to create and control chiral secondary structure necessitates a thorough understanding of how chiral subunits influence the macroscopic structure. Recent studies involve highly sophisticated manipulation of macroscopic chirality through a creative combination of a wide range of synthetic, computational and analytical techniques.
Assuntos
Dendrímeros/química , Substâncias Macromoleculares/química , Simulação por Computador , Ligação de Hidrogênio , Estrutura Molecular , EstereoisomerismoRESUMO
A chiral-base mediated reaction has been used to synthesise the polyethers illustrated and their enantiomers; CD spectroscopy revealed that the homochiral isomers A may be viewed as a C3-symmetric arrangement of fixed paddles while the heterochiral isomers B form a C3-symmetric arrangement of rotating paddles.
Assuntos
Éteres/síntese química , Polímeros/síntese química , Éteres/química , Estrutura Molecular , Polímeros/química , EstereoisomerismoRESUMO
The synthesis, structure, and physical properties of macrocycles have fascinated chemists for many years. Their inherent properties make them useful in areas as diverse as ion transport across membranes, development of new antibiotics, and catalysis. In this Review, the authors examine the chemistry of macrocycles containing non-peptidic amino acid derived molecules; the analysis is discussed in terms of function, rather than structure or synthesis. It is revealed that the diverse and imaginative structures created by synthetic chemists are not being fully exploited in application-driven endeavors.
Assuntos
Aminoácidos/química , Compostos Macrocíclicos/síntese química , Compostos Macrocíclicos/química , Conformação Molecular , EstereoisomerismoRESUMO
Throughout history symmetry and chirality have inspired artists and scientists alike. Given that rotational axes are the only elements of symmetry compatible with chirality, it is not surprising that C2- and C3-symmetrical molecules have attracted considerable attention. In recent years, the aesthetic appeal of C2-symmetrical molecules has been translated into many widely-used applications some of which are of commercial importance by its exploitation in the area of asymmetric catalysis. In contrast, exploitation of the arguably greater aesthetic appeal of C3-symmetric molecules is still in its infancy. This review, which surveys the applications of chiral C3-symmetrical molecules in the areas of asymmetric catalysis, molecular recognition and nanoarchitecture, has been designed with a view to identifying some of the most promising areas of application of these very beautiful molecules.
RESUMO
Five membered carbocycles are important building blocks for many biologically active molecules. Moreover, substituted cyclopentenones (e.g. cyclopentenone prostaglandins) exhibit characteristic biological activity. The efficiency and atom economy of the Pauson-Khand reaction render this process potentially one of the most attractive methods for the synthesis of such compounds. Although it was discovered in its intermolecular form, the scope of the intermolecular Pauson-Khand reaction has always been limited by the poor reactivity and selectivity of the alkene component. The past decade, especially the last three years, has seen concerted efforts to broaden the scope of this reaction. In this overview, we provide a comprehensive and critical coverage of the intermolecular Pauson-Khand reaction based on the reactivity characteristics of different classes of alkenes and a rationalization of successes and misfortunes in this area.
Assuntos
Alcenos/química , Alcinos/química , Cobalto/química , Ciclopentanos/química , Ciclopentanos/síntese química , Ciclopentanos/farmacologia , Substâncias Macromoleculares/química , Estrutura Molecular , Prostaglandinas/química , Prostaglandinas/farmacologiaRESUMO
As a consequence of growing environmental awareness, it is now inappropriate to design a synthetic metal-mediated transformation that involves a noncatalytic use of toxic and expensive transition-metal species. One of the earliest examples of such a metal-mediated transformation is the Pauson-Khand reaction, a [2+2+1] cyclocarbonylation that generates a cyclopentenone. Despite the early descriptions by Pauson and co-workers of catalytic versions of the reaction with octacarbonyldicobalt(0), applications of the Pauson-Khand reaction have to date almost exclusively used approaches that involve stoichiometric quantities of cobalt-carbonyl complexes. In the last decade, and, most markedly, in the last two to three years, however, there have been many exciting and novel developments in the catalytic Pauson-Khand reaction. Furthermore, asymmetric catalysis of the Pauson-Khand reaction has been shown to be a viable process. In view of the impressive developments in Pauson-Khand catalysis in the last two to three years, we present a comprehensive and critical coverage of the catalytic Pauson-Khand reaction that is designed to facilitate its application and to point to exciting future developments.
RESUMO
The syntheses and structures of three cyclophanes containing two (Z)-dehydrophenylalanine residues are reported; the length of the tethers between the two amino acid residues is easily altered and changing this parameter has a significant effect on the solid state structures of the cyclophanes.
Assuntos
Éteres Cíclicos/química , Éteres Cíclicos/síntese química , Fenilalanina/análogos & derivados , Fenilalanina/química , Fenilalanina/síntese química , Cristalografia por Raios X , Conformação Molecular , Estrutura MolecularRESUMO
The conformationally constrained analogues of phenylalanine, tetrahydroisoquinoline-3-carboxylic acid (Tic), Sic, Hic and Nic, and the new amino acid Xic have been incorporated into a potent and highly selective cholecystokinin-2 (CCK(2)) receptor antagonist (2) in place of the phenylalanine residue, producing compounds 15a-e. High selectivities for CCK(2) over CCK(1) were observed for compounds 15a-e. The in vitro profile of the analogue containing the Nic residue (15d) was identical to that of compound 2, whereas the alternative conformational constraints resulted in a significant loss of affinity. The apparent advantage of Nic in the context of these CCK(2) ligands was subsequently demonstrated to be statistically significant.
Assuntos
Compostos Heterocíclicos com 2 Anéis/síntese química , Fenilalanina/química , Receptores da Colecistocinina/antagonistas & inibidores , Tetra-Hidroisoquinolinas , Animais , Córtex Cerebral/efeitos dos fármacos , Córtex Cerebral/metabolismo , Mucosa Gástrica/metabolismo , Cobaias , Compostos Heterocíclicos com 2 Anéis/química , Compostos Heterocíclicos com 2 Anéis/farmacologia , Técnicas In Vitro , Isoquinolinas/química , Ligantes , Camundongos , Pâncreas/efeitos dos fármacos , Pâncreas/metabolismo , Ratos , Receptor de Colecistocinina B , Estômago/efeitos dos fármacosRESUMO
Nowadays it is rare to find an issue of a major chemistry journal without at least one article on solid-phase synthesis. This is hardly surprising: the technique promises an end to arduous work-up procedures and the ability to facilitate the creation of vast libraries of compounds using combinatorial techniques. No longer is the technique only of interest to those involved in peptide synthesis: an enormous variety of product classes have now been prepared on and isolated from the solid phase. It is the "linker" which is the focus of this article. The linker's ultimate function is to release a product from the support into solution: it does this, without exception, with a chemical change to the product at the former linkage site. Some linkers, apparently, are "traceless". But what, in fact, is "tracelessness"? Twenty years ago, in a climate where cleavage of a linker resulted in formation of a polar carboxylic acid as the vestige of the support, the concept was attractive. Today the chemist is faced with a myriad of novel linkers which have the ability to release products bearing most major functionalities at the former linkage site and we will argue here that the term "traceless", although currently in widespread use, is meaningless. Instead, we propose a new categorization of linkers based on the functionality they release upon cleavage, and suggest a nomenclature to underpin this categorization. We anticipate that the article will also serve to highlight areas of linker technology in need of further research.
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Four (binap)(enyne)tetracarbonyldicobalt(0) complexes have been synthesised and their reactivity monitored by variable temperature (31)P NMR spectroscopy. Formation of (binap)dicarbonylhydridocobalt(-1) 12 occurred at temperatures between 35 and 55 degrees C, depending on the nature of the alkene and alkyne components of the enyne. The structure of 12 was determined by X-ray crystallography, and its presence under Pauson-Khand reaction conditions was verified by NMR spectroscopy.
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A novel aldehyde dual-linker system has been developed for the solid phase synthesis of sterically hindered amides. The linker [5-(4-formyl-3-hydroxyphenoxy)pentanoic acid] exploits an intramolecular oxygen-nitrogen acyl transfer mechanism to prepare compounds that are unattainable with current commercially available linkers. A dual linker system, exploiting the hyper-acid labile Sieber amide linker as part of the construct, enabled the initial reductive alkylation reactions of hindered amines and their subsequent acylation with a range of carboxylic acids with varying stereoelectronic properties to be monitored. Simple acylation conditions (HBTU/HOBt/NMM) sufficed to provide near quantitative reaction of test acids with support-bound hindered amines, reaction conditions which failed when commercial linkers were used.
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Chiral base chemistry has been used to create three chiral centres in one pot on a C3-symmetric substrate. The potential of this new approach to C3-symmetric molecules is exemplified by the creation of an enantiopure C3v-symmetric triol, triphosphane and tripyridine. A ruthenium complex of the last compound has been studied by X-ray crystallography.
RESUMO
Understanding the interaction of chiral ligands, alkynes, and alkenes with cobaltcarbonyl sources is critical to learning more about the mechanism of the catalytic, asymmetric Pauson-Khand reaction. We have successfully characterized complexes of the type [Co2(alkyne)(binap)(CO)4] (BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine)) and shown that diastereomer interconversion occurs under Pauson-Khand reaction conditions when alkyne=HC[triple bond]CCO2Me. Attempts to isolate [Co2(alkyne)(binap)(CO)x] complexes with coordinated alkenes led to the formation of cobaltacyclopentadiene species.
Assuntos
Alcinos/química , Naftalenos/química , Compostos Organometálicos/química , Acroleína/análogos & derivados , Acroleína/química , Catálise , Cobalto/química , Isomerismo , Espectroscopia de Ressonância Magnética , Estrutura Molecular , Ródio/químicaRESUMO
The novel complex (eta 6-benzylamine)tricarbonylchromium(0) 11 was prepared in up to 66% yield by direct complexation with Cr(CO)6 in refluxing 1,4-dioxane. Imine derivatives of this complex were readily deprotonated at the benzylic position by diamide 5, and the resultant anions reacted regioselectively with electrophiles (Me3SiCl or MeI) to give fairly good yields of products substituted at the benzylic carbon. Products of up to 87% e.e. were obtained in these reactions, with the highest enantioselectivity being derived from the tert-butyl-substituted imine complex 25.
RESUMO
Six X-ray crystal structures are reported, all containing substituted triphenylmethanol derivative 4 either alone or as its mono or bis(chromium tricarbonyl) complexes. All four chromium complexes studied crystallize with two independent molecules in the crystallographic asymmetric unit. It is demonstrated that from the X-ray crystal structure of the acentric racemic (+/-)-(1pR,1' 'R)(1pS,1' 'S)-[Cr(CO)(3)(eta(6)-t-BuC(6)H(3)(CMeOMe)CPh(2)OH)], (+/-)-3, it is possible to deduce the 4-fold helical structure of the chiral (-)-(1pR,1' 'R) isomer, (-)-3. The bimetallic derivatives demonstrate the ability to control intermolecular interactions by the positioning of relative stereochemistry.
RESUMO
Carbonylcobalt(0) species have been used as linkers between alkynes and a polymer support for the first time. The alkynes may be loaded indirectly onto a phosphine functionalised polymer via their hexacarbonyldicobalt(0) complex, or directly onto a cobalt coated polymer. The alkynes have been released either as alkynes, thus providing a traceless method of immobilising alkynes, or by reaction with an alkene to generate a cyclopentenone via the Pauson-Khand reaction. The cobalt coated polymers produced during this study were shown to catalyse the Pauson-Khand reaction.
RESUMO
Metal isocyanides have been used and studied by organometallic chemists for many years and, as a result, they have a rich and interesting chemistry. The nature of metal-free isocyanides and the methods of making isocyanide complexes, however, has resulted in the vast majority of studies to date being performed with structurally simple isocyanides. We report here a new approach to the synthesis of isocyanide ligands that involves the reaction of a metal carbonyl ligand with the anion of a phosphoramidate. As phosphoramidates can be synthesised in one step from amines, our method means that the structural diversity of readily available amines, particularly chiral amines, can now be incorporated into isocyanide ligands.