Academic literature on the topic 'Isomeros meso'

The unsymmetrical N,O-dibridging NO2– group in the threo (‘rac’) or erythro (‘meso’) isomers of [(en)2Co(μ-NH2,μ-NO2)Co(en)2]4+ is cleaved in hot triflic acid to yield a complex that is predominantly meso. This reacts in water to yield the new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ion. The reactions of the threo- and erythro-μ-NO2– isomers in basic (0.1 M) aqueous solution are each retentive, giving the rac- and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions as products, respectively. The optically resolved erythro complex gives the achiral meso isomer, which completes the threo/rac and erythro/meso chemical correlations between the two series of dinuclear complexes; the meso synthesis completes the pair – the meso- and rac-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions. In HCl, stable μ-Cl, μ-NH2 dinuclear ions are formed from these μ-NH2, μ-OH complexes, with retention. The peroxo or superoxo ions rac-[Co(en)2(μ-NH2,μ-O2)Co(en)2]n+ react with SnII to produce symmetrically bridged intermediates that decay to the common and new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomer on reaction in Me2SO. Both the rac and meso isomers of the μ-hydroxo-μ-peroxo complexes are also reduced by SnII to give intermediates that in Me2SO decay to the respective diols with retention. The new rac-[Co(en)2(μ-OH)2Co(en)2]4+ and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomers complete the characterization of rac/meso pairs in both systems. The corresponding μ-sulfate pairs are also reported.

As models for a poly(methyl methacrylate ) chain which contains a head-to-head unit, the (�)- and meso -isomers of tetramethyl 2,4,5,7-tetramethyloctane-2,4,5,7-tetracarboxylate have been synthesized by oxidative dimerization of a trimethylsilyl ketene acetal. The isomers were characterized by 1H and 13C n.m.r. spectroscopy, and the molecular structure of the meso -isomer was determined by X-ray crystallography. Molecular mechanics (MM2) calculations showed that three conformations of each isomer, about its central bond, had comparable steric energies. Calculated energy barriers to interconversion of the conformers were low.

Addition of dien in slight excess to CrCl3.6H2O in Me2SO (which has been dehydrated at 190�C, then cooled to 170�) produces an isomeric mixture of orange-yellow [Cr( dien )2] Cl3.Me2SO of � approximate composition mer 70%; s- fac 30%; u- fac <2%. The isomers are partially separated by dissolving the mixture in 3M HCl and adding ZnCl2; pure lemon-yellow s- fac -[Cr( dien )2] (ZnCl5) then deposits. Addition of NaClO4 to the mother liquor results in the crystallizationof an orange-yellow mer/u-fac -[Cr( dien )2] (ClO4)3 (>98% mer ) mixture. All three isomers can be obtained as (Hg2Cl7)3- salts, suitable for single-crystal X-ray analysis, and the cation geometries have been established from these X-ray data. The mer and s- fac isomers are distinguished by their different labilities in acidic solution ( mer is relatively inert, s- fac decomposes rapidly), their different infrared and visible absorption spectra, and their different chromatographic behaviour on SP- Sephadex C-25. However, this latter technique failed to detect the u- fac form and was complicated by the facile decomposition of the s- fac isomer. The mer and s- fac forms isomerize rapidly in Me2SO and the equilibrium is largely towards the mer. [Cr( dpt )2]Cl3 [ dpt = di(3- aminopropyl )amine] could be obtained only by direct reaction between anhydrous CrCl3 and dpt , and was subsequently precipitated as the iodide and perchlorate salts. These consisted entirely of the mer isomer, from their isomorphism with the corresponding mer -[Cr( dpt )2 ]3+ salts. The six-membered rings in -[Cr( dien )2 ]3+ are considerably more resistant to hydrolytic decomposition than the rings in mer -[ cr ( dien )2]3+ and mer -[Co( dpt )2]3+.

The synthesis of crown ethers derived from meso-tartaric acid was investigated. The sodium salt of the bis(dimethylamide) of meso-tartaric acid reacted with diethylene glycol ditosylate to give a mixture of 18-crown-6 tetraamide and 27-crown-9 hexaamide crown ethers. The 2R,3S,11S,12R 18-crown-6 isomer crystallized in triclinic space group [Formula: see text] (a = 7.557(2), b = 8.866(2), c = 10.4133(13) Å, α = 94.13(2), β = 95.86(2), γ = 99.26(2)°, R = 0.040 for 2090 observed of 3129 unique reflections). The structures of the remaining products were then assigned from the NMR spectra. The solution conformations of the amide crown ethers were examined by NMR, and provide a rationale for the product distribution obtained. One of the 18-crown-6 isomers and a mixture of the two 27-crown-9 isomers were hydrolyzed to the respective crown ether carboxylic acids, and the stability constants for complexation of cations were determined by potentiometric titration. The meso tetra- and hexacarboxylates are remarkably nonselective and inefficient cation complexing agents, compared to related crown ethers from R,R-(+)-tartaric acid, due to the unfavorable conformational control exerted by the tartaro units. Key words: crown ether synthesis, complexation, crown ether conformation, meso-tartaric acid, crystal structure.

The metallate, PPN+Cr(CO)4NO−, was used for the 1,3-debrominative reductive cyclization of the (±) and meso isomers of dimethyl 2,4-dibromoglutarate and dimethyl 2,4-dibromo-2,4-dimethylglutarate. In both (±) isomers, the reaction is stereospecific in giving the trans cyclopropane product. In the meso case, the reaction is unselective in the first case, but distinctly favors the cis cyclopropane isomer in the second set of compounds. This (±) and meso pair thus represent the first example of near stereospecificity in the debrominations of both 1,3-dibromo diastereomers. Using an enantiomerically enriched dimethyl 2,4-dibromoglutarate, and determining the absolute stereochemistry of the cyclopropane product, it was found that a strict double inversion [Formula: see text] mechanism is involved in the (±) debromination reaction and presumably also in the near-stereospecific meso case reported above.

One of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.

One of the important features of polymethine (cyanine) dyes is isomerization around one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomerization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocyanine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be explained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric effect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes complexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cyanine dyes can be used in various systems of information storage and deserves further investigation using modern research methods.

One of the important features of polymethine (cyanine) dyes is isomerization about one of C–C bonds of the polymethine chain. In this review, spectral properties of the isomers, photoisomer-ization and thermal back isomerization of carbocyanine dyes, mostly meso-substituted carbocy-anine dyes, are considered. meso-Alkyl-substituted thiacarbocyanine dyes are present in polar solvents mainly as cis isomers and, hence, exhibit no photoisomerization, whereas in nonpolar solvents, in which the dyes are in the trans form, photoisomerization takes place. In contrast, the meso-substituted dyes 3,3′-dimethyl-9-phenylthiacarbocyanine and 3,3′-diethyl-9-(2-hydroxy-4-methoxyphenyl)thiacarbocyanine occur as trans isomers and exhibit photoisomerization in both polar and nonpolar solvents. The behavior of these dyes may be ex-plained by the fact that the phenyl ring of the substituent in their molecules can be twisted at some angle, removing the substituent from the plane of the molecule and reducing its steric ef-fect on the conformation of the trans isomer. In some cases, photoisomerization of cis isomers of meso-substituted carbocyanine dyes is also observed (for some meso-alkyl-substituted dyes com-plexed with DNA and chondroitin-4-sulfate; for 3,3′-diethyl-9-methoxythiacarbocyanine in moderate polarity solvents). The cycle photoisomerization–thermal back isomerization of cya-nine dyes can be used in various systems of information storage and deserves further investiga-tion using modern research methods.

Conversion of four isomers of meso-oxyprotohaem IX into the corresponding biliverdin IX was attempted with a reconstituted haem oxygenase system in the presence of NADPH-cytochrome c reductase and NADPH. Only the alpha-isomer of meso-oxyprotohaem IX was converted effectively into biliverdin IX alpha, which was further reduced to bilirubin IX alpha by biliverdin reductase. Only trace amounts of biliverdins IX beta, IX gamma and IX delta were respectively formed from the incubation mixture of the corresponding oxyprotohaemin IX isomers with the complete haem oxygenase system under the same conditions. In a kinetic study, the Km for alpha-meso-oxyprotohaem IX was 3.6 microM, which was 2-fold higher than that for protohaem IX. The maximum velocity (Vmax.) of the conversion of alpha-meso-oxyprotohaem IX into biliverdin IX alpha was twice as fast as that of protohaem IX. These results demonstrate that alpha-meso-oxyprotohaem IX is an intermediate of haem degradation and it was converted stereospecifically into biliverdin IX alpha via verdohaem IX alpha.

We report here a chemical study on a Re analogue of one of the few cationic Tc-99m tracers previously investigated as an agent for effective renal plasma flow (ERPF) measurement. Cationic Tc-99m tracers have the potential for overcoming problems associated with common anionic Tc-99m tracers in patients who have developed a uremic state. The Tc-99m-DACH tracer, prepared from 1,2-diaminocyclohexane (1,2-DACH), is the only cationic renal agent tested in humans and has seven possible isomers. The complex isolated from the reaction of the racemic mixture, (±)-trans-1,2-DACH, and ReIO2(PPh3)2 after conversion to the BPh4- salt was found by X-ray crystallography to be the meso isomer, trans-[ReO2 (trans-R,R-1,2-DACH)(trans-S,S-l,2-DACH)][BPh4]·MeOH·2H2O (1). The structural parameters for 1 are normal. The complex is highly symmetrical, suggesting that the analogous meso Tc-99m-DACH agent is also symmetrical. Studies of other Tc-99m-DACH agents that were made from cis-1,2-DACH or individual trans-1,2-DACH enantiomers show that the biodistribution is not very dependent on the starting 1,2-DACH ligand stereochemistry; these agents must be less symmetrical than the meso Tc-99m-DACH agent analogue of 1. Thus, the overall charge and lipophilicity (similar for all Tc-99m-DACH isomers) exert a greater influence on biodistribution than the specific structural features of the different Tc-99m-DACH isomers.

Orientadores: Ronei Jesus Poppi, Adriana Vitorino Rossi
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica
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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES
In order to contribute to the teaching and learning of chemistry, this research is presented and it proposes an alternative methodology to the study of scientific concepts in optical isomerism. The choice of this subject is due to its great importance in pharmacology due to the fact that many prescribed drugs employ enantiomers of active compounds, bearing in mind the tragedy due to talidomida around the sixties, hence we judge the learning of this concepts, a matter of prime importance for the chemistry student. By knowing that the contentes is considered too abstract and theoretical, the main difficulties presented in the teaching and learning of optical isomerisms are identified in order to propose a study of the following concepts: light, dichroism, polarizers, birrefringence, light polarization from physics and the insertion of these concepts to derive relevant chemical concepts like optical activity, quirality, molecular asymmetry. These issues were worked on by students through experimental activities under a constructivist learning environment in which students acquired their own knowledge through interaction with lab instruments and group discussion. The didactic materials used in this work were all constructed by the author. The series of activities were done in a classroom of first year pharmacy students and they were made towards the application of concepts of isomery to selected issues on modern pharmacological research, expecting to motivate the students for their professional formation. The activities were evaluated through textual discusive analysis. The results allowed us to conclude that the activities involved as well as the didactic materials were suitable in satisfying the expected aims of this research and, therefore, we concluded that this proposal allowed a better teaching and learning of concepts of optical isomerism for introductory university general chemistry courses.
A fim de contribuir para o ensino e aprendizagem de química, desenvolveu-se esta pesquisa que propõe uma metodologia diferenciada para o estudo dos conceitos de isomeria óptica. A escolha por este assunto deve-se a sua grande importância na indústria farmacêutica, devido a muitos medicamentos utilizarem como princípios ativos alguns enantiômeros, e tendo em vista a tragédia acontecida com a talidomida por volta de 1960, julgamos o ensino destes conceitos de fundamental importância. Sabendo que o conteúdo apresentado é considerado muito abstrato e teórico, inicialmente na pesquisa foram identificadas as dificuldades vinculadas ao ensino e aprendizagem de conceitos de isomeria óptica, e baseando-se nisso, foram estudados os conceitos necessários para a aprendizagem destes. Foram abordados os seguintes conceitos: luz, dicroísmo/polaroides, birrefringência e polarização, vinculados a física, e posteriormente a introdução dos conceitos de atividade óptica, quiralidade/assimetria molecular referentes a química. Estes conceitos foram trabalhados por meio de atividades experimentais, numa perspectiva construtivista de ensino, na qual os estudantes construíram seu conhecimento a partir da interação com os materiais didáticos e outros estudantes. Os materiais didáticos utilizados na pesquisa foram planejados e desenvolvidos pela pesquisadora. A aplicação das atividades se deu em uma turma de ingressantes do curso superior de farmácia, e o ensino foi vinculado a sua aplicação nos medicamentos de maneira a tornar o ensino relevante para os estudantes, e assim motivá-los. Tais atividades foram avaliadas por meio de questionários abertos e fechados, os quais foram analisados por meio da Análise Textual Discursiva, os resultados obtidos nos permitiram concluir que as atividades desenvolvidas bem como os materiais didáticos, cumpriram com os objetivos esperados da pesquisa e favoreceram o ensino e aprendizagem dos conceitos de isomeria óptica.

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This work tried to investigate how the didactic strategy of problem-situation solving can assist the learning of the concept of isomerism in college, more specifically in Chemistry Undergraduate level at Universidade Federal Rural de Pernambuco (UFRPE). To this end, two problem-situations, whose thematic were closely related to the concept of isomerism, were elaborated based on Meirieu’s propositions (1998). The first one discussed the historical aspects of the discovery of the chemical phenomena, and the second talked about a medicinal application of isomeric compounds. During the intervention, a text produced on the aforementioned work and commercial molecular models were used as didactic materials aiming to assist students on problem solving problem-situations. Data collection was done using, among other instruments, printed questionnaire and semi-structured interview, recorded in audio. The resolution of problem-situations by the conditions suggested on students participating in the search - material indicated, working in groups and interacting with the teacher-researcher - contribute significantly to the understanding of the concept of isomerism and its features, such as historical context (discussed in the first problemsituation) and the application of these compounds (the second problem-situation), as shown by the data obtained after adequately analyzed.
Este estudo procurou investigar como a estratégia didática de resolução de situaçõesproblema pode auxiliar no aprendizado do conceito de isomeria no ensino superior, mais precisamente no Curso de Licenciatura em Química da Universidade Federal Rural de Pernambuco (UFRPE). Para isso, a partir das indicações propostas por Merieu (1998), foram elaboradas duas situações-problema, que tiveram suas temáticas ligadas ao conceito de isomeria – uma delas abordou os aspectos históricos da descoberta do fenômeno químico e a outra uma determinada aplicação medicinal de compostos isoméricos. Durante a intervenção, um texto (produzido neste trabalho) e modelos moleculares comerciais foram utilizados como materiais didáticos visando auxiliar os estudantes na resolução das situações-problema. A obtenção dos dados foi feita usando, entre outros instrumentos, questionário impresso e entrevista semi-estruturada, esta gravada em áudio. A resolução das situações-problema, mediante as condições sugeridas pelo método do trabalho aos estudantes participantes da pesquisa – material didático indicado, trabalho em grupo e interação com o professor-pesquisador, contribuiu de forma significante para o entendimento do conceito de isomeria e suas particularidades, tais como o contexto histórico (abordado na primeira situação-problema) e as aplicações destes compostos (segunda situação-problema), como mostram os dados obtidos depois de analisados adequadamente.

Part I. The unsolvated porphyrin compound (Fe$\sp{\rm III}$(TPP)(OSO$\sb2$CF$\sb3$)) has been structurally characterized by single-crystal X-ray diffraction in a monoclinic phase at 293 K and in a triclinic phase at 293, 188 and 103 K. While only one type of molecular site is found in the monoclinic phase, the temperature-dependent structural parameters and magnetic susceptibility data (5.82 $\mu\sb{\rm B}$ at 293 K; 4.86 $\mu\sb{\rm B}$ at 20 K) together indicate the existence of two crystallographically and magnetically distinct spin-admixed crystal lattice sites in the triclinic phase. One site (molecule 1) is unique in that its structure is temperature dependent, whereas the second site (molecule 2) has a structure which is essentially independent of temperature. This distinct site assignment has been further investigated by Mossbauer and EPR spectroscopies which suggest different spin ground states for molecules 1 and 2. This triclinic phase is also the first report of molecule pairs of the same porphyrin complex interacting in very different ways in the same crystal lattice in that molecule 1 pairs form $\pi$-$\pi$ dimers, whereas molecule 2 pairs do not. Part II. A new binucleating picket-fence porphyrin ligand, 5,10,15,20-Tetrakis (o-4-methylimidazole-5-ethylcarboxyl-2-oxy)phenyl) porphyrin, N$\sb4$-PH$\sb2$, has been synthesized and characterized to be used eventually as a vehicle to study the structure, function and reactivity patterns of the active site of cytochrome c oxidase. The enzyme itself contains an (Fe(porphyrin)$\cdots$Cu) binuclear active site of unknown structure. The properties which set N$\sb4$-PH$\sb2$ apart from other binucleating picket-fence porphyrin ligands previously prepared as active site ligating molecules are the four authentic imidazole nitrogen donor atoms in the potential binding site for the Cu center and a separation between potential metal binding sites of only 3.5 to 4.0 A, made possible by the ligand's exceptionally "short" picket-fence arms derived from imidazole-ether linkages.

O estudo de estereoquimica tem grande dificuldade entre os alunos, principalmente pelo fato de ser abstrato, decorativo e fora de contexto, quando o aluno participa da aula confeccionando o seu proprio material didatico com certeza havera um melhor entendimento, com base nessa premissa, desenvolveu-se um modelo molecular confeccionado em papel e recortado com tesoura, para o entendimento do conteudo de estereoquimica, tais como quiralidade, plano de simetria, enantiomero, diastereoisomero, meso-isomero, entre outros.,