RESUMO
The effect of an extra EWG in the reactivity and regioselectivity in Diels-Alder reactions of ß-cyanolevoglucosenone and 4 different dienes was studied by a joint computational and experimental study. Conceptual DFT analysis successfully predicted an important enhancement in the reactivity, and correctly anticipated the regioselectivity in the reactions with isoprene. However, this static treatment failed when dealing the regiochemical preference of the reactions involving a substituted anthracene as diene. MPW1K/6-31G* calculations correctly reproduced the experimental observations. Based on the collected data, we found that when dealing with dienes and dienophiles with no clear electronically activated position, the ease of pyramidalization of the interacting atoms dictates the regioselectivity of the DA reaction.
Assuntos
Alcenos/química , Butanos/química , Carboidratos/química , Biologia Computacional , Reação de Cicloadição , Cicloexenos/química , Antracenos/química , Butadienos/química , Ciclopentanos/química , Hemiterpenos/química , Cetonas/química , Pentanos/química , EstereoisomerismoRESUMO
A conformationally restricted 2-O-benzyl-3,5-O-di-tert-butylsilylene-ß-D-thiogalactofuranoside donor was prepared from benzyl α-D-galactofuranoside and its donor capability was studied for stereoselective 1,2-cis α-D-galactofuranosylation. An unusual chemical behavior in benzylation and hydrogenolysis reactions was observed after the introduction of the 3,5-O-di-tert-butylsilylene protecting group into the galactofuranosyl moiety. The influence of the solvent, temperature, and activating system was evaluated. The NIS/AgOTf system, widely used in 1,2-cis ß-arabinofuranosylation, was not satisfactory enough for 1,2-cis galactofuranosylation. However, moderate to high α-selectivity was obtained with all the acceptors employed when using p-NO2PhSCl/AgOTf as a promoting system, in CH2Cl2 at -78°C. The order of the addition of the reactants (premixing or preactivation) did not affect substantially the stereochemical course of the glycosylation reaction. The α-D-Galf-(1â6)-D-Man linkage was achieved with complete diastereoselectivity by preactivation of the conformationally constrained thioglycoside donor.
Assuntos
Tiogalactosídeos/química , Tioglicosídeos/química , Butanos/química , Configuração de Carboidratos , Técnicas de Química Sintética , Glicosilação , Silanos/química , Solventes , Estereoisomerismo , Tiogalactosídeos/síntese químicaRESUMO
Mn4(IV)(µ3-N(t)Bu)4(N(t)Bu)4 is obtained from a previously reported asymmetric Mn(IV/V)-Li-(NR)(N) cluster by the removal of Li from the starting cluster by ion metathesis, which triggers reductive elimination of azo-tert-butane to give a tetranuclear heterocubane cluster.
Assuntos
Butanos/química , Manganês/química , Cristalografia por Raios X , Elétrons , Íons/química , Conformação Molecular , OxirreduçãoRESUMO
This study aimed at assessing the influence of different pressurized fluids treatment on the enzymatic activity and stability of a lyophilized ß-galactosidase. The effects of system pressure, exposure time and depressurization rate, using propane, n-butane, carbon dioxide and liquefied petroleum gas on the enzymatic activity were evaluated. The ß-galactosidase activity changed significantly depending on the experimental conditions investigated, allowing the selection of the proper compressed fluid for advantageous application of this biocatalyst in enzymatic reactions. The residual activity ranged from 32.1 to 93.8 % after treatment. The storage stability of the enzyme after high-pressure pre-treatment was also monitored, and results showed that the biocatalyst activity presents strong dependence of the fluid used in the pretreatment. The activity gradually decreases over the time for the enzyme treated with LGP and propane, while the enzyme treated with n-butane maintained 96 % of its initial activity until 120 days. For CO(2), there was a reduction of around 40 % in the initial activity 90 days of storage. The enzyme treated with n-butane also showed a better thermostability in terms of enzymatic half-life.
Assuntos
Proteínas Fúngicas/química , Kluyveromyces/enzimologia , Pressão , beta-Galactosidase/química , Butanos/química , Dióxido de Carbono/química , Estabilidade Enzimática , Propano/químicaRESUMO
The thermal [4+2] cycloadditions of 3-acetyl-, 3-carbamoyl, and 3-ethoxycarbonylcoumarins with 2,3-dimethyl-1,3-butadiene under solvent free conditions are reported, as well as the epoxidation reactions of some adducts. Discussion is focused on the structural features of the Diels-Alder adducts and their epoxides, based upon NMR, X-ray, and mass spectral data, and supported by ab initio theoretical calculations.
Assuntos
Alcenos/química , Butanos/química , Cumarínicos/química , Ciclização , Espectroscopia de Ressonância Magnética , Espectrometria de Massas , Modelos Moleculares , Estrutura Molecular , Solventes , TemperaturaRESUMO
This work investigates the continuous production of fatty acid ethyl esters from soybean oil in compressed fluids, namely carbon dioxide, propane and n-butane, using immobilized Novozym 435 as catalyst. The experiments were performed in a packed-bed bioreactor evaluating the effects of temperature in the range of 30-70 degrees C, from 50 to 150 bar, oil to ethanol molar ratio of 1:6-1:18 and solvent to substrates mass ratio of 4:1-10:1. In contrast to the use of carbon dioxide and n-butane, results showed that lipase-catalyzed alcoholysis in a continuous tubular reactor in compressed propane might be a potential route to biodiesel production as high reaction conversions were achieved at mild temperature (70 degrees C) and pressure (60 bar) conditions in short reaction times.
Assuntos
Glycine max/metabolismo , Lipase/química , Óleos de Plantas/química , Fontes de Energia Bioelétrica , Biotecnologia/métodos , Butanos/química , Candida/enzimologia , Dióxido de Carbono/química , Catálise , Enzimas Imobilizadas/química , Ésteres/química , Proteínas Fúngicas , Pressão , Propano/química , TemperaturaRESUMO
The stabilities of the gauche and anti conformations of butane, 1,2-dicyanoethane (DCE), and 1,2-dinitroethane (DNE) have been investigated through theoretical calculations. The gauche effect-the tendency of keeping close vicinal electronegative substituents (thetaX-C-C-X approximately 60 degrees ) in an ethane fragment-is expected to drive the conformational equilibrium of DCE and DNE toward the gauche conformation. It was found that, for butane, where the gauche effect is supposed to be poor/null, the hyperconjugation effect contributes mostly to the anti stabilization in opposition to the traditional sense that the methyl groups repel each other, and this should govern its conformational equilibrium. For DCE the equilibrium was shifted to the anti conformer, essentially due to a gauche repulsion, while for DNE, despite the higher electronic delocalization energies, a predominance of the gauche conformer was obtained, and this was attributed mainly to the attractive dipolar interaction between the two nitro groups. A full orbital energy analysis was performed using the natural bond orbital approach, which showed that bond bending and anti-C-H/C-X* hyperconjugation models, usually applied to explain the origin of the gauche effect in fluorinated derivatives, are not adequate to completely explain the conformational behavior of the titled compounds.
Assuntos
Algoritmos , Etano/análogos & derivados , Butanos/química , Carbono/química , Cianetos/química , Elétrons , Etano/química , Hidrocarbonetos Fluorados/química , Isomerismo , Modelos Moleculares , Conformação Molecular , Nitroparafinas/química , TermodinâmicaRESUMO
Bioassay-guided purification of the organic crude extract of Alternaria solani resulted in the isolation of three metabolites responsible for causing necrosis on potato leaves. These phytotoxins were identified as 2-(2",3"-dimethyl-but-1-enyl)-zinniol (1), 8-zinniol methyl ether (2). and 8-zinniol methyl ether based on their spectroscopic data (IR, MS, 1H and 13CNMR). Metabolites 1 and 2 have been identified as new phytotoxins structurally related to zinniol (4). Additionally, 5-(3',3'-dimethylallyloxy)-7-methoxy-6-methyl-phthalide and 8-zinniol-2-(phenyl)-ethyl ether (3) were also isolated during the purification process.
Assuntos
Alternaria/química , Butanos/isolamento & purificação , Butanos/toxicidade , Éteres/isolamento & purificação , Éteres/toxicidade , Xilenos/isolamento & purificação , Xilenos/toxicidade , Butanos/química , Éteres/química , Necrose , Folhas de Planta , Solanum tuberosum , Análise Espectral , Xilenos/químicaRESUMO
The 5-substituted tetrazole ring was reacted in the gas phase with an acyl ion generated as the secondary reactive chemical ionization plasma in the mass spectrometer. The product ions obtained, among others, were proposed as the rearranged 2,5-disubstituted-1,3,4-oxadiazole nucleus. Its structure was demonstrated by comparison of the product ion spectrum of the 2,5-disubstituted-1,3,4-oxadiazole derivative obtained by condensed-phase reaction and the product ion spectrum of the oxadiazole derivative generated in situ by reaction of the 5-substituted tetrazole derivative with the acyl plasma. It was proposed that the mechanism of this transformation involved the presence of an acylated tetrazole intermediary, which could not be isolated in the condensed phase, followed by the rearrangement, with nitrogen loss, to afford the oxadiazole derivative. Under our conditions we were able to isolate the intermediate ion in the first field free region and demonstrate its structure by collision induced dissociation-tandem mass spectrometry.