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281.
Garrabou X Calveras J Joglar J Parella T Bujons J Clapés P 《Organic & biomolecular chemistry》2011,9(24):8430-8436
Aldol addition reactions of dihydroxyacetone (DHA) to N-Cbz-amino aldehydes catalyzed by L-rhamnulose-1-phosphate aldolase (RhuA) in the presence of borate buffer are reported. High yields of aldol adduct (e.g. 70-90%) were achieved with excellent (>98?:?2 syn/anti) stereoselectivity for most S or R configured acceptors, which compares favorably to the reactions performed with DHAP. The stereochemical outcome was different and depended on the N-Cbz-amino aldehyde enantiomer: the S acceptors gave the syn (3R,4S) aldol adduct whereas the R ones gave the anti (3R,4R) diastereomer. Moreover, the tactical use of Cbz protecting group allows simple and efficient elimination of borate and excess of DHA by reverse phase column chromatography or even by simple extraction. This, in addition to the use of unphosphorylated donor nucleophile, makes a useful and expedient methodology for the synthesis of structurally diverse iminocyclitols. The performance of aldol additions of dihydroxyacetone phosphate (DHAP) to N-Cbz-amino aldehydes using RhuA and L-fuculose-1-phosphate aldolase (FucA) catalyst in borate buffer was also evaluated. For FucA catalysts, including FucA F131A, the initial velocity of the aldol addition reactions using DHAP were between 2 and 10 times faster and the yields between 1.5 and 4 times higher than those in triethanolamine buffer. In this case, the retroaldol velocities measured for some aldol adducts were lower than those without borate buffer indicating some trapping effect that could explain the improvement of yields. 相似文献
282.
Bauzá A Quiñonero D Frontera A Deyà PM 《Physical chemistry chemical physics : PCCP》2011,13(45):20371-20379
Substituent effects in halogen bonding complexes involving aromatic rings are investigated. We have analyzed how the interaction energy (the RI-MP2/aug-cc-pVDZ level of theory) is affected by the substitution in both halogen bond donor and acceptor aromatic moieties. In addition, we have used two different aromatic electron donor molecules pyridine and cyanobenzene, which allow us to study the effect of having the electron donor nitrogen atom forming part of the ring or outside the ring (-CN). Interestingly, the effect of the substituents on the interaction energies is similar in both cases. We have obtained the Hammett's plots for four combinations of aromatic donors and acceptors and in all cases we have obtained good regression plots (interaction energies vs. Hammett's σ parameter). We have also studied and compared bifurcated halogen bonds using both possible combinations, that is two donors and one acceptor and vice versa. In addition, we have analyzed the effect of the solvent on the interaction energies using COSMO. Finally, we have used Bader's theory of "atoms-in-molecules" to demonstrate that the electron density computed at the bond critical point that emerges upon complexation can be used as a measure of bond order in this noncovalent interaction. 相似文献
283.
Estarellas C Frontera A Quiñonero D Deyà PM 《The journal of physical chemistry. A》2011,115(26):7849-7857
Several complexes of fluorine-substituted ethyne, ethene, butadiene, benzene, and [n]radialenes (n = 3-5) with two anions have been optimized at the RI-MP2/aug-cc-pVTZ level of theory. The additivity of the anion-π interaction was studied depending on the number of double bonds and fluorine atoms. Interesting nonadditivity effects were observed in the aromatic and antiaromatic complexes, which were analyzed by partitioning the total interaction energy into individual components, using Bader's theory of "atoms in molecules" and changes in the aromatic character of the ring upon complexation. 相似文献
284.
A relationship between intrinsic viscosity and molecular weight for fractionated cellulose is established. The exponent a of the Mark-Houwink equation is 0.80. The unperturbed dimensions of cellulose are discussed on the basis of the Stockmayer-Fixman and Kurata-Stockmayer viscometric theories, and it is determined that cellulose is not a very rigid macromolecule in nature. This conclusion disagrees with the results obtained by statistical mechanics procedures in which the restriction of β-glucose residues to the C1 conformation was kept. The steric hindrance factor, σ is about 2, and it seems to be independent of the substituents in cellulose if it is evaluated in both cellulose and cellulose derivatives from viscometric data. From statistical mechanics data reported in the literature, σ is estimated as 4.4 for cellulose and cellulose derivatives, such as cellulose nitrate, and this means that the steric hindrances of the substituents have little influence on the rigidity of cellulose derivatives. This view disagrees with the results obtained from the non-Newtonian behavior of cellulose and cellulose nitrate in dilute solutions. By applying the Noda-Hearst theory, the conclusion is reached that cellulose nitrate is a more extended macromolecule than cellulose. 相似文献
285.
Abel Carreras Andreas Lorbach Guillermo C. Bazan Pere Alemany Guang Wu Miguel A. Garcia-Garibay Emily F. Maverick 《Helvetica chimica acta》2023,106(2):e202200142
A single-crystal X-ray study of a fullerene-imidazole adduct at nine temperatures (80 K≤T≤480 K), accompanied by energy calculations, strongly suggested thermal motion of the C60 moiety with respect to the imidazolium heterocycle. Analysis of the anisotropic displacement parameters, calculations of frequencies, and the refinement of disorder models for the crystal at four temperatures (230 K≤T≤380 K) lead to the conclusion that the rotator is moving at all temperatures. The rotation barrier is low, with one preferred crystallographic site and several other energy minima. 相似文献
286.
Maria L. Cerrada Oscar Prieto José M. Pereña Rosario Benavente Ernesto Pérez 《Macromolecular Symposia》2003,198(1):91-102
Blends covering the entire range of compositions of a metallocenic ethylene-1-octene, CEO, copolymer and two conventional isotactic polypropylenes, iPP, of different molecular weights have been prepared, analyzing the effect of composition and molecular weight on the crystallization (studied by DSC and X-ray diffraction) and viscoelastic behavior (DMTA). It was found that those blends rich in the iPP component show a behavior practically coincident with the weighted addition of the two components. On the contrary, significant deviations were found for the blends where the CEO copolymer is the major component. These deviations are considerably more important in the case of the blends with the iPP of higher molecular weight. Moreover, both components are not miscible, exhibiting the glass transitions of the two neat components. The area under the loss tangent curves provides a preliminary information about how the toughness is enhanced using this type of impact modifier, though it provokes a significant reduction of stiffness. 相似文献