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91.
2-Oxopiperazine-based gamma-turn conformationally constrained peptides: synthesis of CCK-4 analogues
Herrero S García-López MT Latorre M Cenarruzabeitia E Del Río J Herranz R 《The Journal of organic chemistry》2002,67(11):3866-3873
2-Oxopiperazine derivatives 1 have been designed as mimetics of gamma-turn conformationally constrained tripeptides. The synthetic pathway devised for the preparation of both epimers of 1 at C(5) involves a reductive amination of cyanomethyleneamino pseudopeptides with amino acid derivatives, followed by regiospecific lactamization of the resulting C-backbone branched pseudopeptides. The versatility of this methodology is illustrated in the synthesis of analogues of the tetrapeptides Boc-[Nle(31)]-CCK-4 and Boc-[Lys(o-tolylaminocarbonyl)(31)]-CCK-4. The introduction of the new conformational restriction into these Boc-CCK-4 analogues led to a loss of 2 or 3 orders of magnitude in the affinity at CCK receptors. These results suggest the absence of a gamma-turn in the bioactive conformation of the C-terminal tripeptide of CCK-4. 相似文献
92.
Reactions of enamines, derived from 2-nitroanilines and alpha-substituted aldehydes, with carbon monoxide (6 atm) in the presence of a catalytic amount of bis(dibenzylideneacetone)palladium(0) (Pd(dba)(2)) and 1,3-bis(diphenylphosphino)propane (dppp) afford readily separated mixtures of 1,2-dihydroquinoxalines and 3,4-dihydroquinoxalinones. Addition of a catalytic amount of 1,10-phenanthroline to the reaction mixture substantially improved the yield of products. [reaction: see text] 相似文献
93.
Aydeé Fuentes Rafael Martínez-Palou Hugo A. Jiménez-Vázquez Francisco Delgado Alicia Reyes Joaquín Tamariz 《Monatshefte für Chemie / Chemical Monthly》2005,136(2):177-192
Summary. The (Z)-N-substituted 4-methylene-5-propylidene-2-oxazolidinone dienes were prepared by a one-step synthesis, starting from 2,3-hexanedione and isocyanates. Diels-Alder cycloadditions of these dienes were carried out in the presence of the dienophiles methyl vinyl ketone, methyl propiolate, and a captodative olefin, under conditions such as solvents of high polarity, Lewis acid catalysis, and non-conventional energy sources. The reactions carried out either with mixtures of H2O/MeOH or under BF3·Et2O catalysis yielded the highest regio- and stereoselectivities. The use of ionic liquids, microwaves, and ultrasound did not significantly increase the selectivity. 相似文献
94.
Colloid and Polymer Science - A statistical simulation method, designated as a “mathematical experiment”, is used to establish some characteristics of the adsorption models most widely... 相似文献
95.
Conejo-García A Campos J Eder C Entrena A Gallo MA Espinosa A 《The Journal of organic chemistry》2005,70(14):5748-5751
[structure: see text] Two-step syntheses of triple-bridged triscationic bicyclophanes are presented. Molecular modeling has been undertaken by means of ab initio calculations (6-31G level) on the triquinolina triscationic bicyclophane. This compound exists as two diastereomeric sets of enantiomers, one with C3 symmetry and the other with C1 symmetry. The C3-symmetrical derivative is 1.94 kcal mol(-1) more stable than its C1-symmetrical one. This energy difference is sufficient to consider the former and its enantiomer the only two conformations existing in solution at room temperature. 相似文献
96.
Espinosa-García J 《Journal of the American Chemical Society》2004,126(3):920-927
The reactivity of the hydroperoxyl radical with coenzyme Q, as a prototypical chemical reaction involved in biological antioxidant actions, was studied theoretically. Two pathways were analyzed: the hydrogen abstraction reaction from the phenolic hydrogen on the reduced form (ubiquinol), and OOH addition on the oxidized form (ubiquinone). Optimized geometries, harmonic vibrational frequencies, and energies of the stationary points (reactants, intermediate complexes, transition states, and products) for each pathway were calculated at the BHandHLYP/6-31G level of theory. The reaction paths for the two mechanisms were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory with multidimensional small-curvature tunneling. We found that the reactivity of the OOH radical is dominated by the hydrogen abstraction mechanism on ubiquinol, with a rate constant of 5.32 x 10(5) M(-1) s(-1), at 298 K. This result strongly contrasts with that, also obtained by our group, for the more reactive OH radical, which attacks ubiquinone by an addition mechanism, with a diffusion-controlled rate of 6.25 x 10(10) M(-1) s(-1), at 298 K. 相似文献
97.
Guadalupe Alcerreca Rubén Sanabria René Miranda Gabriel Arroyo Joaquín Tamariz Francisco Delgado 《合成通讯》2013,43(7):1295-1301
Several benzaldehydes were condensed with barbituric acid under infrared irradiation, in absence of solvent, affording the corresponding 5-benzylidene barbituric acids. 相似文献
98.
Dr. Jesús Campos Dr. Joaquín López‐Serrano Dr. Riccardo Peloso Prof. Ernesto Carmona 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(19):6432-6457
Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition‐metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition‐metal complexes containing M?CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M?CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl‐bridged complexes and reactivity. 相似文献
99.
100.
We present a robust algorithm to synchronize two different single-input/single-output (SISO) nonlinear systems connected in a master/slave scheme, where the relative degree of the master system (r
m) is greater than or equal to the relative degree of the slave (r
s). The sliding mode control technique is used to design the coupling signal. This discontinuous controller renders the closed-loop system robust with respect to matched bounded disturbances. The synchronization objective is to match the first r
s normal coordinates. Depending on the characteristics of the involved systems, the closed loop system can display full or partial, identical or generalized synchronization. The performance of the proposed controlled synchronization is illustrated numerically and experimentally. 相似文献