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231.
Alberto Martinez-Cuezva Aurelia Pastor Marta Marin-Luna Carmen Diaz-Marin Delia Bautista Mateo Alajarin Jose Berna 《Chemical science》2021,12(2):747
A detailed mechanistic study of the diastereoselective CsOH-promoted cyclization of interlocked fumaramides to give β-lactams is described. The mechanistic analysis comprises the experimental evaluation of the structure-reactivity relationship for a wide range of fumaramides [2]rotaxanes (Hammet-plots), KIE studies with deuterium-labelled interlocked fumaramides and computational analysis of two alternative mechanistic pathways for the cyclization process. The obtained results confirm that: (a) the rate-determining step is the deprotonation of the N-benzyl group of the thread by the amidate group of the macrocycle generated by the external base, (b) the polyamide macrocycle plays an important role not only as activating element but also as the stereodifferenciating factor responsible for the observed diastereoselection and (c) the higher flexibility of the adamantyl core speeds up the cyclization process in diadamantyl-derived rotaxanes.A mechanistic study of the diastereoselective cyclization of interlocked fumaramides to give β-lactams unveils the key factors for successfully taming the process. 相似文献
232.
Dieter Seebach Bernd Amatsch Ren Amstutz Albert K. Beck Max Doler Martin Egli Robert Fitzi Markus Gautschi Bernardo Herradn Pirmin C. Hidber John J. Irwin Rita Locher Miguel Maestro Thomas Maetzke Antoni Mourio Elmar Pfammatter Dietma A. Plattner Christof Schickli W. Bernd Schweizer Paul Seiler Gerhar Stucky Walter Petter Jaime Escalante Eusebio Juaristi Delia Quintana Carlos Miravitlles Elies Molins 《Helvetica chimica acta》1992,75(3):913-934
The X-ray structures of fifteen 1, 3-imidazolidine, 1, 3-oxazolidine, 1, 3-dioxan-4-one, and hydropyrimidine-4(1H)-one derivatives are described (Table 2) and compared with known structures of similar compounds (Figs. 1–20). The differences between structures containing exocyclic N-acyl groups and those lacking this structural element arise from the A1,3 effect of the amide moieties. Even t-Bu groups are forced into axial positions of six-ring half-chair or into flag-pole positions of six-ring twist-boat conformers by this effect (Figs. 16–20). In the N-acylated five-membered heterocycles, a combination of ring strain and A1, 3 strain leads to strong pyramidalizations of the amide N-atoms (Table 1) such that the acyl groups wind up on one side and the other substituents on the opposite side of the rings (Figs. 4–9 and Scheme 3). Thus, the acyl (protecting!) groups strongly contribute to the steric bias between the two faces of the rings. Observed, at first glance surprizing stereoselectivities of reactions of these heterocycles (Schemes 1 and 2) are interpreted (Scheme 3) as an indirect consequence of the amide A1, 3 strain effect. The conclusions drawn are considered relvant for a better understanding of the ever increasing role which amide groups play in stereoselective syntheses. 相似文献
233.
Synthetic procedures to high oxidation-state complexes of niobium incorporating the bicyclic guanidinate 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate, [hpp]−, are described. The ligand source was either the N-trimethylsilylated guanidine or the lithium guanidinate and the reaction proceeded via elimination of trimethylsilylchloride or transmetallation from NbCl5, respectively. A 1:1 ratio of reagents afforded the mono-ligand product, Nb(hpp)Cl4 (1) and crystallisation from acetonitrile afforded the solvated species Nb(hpp)Cl4(MeCN) (1a), demonstrating the ability of the metal centre in 1 to bind small substrate molecules. A 2:1 ratio of lithium guanidinate to NbCl5 resulted in formation of the seven-coordinate, bis-ligand compound, Nb(hpp)2Cl3 (2). These products represent the first examples of guanidinate compounds in which niobium is stable in the +5 oxidation-state, believed to result from enhanced electron donation caused by the bicyclic framework of the ligand. The molecular structures of 1, 1a and 2 are reported, presenting for the first time an opportunity to describe bonding parameters within compounds of this type. 相似文献
234.
Moise Maria-Ioana Marutoiu Constantin Badea Delia Nica Gavrila Cornelia-Anca Patroescu Constantin 《平面色谱法杂志一现代薄层色谱法》2004,17(2):147-148
JPC – Journal of Planar Chromatography – Modern TLC - Capsaicin was extracted from red hot pepper (Capsicum annuum) fruits and from commercial red hot pepper powder and separated on... 相似文献
235.
236.
A method based on the reaction of an E-phosphazide, an intermediate in the Staüdinger reaction between triphenylphosphine and an azide, with heterocumulenes allows the one-pot, two-component synthesis of a number of pyrrole-imidazole derivatives. The procedure, which involves sequential treatment of the appropriate α-azido ester with triphenylphosphine and isocyanate leads to the hydantoin product after aqueous work-up. The cyclization conditions can also be adapted for the synthesis of thiohydantoins by using isothiocyanates. These hybrids pyrrole-thiohydantoins undergo a novel oxidative spirocyclization by action of DDQ to give a tricyclic derivative (pyrrole-pyrrolidine-imidazole), which displays an interesting cytotoxic activity. 相似文献
237.
Performance of DFT hybrid functionals in the theoretical treatment of H‐bonds: Analysis term‐by‐term
Patricia Guadarrama Delia Soto‐Castro Jesús Rodríguez‐Otero 《International journal of quantum chemistry》2008,108(2):229-237
The performance of DFT to reproduce noncovalent interactions like H‐bonds was assessed. Three DFT hybrid functionals (B3LYP, BHandHLYP from Jaguar5.5, and BHandHLYP from G03) were used to calculate the interaction energies and H‐bond distances of several host–guest complexes theoretically designed. Two reference systems (whose experimental data were available) were also calculated at the same levels of theory. In all the cases B3LYP and BHandHLYP from G03 gave rather the same results of interaction energy and distances, whereas the functional BHandHLYP from Jaguar5.5 overestimated the interaction energies and underestimated the H‐bond distances. Considering the construction of each functional, the terms responsible for such differences are the gradient correction to the exchange functional (Becke88) and the correlation functional (VWN, LYP) and not the Hartree–Fock contribution. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
238.
Cristina‐Delia Nechifor Mihai Postolache Raluca Marinica Albu Andreea Irina Barzic Dana‐Ortansa Dorohoi 《先进技术聚合物》2019,30(8):2143-2152
Some optical properties of poly (vinyl alcohol) (PVA) films are investigated prior and after the rubbing and stretching of the samples. Birefringence of the prepared foils is enhanced as their stretching degree and rubbing become larger. These processing factors are also affecting the values of the ordinary and extraordinary refractive indices. The induced anisotropy in PVA foils is evidenced in the morphological characteristics observed by optical microscopy. The spreading and adhesion behaviors of a nematic liquid crystal on PVA foils are analyzed by means of contact angle measurements. Preliminary testing of the nematic on the PVA films revealed significant variations in transmitted light intensity during sample rotation under crossed polarizers. The high contrast between dark and bright patterns is indicative of uniform and homogenous alignment of nematic on the rubbed and stretched PVA foils. 相似文献
239.
The formation of solid solution and ZrO2 phase stabilization were investigated by Raman spectroscopy and X‐ray diffraction (XRD) in calcium‐containing and cadmium‐containing zirconium oxide samples heated at 1073 K in air. The adopted preparation procedure led to the incorporation of calcium and cadmium in solid solution into the zirconia structure. The solid solution favored the tetragonal and cubic zirconia phases at the expense of the thermodynamically stable monoclinic modification. Combined macro‐ and micro‐Raman spectroscopy disclosed that instead of forming a homogeneous phase t″, intermediate between the tetragonal t′ and the cubic phase, the tetragonal and cubic phases coexisted in the range 9.49–13.89 mol% for Ca and 11.88–17.23 mol% for Cd. At higher dopant contents the cubic form stabilized. The impurity content necessary to stabilize the high‐symmetry phases was defined. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
240.
Cottiglia F Bonsignore L Casu L Deidda D Pompei R Casu M Floris C 《Natural product research》2005,19(2):117-123
Two new phenolic glycosides, 4-hydroxy-3-(3-methyl-2-butenyl)phenyl beta-D-glucopyranoside (nebrodenside A) and O-coumaric acid beta-D-allopyranoside (nebrodenside B), were isolated from the aerial parts of Ephedra nebrodensis. In addition, O-coumaric acid glucoside, (-)-epicatechin, and (-)-ephedrine were also isolated. The structures were deduced from extensive 1D and 2DNMR spectroscopy (1H, 13C, DQF-COSY, TOCSY, GHSQC, GHMQC, ROESY) as well as mass spectrometry (EI and HR-MALDI). (-)-Epicatechin showed weak antiviral activity against Influenza A virus and very weak cytotoxicity against MDCK cells. 相似文献