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71.
Gillespie KM Sanders CJ O'Shaughnessy P Westmoreland I Thickitt CP Scott P 《The Journal of organic chemistry》2002,67(10):3450-3458
Racemic 2,2'-diamino-6,6'-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu(2)L(2)](2+)) and monometallic ([CuL](+)). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88-98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5-54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu(II) is probably also a competent system. 相似文献
72.
Ian L. Cooper Christopher N. M. Pounder 《International journal of quantum chemistry》1983,23(1):257-270
Local Coulomb correlation hole distribution functions may be used to assess the extent to which electron correlation effects are present in large scale SCF + CI wave functions. From a set of modified virtual orbitals, ordered according to their interaction with the SCF configuration, we have constructed a limited SCF + CI wave function with improved convergence characteristics with respect to that formed from the canonical virtual orbital set. These wave functions, of the same size yet with different energies, have been used to examine the range and depth of local Coulomb correlation holes in FCN. In all cases, the depth of the local Coulomb hole is no more than 10% or so of that of the corresponding Fermi hole, and the range Fermi correlation is generally less than that of Fermi correlation. This is particularly marked in the high density regions around the nuclei. The significance of our results is discussed in relation to a recent proposal for the incorporation of Coulomb correlation into the local exchange method. 相似文献
73.
The importance of applying a variety of experimental techniques to unravel the nature of solvated electrons is emphasized. Compton profiles are evaluated for these species from a range of models. Some comparisons are made with positron annihilation studies. 相似文献
74.
Resynthesis of NSC 341,964, which had been assigned structure 1 (1-[[3-(7-chloro-4-oxo-4H-3,1-benzoxazin-2-yl)phenyl]methyl]pyridinium chloride) was approached via 7-chloro-2-(3-methylphenyl)-4H-3,1-benzoxazin-4-one ( 5 ) obtained from 3-methylbenzoyl chloride ( 2 ) and 2-amino-4-chlorobenzoic acid ( 3 ) followed by dehydration in acetic anhydride. Radical bromination provided 6 which with pyridine afforded the bromide analog 7 of 1 . Ion exchange, however, gave ring-opened benzoic acid 8 rather than 1 . The original sample of NSC 341,964 also proved to be ring-opened material. However, 7 upon standing exhibited slow hydrolysis to 8 so that the structure of the original NSC 341,964 remains uncertain. A more direct route to compound 8 is also described. 相似文献
75.
The roles of template complexation and ligand binding conditions on recognition in bupivacaine molecularly imprinted polymers 总被引:3,自引:0,他引:3
A model for the molecular basis for ligand recognition in bupivacaine imprinted methacrylic acid-ethylene glycol dimethacrylate co-polymers has been developed based upon a series of (1)H-NMR studies in conjunction with HPLC and radioligand binding analyses. (1)H-NMR studies indicated that functional monomer-template complexes survive the polymerisation process, at least up until the stage of gelation. Polymers were synthesised and characterised by surface area analysis (BET), FT-IR and SEM. A combination of zonal and frontal chromatographic studies in aqueous and non-polar media indicate that selectivity arises from a combination of hydrophobic and electrostatic interactions. However, in the concentration regime employed for LC-based studies, ligand recognition in aqueous media was shown to be predominantly non-specific and hydrophobic in character. Radioligand binding studies, in lower ligand binding concentration regimes, permitted closer examination of the higher affinity binding sites. It was shown that the presence of a polar modifier in a non-polar solvent, or an organic modifier in water, produced enhanced selectivity. Variable temperature studies showed that the temperature of binding influences selectivity as well as the apparent number of sites available and that this effect is different in organic and aqueous environments. This indicates that the system studied is more complex in character than is generally appreciated. A comparison of the techniques employed here indicates that although chromatographic studies provide a valuable first-round screen for polymer-ligand selectivities, the level of detail obtainable using radioligand binding studies (lower concentrations and true equilibrium binding) makes them superior for detailed evaluations of molecularly imprinted polymers. 相似文献
76.
Marine organisms produce a fascinating range of structurally diverse secondary metabolites, which often possess unusual and sometimes unexpected biological activities. This structural diversity makes these marine natural products excellent molecular probes for the investigation of biochemical pathways. Recently, a number of novel and stereochemically complex macrolides, having a large macrolactone (22- to 44-membered) ring, that interact with the actin cycloskeleton have been isolated from different marine sources. Actin, like tubulin, is a major component of the cytoskeleton and has important cellular functions. Although the details of these interactions are still under investigation, these marine macrolides are becoming increasingly important as novel molecular probes to help elucidate the cellular functions of actin. Owing to their potent antitumor activities, these compounds, for example the aplyronines, also have potential for preclinical development in cancer chemotherapy. Their appealing molecular structures, with an abundance of stereochemistry, and biological significance, coupled with the extremely limited availability from the marine sources, have stimulated enormous interest in the synthesis of these compounds. This review summarizes the biological properties of these unusual marine natural products and features the recently completed total syntheses of swinholide A, scytophycin C, aplyronine A, mycalolide A--all of these being potent cytotoxic agents that target actin--and a diastereoisomer of ulapualide A. Rather than detailing each individual step of these multistep total syntheses, the different synthetic strategies, key reactions, and methods adopted for controlling the stereochemistry are compared. 相似文献
77.
Geraldine S.P. Ritchie Alan M. Posner Ian M. Ritchie 《Journal of Electroanalytical Chemistry》1981,123(2):397-407
Variation of the lifetime of a mercury drop with potential was used to determine the pzc of mercury in the presence of soil humic acids and their aluminium complexes. In all cases there was an overall net shift in the pzc in the cathodic direction. Variation in the extent of the shift with pH and concentration indicated greater adsorption of negatively charged species. The shift was smaller in the presence of aluminium humates, probably due to a decrease in the negative charge of the humic acid molecules after complexing. The use of electroanalytical techniques for metal speciation studies in soils and natural water, if humic materials are known to be present, might therefore be limited. 相似文献
78.
Kim C. Pich Roger Bishop Donald C. Craig Ian G. Dance A. David Rae Marcia L. Scudder 《Structural chemistry》1993,4(1):41-51
In a further exploration of the structural factors causing alicyclic diols such asexo-2,exo-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (1) to adopt the helical tubuland inclusion host structure, the title compound (8) has been synthesized and its crystal structure determined. This 9-oxa analogue of (1) has a totally different structure [C10H18O3; orthorhombic;P21212;a 18.989(4),b 19.064(3),c 14.256(3) Å;Z 20; with finalR 0.065] due to involvement of the ether oxygen atom in the hydrogen bonding network. The structure of the diol (8) approximatesP¯421
c symmetry, and 16 of the 20 molecules per cell create a hostlike structure of this symmetry, which consists of tightly bound hydrogen bonded pillars parallel toc. The remaining guestlike molecules occupy inequivalent pseudo ¯4 sites separated byc/2 to form weakly bound columns parallel toc. Hydrogen bonds also occur between the pillars and columns. An ordered structure with sensible intermolecular contacts can be formed inP212121 with thec axis doubled. A difference betweena andb axial lengths correlates with a difference in occupancies of pseudo-¯4-related sites for the guest-like component of the structure. 相似文献
79.
A synthesis of N-acetylcolchinol, a key intermediate in the synthesis of ZD6126, was developed. The enantiodifferentiating step required the catalytic asymmetric hydrogenation of an enamide. After screening a range of metal and ligand combinations it was found that (S,S)-iPr-FerroTANE Ru(methallyl)2 and [(S,S)-tBuFerroTANE Rh(COD)]BF4 gave both high enantioselectivity (>90% ee) and high catalyst utility (molar S/C = 1000). 相似文献
80.
Mulligan A Lane I Rousseau GB Hecht L Johnston SM Lennon D Kadodwala M 《Chemical communications (Cambridge, England)》2004,(21):2492-2493
Using a novel non-linear optical technique enantiomeric excess within a translationally disordered overlayer on a metal surface has been monitored for the first time. 相似文献