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991.
Mecheril V. Nandakumar 《Tetrahedron》2005,61(41):9775-9782
A Pd2dba3/P(i-BuNCH2CH2)3N catalyzed one-pot synthesis of unsymmetrically substituted trans-4-N,N-diarylaminostilbenes and both symmetrically and unsymmetrically substituted N,N-diarylaminostyrene derivatives is reported. The procedure involves two or more palladium catalyzed sequential coupling reactions (an amination and an inter-molecular Heck reaction) in one-pot using the same catalyst system with two different aryl halides, including aryl chlorides and hetero aryl halides as the coupling partners. 相似文献
992.
993.
994.
The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer–dimer equilibrium where a Mg2+-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg2+) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 °C, 5 mM pyruvate (with 2 mM Mg2+) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg2+ is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from >108 to <5 × 105 or 3 × 107 M−1, respectively. With 2 mM Mg2+ at 15–25 °C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with KA′106 M−1 (ΔG′=−33.7±0.2 kJ mol−1 and ΔH=+16.3 kJ mol−1 at 20 °C with ΔCp=−1.4 kJ K−1 mol−1). The binding of PEP to EI(H189A) is synergistic with that of Mg2+. Thus, physiological concentrations of PEP and Mg2+ increase, whereas pyruvate and Mg2+ decrease the amount of dimeric, active, dephospho-enzyme I. 相似文献
995.
The ENDOR spectrum of ~(14)N-~(63)Cu-HAP complex in DMSO/EtOH (5:1) freezing solution at 20 K has been studied using orientational selective method in this paper. The anisotropic superhyperfine coupling tensor and qusdrupole coupling tensor of ligand ~(14)N nucleus, and the superhyperfine coupling tensor of various ~1H nuclei have been measured precisely. Comparing the superhyperfine coupling tensor of ~(14)N-nucleus with previous work shows that the analytical method of spectrum is reasonable and the data are reliable in our previous work. 相似文献
996.
p-tert-Butylcalix[8]arene bonded capillaries for open-tubular capillary electrochromatography were prepared with γ-glycidoxypropyltrimethoxysilane as a bridge. The bonded capillary displayed low and steady electroosmotic flow (EOF) values over the pH range from 4 to 9. Detection limits for direct spectrophotometric detection at 277 nm for benzenediols (at a signal to noise ratio of 2) were 0.96 mg l−1 for the unbonded capillary and 1.48 mg l−1 for the bonded capillary, showing that the bonded layer did not show significant absorbance and hence decreased sensitivity. The bonded capillaries showed good separation selectivity for o-, m- and p-benzenediols, α- and β-naphthols, and α- and β-naphthylamines. This selectivity was attributed to significant interactions between the analytes and the bonded p-tert-butylcalix[8]arene, which contributed to the electrochromatographic separation mechanism. The bonded capillaries gave high stability and reproducibility. 相似文献
997.
Jin Hong 《Journal of Molecular Structure》2003,655(3):435-441
X-ray crystallographic analysis and density functional B3LYP/6-31G(d) calculation confirm that dimeric water is embedded within a hydrophobic cavity of tetra-(p-tert-butyl)thiacalix[4]arene and stabilized by hydrogen bondings of aromatic π?H2O(1) and methyl?H2O(2) in the dimeric water inclusion complex with binding energies of 1.4 and 0.9 kcal mol−1 respectively, and by hydrogen bonding formed between H2O(2) and four phenolic OH groups from an adjacent tetra-(p-tert-butyl)thiacalix[4]arene, with binding energy of 3.8-4.2 kcal mol−1. 相似文献
998.
Shuji YasuikeSatoru Okajima Kentaro YamaguchiHiroko Seki Jyoji Kurita 《Tetrahedron》2003,59(27):4959-4966
Racemic 2,2′-bis[diarylstibano]-1,1′-binaphthyls [(±)-BINASbs] and 2,2′-bis[di(p-tolyl)bismuthano]-1,1′-binaphthyl [(±)-BINABi], which are the antimony and bismuth congeners of BINAP, have been prepared from 2,2′-dibromo-1,1′-binaphthyl (DBBN) via 2,2′-dilithio-1,1′-binaphthyl intermediate by treatment with the appropriate metal halides [(p-Tol)2SbBr, Ph2SbBr and (p-Tol)2BiCl]. The optical resolution of the (±)-BINASbs could be achieved via the separation of a mixture of the diastereomeric Pd-complexes derived from the reaction of (±)-BINASbs with di-μ-chlorobis{(S)-2-[1-(dimethylamino)-ethyl]phenyl-C1,N}dipalladium(II). Optically active (R)-BINASb and (R)-BINABi could be also obtained from optically active (R)-DBBN by the same procedure. The enantiopure BINASbs have been shown to be effective chiral ligands for the rhodium-catalyzed asymmetric hydrosilylation of ketones. 相似文献
999.
Zi-Qiang GuMin Li 《Tetrahedron letters》2003,44(15):3203-3205
A concise, multi-gram scale method for producing the bioactive and enantiomerically pure epimers, (2S,4R)- and (2S,4S)-glutamic acids, in a single synthetic scheme is described. 相似文献
1000.
氯代甲苯双电荷离子的单分子解离反应研究 总被引:1,自引:0,他引:1
研究了在70 eV电子轰击电离条件下,氯代甲苯及氯化苄产生的双电荷离子[C_7H_7Cl]~(2+)、[C_7H_6Cl]~(2+·)和[C_7H_5Cl]~(2+)为母体的两种类型单分子解离反应.主要讨论了亚稳双电荷离子的异构化反应、失H解高的“偶电子规则”以及单分子电荷分离过渡态的结构. 相似文献