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排序方式: 共有351条查询结果,搜索用时 109 毫秒
101.
Tuan P. Tran Edmund L. Ellsworth Brian M. Watson Joseph P. Sanchez H. D. Hollis Showalter John R. Rubin Michael A. Stier Judy Yip Dai Q. Nguyen Paul Bird Rajeshwar Singh 《Journal of heterocyclic chemistry》2005,42(4):669-674
A new synthesis of a series of 3‐amino‐1H‐quinazoline‐2,4‐diones is described. The 1H‐quinazoline‐2,4‐dione 10 was made starting with fluorobenzoic acid in three high yielding steps. The key step of this synthesis involved the generation of the dianion of urea 7 and the subsequent intramolecular nucleophilic displacement of the 2‐fluoro to form the quinazolinedione ring. The 3‐amino moiety was incorporated using (2,4‐dinitro‐phenyl)‐hydroxylamine as the aminating reagent. 相似文献
102.
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104.
A silica gel-immobilized [(Me(3)tacn)Ru(III)(CF(3)COO)(2)(H(2)O)]CF(3)CO(2) complex (1-SiO(2), Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) was prepared by simple impregnation, and the catalyst was characterized by powdered X-ray diffraction, nitrogen adsorption/desorption, Raman, and diffuse reflectance UV-vis spectroscopies. The supported Ru catalyst can effect facile oxidation of alcohols by tert-butyl hydroperoxide (TBHP). Primary and secondary benzyl, allylic, and propargylic alcohols were transformed to their corresponding aldehydes and ketones in excellent yields; no oxidation of the C=C and Ctbd1;C bonds was observed for the allylic and propargylic alcohol oxidations. Likewise alkene epoxidation by TBHP can be achieved by 1-SiO(2); cycloalkenes such as norbornene and cyclooctene were oxidized to their exo-epoxides exclusively in excellent yields (>95%). The 1-SiO(2) catalyst can be recycled and reused for consecutive alcohol and alkene oxidations without significant loss of catalytic activity and selectivity; over 9000 turnovers have been attained for the oxidation of 1-phenyl-1-propanol to 1-phenyl-1-propanone. 4-Substituted phenols were oxidized by the "1 + TBHP" protocol to give exclusively ruthenium-catecholate complexes, which were characterized by UV-vis and ESI-MS spectroscopies. No (tert-butyldioxy)cyclohexadienone and other radical coupling/overoxidation products were produced using the "1 + TBHP" protocol. The formation of ruthenium-catecholate is proposed to proceed via ortho-hydroxylation of phenol. 相似文献
105.
Yip VL Varrot A Davies GJ Rajan SS Yang X Thompson J Anderson WF Withers SG 《Journal of the American Chemical Society》2004,126(27):8354-8355
Among the numerous well-characterized families of glycosidases, family 4 appears to be the anomaly, requiring both catalytic NAD+ and a divalent metal for activity. The unusual cofactor requirement prompted the proposal of a mechanism involving key NAD+-mediated redox steps as well as elimination of the glycosidic oxygen. Primary kinetic isotope effects for the 2- and 3-deutero substrate analogues, isotopic exchange with solvent, and structural analysis of a 6-phospho-beta-glucosidase, BglT (E.C. 3.2.1.6), provided evidence in support of the proposed mechanism, which has striking resemblances to that of the sugar dehydratases. Furthermore, analysis of the stereochemical outcome indicated that family 4 enzymes are retaining glycosidases. 相似文献
106.
We employed negatively charged fluorescein (FL), positively charged rhodamine 6G (R6G), and neutral Nile Red (NR) as molecular probes to investigate the influence of Coulombic interaction on their deposition into and rotational mobility inside polyelectrolyte multilayer (PEM) films. The entrapment efficiency of the dyes reveals that while Coulombic repulsion has little effect on dye deposition, Coulombic attraction can dramatically enhance the loading efficiency of dyes into a PEM film. By monitoring the emission polarization of single dye molecules in polyethylenimine (PEI) films, the percentages of mobile R6G, NR, and FL were determined to be 87 +/- 4%, 76 +/- 5%, and 68 +/- 3%, respectively. These mobility distributions suggest that cationic R6G enjoys the highest degree of rotational freedom, whereas anionic FL shows the least mobility because of Coulombic attraction toward cationic PEI. Regardless of charges, this high percentage of mobile molecules is in stark contrast to the 5-40% probe mobility reported from spun-cast polymer films, indicating that our PEI films contain more free volume and display richer polymer dynamics. These observations demonstrate the potential of using isolated fluorescent probes to interrogate the internal structure of a PEM film at a microscopic level. 相似文献
107.
Mark John P. Mandigma Jonas
urauskas Callum I. MacGregor Lee J. Edwards Ahmed Shahin Ludwig d'Heureuse Philip Yip David J. S. Birch Thomas Gruber Jrg Heilmann Matthew P. John Joshua P. Barham 《Chemical science》2022,13(7):1912
We report an organophotocatalytic, N–CH3-selective oxidation of trialkylamines in continuous flow. Based on the 9,10-dicyanoanthracene (DCA) core, a new catalyst (DCAS) was designed with solubilizing groups for flow processing. This allowed O2 to be harnessed as a sustainable oxidant for late-stage photocatalytic N–CH3 oxidations of complex natural products and active pharmaceutical ingredients bearing functional groups not tolerated by previous methods. The organophotocatalytic gas–liquid flow process affords cleaner reactions than in batch mode, in short residence times of 13.5 min and productivities of up to 0.65 g per day. Spectroscopic and computational mechanistic studies showed that catalyst derivatization not only enhanced solubility of the new catalyst compared to poorly-soluble DCA, but profoundly diverted the photocatalytic mechanism from singlet electron transfer (SET) reductive quenching with amines toward energy transfer (EnT) with O2.An N–CH3-selective trialkylamine oxidation to N-formamides is reported in continuous flow using gaseous O2. A novel, enhanced-solubility dicyanoanthracene organophotocatalyst switched the photochemical mechanism from electron to energy transfer. 相似文献
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109.
Phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS) have been separated by supercritical fluid chromatography coupled with evaporative light scattering and mass spectrometric detection. Four different silica based stationary phases were investigated: diol, cyanopropyl, 2-ethylpyridine, and 4-ethylpyridine. A gradient mobile phase of pressurized carbon dioxide modified with methanol was employed. The individual effect of basic (isopropylamine), acidic (trifluoroacetic acid), and ionic (ammonium acetate) additives incorporated into the primary modifier on the separation of these four lipids from the four stationary phases was investigated. No additive was required to effect the elution of PC and PE (which differed only in polar head group) from any of the four phases. When the two ethylpyridine phases were used, near baseline resolution of two species in each lipid case that differed in fatty acid content were seen. PS and PI could be successfully eluted from each phase when isopropylamine was added to the mobile phase, although the PS peak was always weak and very broad. Ammonium acetate was a more effective additive for phospholipid separation than trifluoroacetic acid. All chromatographic separations were reproducible and required less than 15 min. 相似文献
110.