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991.
The first total synthesis of (-)-sordarin (1) was accomplished exploiting the following key reactions: (i) Ag(I)-catalyzed oxidative radical cyclization of a cyclopropanol derivative leading to a bicyclo[5.3.0]decan-3-one skeleton; (ii) Pd(0)-catalyzed intramolecular allylation reaction resulting in the entire strained bicyclo[2.2.1]heptan-2-one framework of sordaricin (2); (iii) selective dihydroxylation of terminal alkenes by the combined use of OsO(4) and PhB(OH)(2); and (iv) beta(1,2-cis)-selective glycosidation via a 1,3-anchimeric assistance from a 4-methoxybenzoyl group.  相似文献   
992.
Application of the HPLC hyphenated techniques of LC-MS, LC-NMR and solvent-elimination LC-IR was demonstrated by the identification of the degradation products of a third generation cephalosporin antibiotic, cefpodoxime proxetil, in solid state, drug formulation and solution. Molecular weight and fragment information were obtained by LC-MS, and detailed structural information was confirmed by LC-NMR. Information on the carboxyl functional group obtained by solvent-elimination LC-IR was useful for confirmation of the ester hydrolysis. The degradation products were successfully identified without complicated isolation or purification processes.  相似文献   
993.
Tripod-shaped trithiols 1-3, containing CH2SH groups at the three bridgehead positions of the adamantane framework and a halogen-containing group [Br (1), p-BrC6H4 (2), or p-IC6H4 (3)] at the fourth bridgehead, were synthesized, and self-assembled monolayers (SAMs) were prepared on atomically flat Au111 surfaces. The three-point chemisorption of these tripods was confirmed by polarization modulation infrared reflection absorption spectroscopy, which showed the absence of a S-H stretching band. Scanning tunneling microscopy of the SAM of 1 exhibited a hexagonal arrangement of the adsorbed molecule with a lattice constant of 8.7 angstroms. A unidirectionally oriented, head-to-tail array of 1, which allows the close approach of neighboring molecules, is proposed as a reasonable model of the two-dimensional crystal, where the adsorbed sulfur atoms form a quasi-(radical3 x radical3)R30 degrees lattice. The charge of the electrochemical reductive desorption of the SAM of 1 was in good agreement with the expected surface coverage, while the SAMs of 2 and 3 showed somewhat less (ca. 70%) charge. The large negative reduction peak potentials, observed for the SAM of 1, are taken to indicate a tight anchoring of this tripod by three sulfur atoms.  相似文献   
994.
Cross-contamination is a critical issue for pharmaceutical manufacturing, especially for beta-lactam antibiotics. Thus, an analytical method for the simultaneous determination of beta-lactam antibiotics cefmetazole (CMZ) and cefpodoxime proxetil (CPDXPR) contaminants in non-beta-lactam pharmaceuticals was developed using high-performance liquid chromatography-tandem mass spectrometry. The developed method was found to be sensitive at the detection limit of 0.002 ppm for both compounds. Mean recoveries of CMZ and CPDXPR from olmesartan medoxomil (OLM) tablets were 96.7 to 102.2% and 88.9 to 94.2%, respectively. The developed method was successfully applied for the verification of CMZ and CPDXPR contamination to actually manufactured OLM tablets.  相似文献   
995.
The 1H nuclear spin-lattice relaxation time (T1) of H2 and H2@C60 in organic solvents varies with solvent, and it varies proportionally for H2 and for H2@C60. Since intermolecular magnetic interactions are ruled out, the solvent must influence the modulating processes of the relaxation mechanisms of H2 both in the solvent cage and inside C60. The temperature dependence of T1 also is very similar for H2 and H2@C60, T1 going through a maximum by varying the temperature in solvents which allow a wide range of temperatures to be explored. This behavior is attributed to the presence of dipolar and spin-rotation mechanisms which have an opposite dependence on temperature.  相似文献   
996.
The peptidoglycan (PG) bacterial cell wall glycoconjugate has been well known as a strong immunopotentiator. Partial structures of PG were chemically synthesized for elucidation of precise biological activities. Effective construction of distinct repeating glycans of PG was accomplished by the coupling of a key disaccharide glucosaminyl-beta(1-4)-muramic acid unit. Stereoselective glycosylation of disaccharide units was achieved by neighboring group participation of the N-Troc (Troc = 2,2,2-trichloroethoxycarbonyl) group and appropriate reactivity of N-Troc-glucosaminyl trichloroacetimidate. By using an efficient synthetic strategy, mono-, di-, tetra- and octasaccharide fragments of PG were synthesized in high yields. The biological activity of synthetic fragments of PG was evaluated by induction of tumor necrosis factor-alpha (TNF-alpha) from human monocytes, and toll-like receptor 2 (TLR2) and Nod2 dependencies by using transfected HEK293 cells, respectively. Here we reveal that TLR2 was not stimulated by the series of synthetic PG partial structures, whereas Nod2 recognizes the partial structures containing the MDP moiety.  相似文献   
997.
The photoinduced desorption of NO molecules on a Ag surface was studied theoretically using a recently developed method based on the nonequilibrium Green's function approach combined with the density functional theory. Geometry optimizations for the stable NO dimer phase were carried out, and two structures of adsorbed dimers were identified. We calculated the reaction probabilities as a function of incident photon energy for each of the dimers and compared them with experimental action spectra. The two main features of the action spectra, (i) a long tail to the long wavelength (approximately 600 nm) and (ii) a rapid increase at approximately 350 nm, were well reproduced. By theoretical analysis, we found the importance of quantum interference for the interfacial charge transfer between the metal substrate and the adsorbate, as well as the contribution of secondary electrons. Our calculations suggest that the photoactive species is dimeric and that the resonant level is single for the photodesorption of NO.  相似文献   
998.
Infrared photodissociation action spectra of protonated ammonia cluster ions, NH(4) (+)(NH(3))(n) (n=5-8), were measured in the range of 1020-1210 cm(-1) by using a tunable infrared free electron laser. Analyses by the density functional theory (DFT) show that the spectral features observed can be assigned to the nu(2) vibrational mode of the NH(3) molecules in NH(4) (+)(NH(3))(n). Size dependence of the spectra supports structural models obtained by the DFT calculations, in which the NH(4) (+) ion is solvated by the four nearest-neighbor NH(3) molecules. For NH(4) (+)(NH(3))(5), the spectrum between 1000 and 1700 cm(-1) was measured. The nu(4) bands of the NH(3) molecules and the NH(4) (+) ion were found in the range of 1420-1700 cm(-1).  相似文献   
999.
Alzheimer's disease is characterized by the deposition of senile plaques that consist primarily of amyloid beta peptides. There is substantial evidence that amyloid beta is oxidized in vivo, which has led to the suggestion that oxidative stress is an important mediator of Alzheimer's disease. Metal-catalyzed oxidation can mimic in vivo oxidation of amyloid beta because the metal ion binds to the amino acid residues at the site of oxidation, which then deliver reactive oxygen species to that site. Based on electrospray mass spectrometry, it has been suggested that metal-catalyzed oxidation occurs on histidines-13 and -14. Unfortunately, the amyloid beta peptides provide complex spectra, so it is difficult to definitively characterize the sites of oxidation. Trypsin digestion of both native and oxidized amyloid beta1-16 and amyloid beta1-40 resulted in the formation of tryptic peptides corresponding to amyloid beta6-16, which could be separated by liquid chromatography (LC). Sites of oxidation were then unequivocally characterized as histidine-13 and histidine-14 by LC/tandem mass spectrometric (MS/MS) analysis of the tryptic peptides. The ability to analyze the specific amyloid beta6-16 tryptic fragments derived from full-length amyloid beta peptides will make it possible to determine whether oxidation in vivo occurs at specific histidine residues and/or at other amino acid residues such as methionine-35. Using methodology based on LC/MS/MS it will also be possible to analyze the relative amounts of oxidized peptides and native peptide in cerebrospinal fluid from patients with Alzheimer's disease as biomarkers of oxidative stress.  相似文献   
1000.
[Structure: see text] A temporal N-silylation protocol in the catalytic aryl amination reaction has been devised to prepare nitrogen-bridged calixarene analogues. The protocol involves a smooth in situ N-silylation before aryl amination reaction, followed by spontaneous cleavage of the N-Si bond in the usual workup process, to furnish secondary aromatic amines as the cross-coupled product with no silyl group on the nitrogen atom. A successful application to the preparation of regioselectively N-methylated azacalix[8]arene is described, together with the crystallographic analysis.  相似文献   
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