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排序方式: 共有379条查询结果,搜索用时 31 毫秒
91.
92.
Takamasa Tanabe 《Tetrahedron letters》2004,45(17):3477-3480
Verbenachalcone 1, isolated from the aerial part of Verbena littoralis H. B. K. (Verbenaceae), has successfully been synthesized by employing anodic oxidation of the phenol derivative 3, as the key step. The bromine and chlorine substituents of 3 effected regioselective introduction of a methoxy group, as well as modulating the oxidation reaction. A flow-cell type apparatus of anodic oxidation provided the desired diaryl ether 4 in good yield. 相似文献
93.
In order to trap the carbocation intermediates formed during the squalene cyclization cascade, squalene analogs with threo- and erythro-diols at the 6,7- and 10,11-positions were incubated with the recombinant squalene cyclase from Alicyclobacillus acidocaldarius, leading to the construction of the triterpenes with tetrahydropyran, octahydrochromene, decahydronaphthalene with a carbonyl group, dodecahydrobenzo[f]chromene, tetradecahydronaphtho[2,1-b]oxepine and malabaricane skeletons, almost of which are novel compounds. These products indicate that 6-membered monocyclic, 6/6-fused bicyclic and 6/6/5-fused tricyclic cations were involved in the cyclization reaction in addition to acyclic cation. All the trapped cations were the stable tertiary cation, but not the secondary one, indicating that the polycyclization reaction proceeds with a Markovnikov closure. The product profiles revealed that the cyclization reactions proceeded with the product and substrate specificities in addition to enantioselectivity. Mechanistic insight into the observed stereochemical specificities indicated that the pre-organized chair-conformation of squalene-diols is tightly constricted by the cyclase and a free motion or a conformational change is not allowed in the reaction cavity, thus, the substrate and product specificities are dominantly directed by the least motion of the nucleophilic hydroxyl group toward the intermediary carbocation; a small rotation of the hydroxyl group afforded the cyclization products in a good yield, but a large rotation of the hydroxyl group gave a marginal or no detectable amount of products. 相似文献
94.
Minoru Imoto Takamasa Maeda Tatsuro Ouchi 《Journal of polymer science. Part A, Polymer chemistry》1979,17(2):385-392
Benzaldehyde (PhCHO) is found to be able to initiate the radical polymerization of methyl methacrylate (MMA). The rate of polymerization is expressed by the following equation: Rp = const[PhCHO]0.5[MMA]1.5. The overall activation energy is estimated to be 56.3 kJ mole?1. The mechanism of polymerization is discussed. 相似文献
95.
Seiji Kurihara Masaki Moritsugu Sun-nam Kim Takamasa Nonaka 《European Polymer Journal》2007,43(12):4951-4960
A photochemically tunable photonic crystal was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO2 inverse opal structure. The SiO2 inverse opal film infiltrated with the polymer LC reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the reflection band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anisotropic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable not only for various optical materials, but also for introduction of defects in the photonic crystals. 相似文献
96.
Matsunaga S Yoshida T Morimoto H Kumagai N Shibasaki M 《Journal of the American Chemical Society》2004,126(28):8777-8785
Full details of a direct catalytic asymmetric Mannich-type reaction of a hydroxyketone using a Et2Zn/(S,S)-linked-BINOL complex are described. By choosing the proper protective groups on imine nitrogen, either anti- or syn-beta-amino alcohol was obtained in good diastereomeric ratio, yield, and excellent enantiomeric excess using the same zinc catalysis. N-Diphenylphosphinoyl (Dpp) imine 3 gave anti-beta-amino alcohols in anti/syn = up to >98/2, up to >99% yield, and up to >99.5% ee, while Boc-imine 4 gave syn-beta-amino alcohols in anti/syn = up to 5/95, up to >99% yield, and up to >99.5% ee. The high catalyst turnover number (TON) is also noteworthy. Catalyst loading was successfully reduced to 0.02 mol % (TON = up to 4920) for the anti-selective reaction and 0.05 mol % (TON = up to 1760) for the syn-selective reaction. The Et2Zn/(S,S)-linked-BINOL complex exhibited far better TON than in previous reports of catalytic asymmetric Mannich-type reactions. Mechanistic studies to clarify the reason for the high catalyst efficiency as well as transformations of Mannich adducts are also described. 相似文献
97.
N(G)-Monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (ADMA), and N(G),N(G)'-dimethyl-L-arginine (SDMA) are emerging cardiovascular risk factors. A high-performance liquid chromatographic method with fluorescence detection for the simultaneous determination of L-NMMA, ADMA and SDMA is described. The assay employed 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) as a fluorescent derivatization reagent. After solid phase extraction with cation-exchange column, the methylated arginines were converted to fluorescent derivatives with NBD-F, and the derivatives were separated within 32 min on a reversed-phase column. Nomega-Propyl-L-arginine was Used as an internal standard. Extrapolated detection limits were 12 nM (12 fmol per injection) for L-NMMA and 20 nM (20 fmol per injection) for ADMA and SDMA, respectively, with a signal-to-noise ratio of 3. The calibration curves for L-NMMA, ADMA and SDMA were linear within the range of 50-5000 fmol. The method was applied to the quantitative determination of L-NMMA, ADMA and SDMA in 200 microl of rat plasma. The concentrations of L-NMMA, ADMA and SDMA in rat plasma were 0.16 +/- 0.03, 0.80 +/- 0.25 and 0.40 +/- 0.21 microM, respectively (n = 5). 相似文献
98.
Takamasa Ishigaki Shigeru Yamauchi Junichiro Mizusaki Kazuo Fueki Hiroyuki Naito Tatsuya Adachi 《Journal of solid state chemistry》1984,55(1):50-53
The tracer diffusion coefficient, , of oxide ions in LaFeO3 single crystal was determined over the temperature range of 900–1100°C by the gas-solid isotopic exchange technique using 18O as a tracer. For the determination of , the depth profile of 18O was measured by means of a secondary ion mass spectrometer (SIMS). The surface exchange reaction was found to be slow and the surface exchange rate constant, k, was determined together with . It was found that at 950°C is proportional to P?0.58O2, where PO2 is an oxygen pressure. The vacancy mechanism was determined for the diffusion of oxide ions from the PO2 dependence. The vacancy diffusion coefficient, DV, for LaFeO3 was nearly the same as that for LaCoO3 at the same temperature. The activation energy for migration of oxide ion vacancies was 74 kJ · mole?1 for both oxides. 相似文献
99.
Takamasa Nonaka Rikiichi Tagawa Haruo Nishida Hiroaki Egawa 《Journal of polymer science. Part A, Polymer chemistry》1984,22(12):3815-3825
Water-solube polymer (PST) containing triethylenetetramine side chain was prepared by the amination of chloromethylated polystyrene with triethylenetetramine in 1,4-dioxane. The polymerization of vinyl monomers was carried out in the water–organic solvent system containing PST and a very small amount of peroxide. The polymerization of methyl methacrylate proceeded smoothly in the presence of both peroxide and PST. It was found that the polymerization was initiated with the radicals generated by the interaction between hydroperoxide and amino groups of PST. 1,4-Dioxane hydroperoxide showed a high activity for the polymerization of methyl methacrylate. The maximum rate of the polymerization was observed at 60°C and in an approximately neutral solution. The addition of suitable amount of Cu(II) accelerated the polymerization of methyl methacrylate. The selective polymerization of vinyl monomers was observed in this system. 相似文献
100.
Masayuki Kitamura Noritaka Usami Takamasa Sugawara Kenrato Kutsukake Kozo Fujiwara Yoshitaro Nose Toetsu Shishido Kazuo Nakajima 《Journal of Crystal Growth》2005,280(3-4):419-424
The floating zone technique was employed to grow multicrystalline Si with controlled grain boundary configuration. Purposely designed bi-crystals were utilized as seed crystals to investigate the effect of the tilt angle from the perfect twin boundary on the growth behavior. When the growth was initiated from a bi-crystal with a Σ3 twin boundary, no particular change took place on the grain boundary configuration during growth. On the other hand, the decrease of the tilt angle during growth was observed when the growth was initiated from a bi-crystal with a tilted boundary from Σ3. This was accompanied by the appearance of new crystal grains. The reduction of the total interface energy would be a possible driving mechanism for this phenomenon. 相似文献