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71.
Annulation Approach to Doubly Linked (A‐type) Oligocatechins: Syntheses of (+)‐Procyanidin A2 and (+)‐Cinnamtannin B1
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Yuji Ito Prof. Dr. Ken Ohmori Prof. Dr. Keisuke Suzuki 《Angewandte Chemie (International ed. in English)》2014,53(38):10129-10133
The first stereoselective syntheses of doubly linked (A‐type) oligocatechins, (+)‐procyanidin A2 and (+)‐cinnamtannin B1, have been achieved. Ethylenedioxy‐bridged flavans served as excellent platforms, thus allowing annulation with nucleophilic catechin units in a stereoselective manner. An additional key was the new synthetic approach to selectively protected nucleophilic catechin, thus enabling regioselective construction of the key dioxabicyclo skeleton of the A‐type oligocatechins. 相似文献
72.
The photolysis of iron carbonyl (Fe(CO)5) adsorbed on titanium dioxide (TiO2, anatase) was studied by FT-IR spectroscopy. When adsorbed Fe(CO)5 is illuminated by visible and near-UV light, the IR spectrum of its photolysis products is hardly observed, indicating that most of the Fe(CO)5 is photodecomposed to iron(0) or iron oxides on TiO2. The carbon monoxide (CO) evolution rate upon illumination depends on the wavelength of light; 433 nm light is more effective for CO evolution than 366 nm light. This result implies that the band-gap excitation of TiO2 has little effect on the photolysis of adsorbed Fe(CO)5, since the absorption edge of TiO2 (anatase) lies at around 400 nm. The effects of substrates on the photolysis of adsorbed Fe(CO)5 are discussed with reference to previous results obtained for aluminium oxide (Al2O3) and silicon dioxide (SiO2), on which the photolysis leads to the formation of Fe2(CO)9 or Fe3(CO)12. 相似文献
73.
74.
S. Tomiyoshi H. Yoshida H. Ohmori T. Kaneko H. Yamamoto 《Journal of magnetism and magnetic materials》1987,70(1-3):247-248
Temperature variations of resistivities are measured on the single crystals of Mn3Sn with a hexagonal crystal structure. Anomalies of resistivity along the c-axis are observed at 230 and 420 K which correspond to the transition temperature from a tringle-helix to triangular spin structure state and the Néel temperature, respectively. 相似文献
75.
76.
The aim of this study was to develop a practical route for preparing a fluorine-18 ([18F]) labelled ligand ([18F]1) containing [18F]fluorobenzene ring by employing the reaction of diphenyliodonium salt with [18F]F−. Diphenyliodonium tosylate (2) was synthesized from tributylphenylstannyl compound (6) with [hydroxy(tosyloxy)iodo]benzene (7). Using this method, [18F]DAA1106 ([18F]3a), a positron emission tomography ligand for imaging peripheral-type benzodiazepine receptor, was prepared. 相似文献
77.
Tamiya M Ohmori K Kitamura M Kato H Arai T Oorui M Suzuki K 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(35):9791-9823
A general approach to the regio- and stereoselective total synthesis of the benanomicin-pradimicin antibiotics (BpAs) is described. Construction of the aglycon has been achieved by 1) the diastereoselective ring-opening of a biaryl lactone by using (R)-valinol as a chiral nucleophile and 2) the stereocontrolled semi-pinacol cyclization of the aldehyde acetal by using SmI(2) in the presence of BF(3)OEt(2) and a proton source to afford the ABCD tetracyclic monoprotected diol. This strategy enabled us to control the two stereogenic sites in the B ring (C-5 and C-6) and the regioselective introduction of the carbohydrate moiety. The ABCD tetracycle could serve as an ideal platform for the divergent access to various BpAs. The amino acid (D-alanine) was introduced onto the ABCD tetracycle. Glycosylation was promoted by the combination of Cp(2)HfCl(2) and AgOTf (1:2 ratio). Construction of the E ring followed by deprotection completed the first total synthesis of benanomicin A (2 a), benanomicin B (2 b), and pradimicin A (1 a). The route is flexible enough to allow the synthesis of other congeners differing in their amino acid and carbohydrate moieties. 相似文献
78.
Characterization and mechanistic study of a radical SAM dehydrogenase in the biosynthesis of butirosin 总被引:1,自引:0,他引:1
Yokoyama K Numakura M Kudo F Ohmori D Eguchi T 《Journal of the American Chemical Society》2007,129(49):15147-15155
BtrN encoded in the butirosin biosynthetic gene cluster possesses a CXXXCXXC motif conserved within the radical S-adenosyl methionine (SAM) superfamily. Its gene disruption in the butirosin producer Bacillus circulans caused the interruption of the biosynthetic pathway between 2-deoxy-scyllo-inosamine (DOIA) and 2-deoxystreptamine (DOS). Further, in vitro assay of the overexpressed enzyme revealed that BtrN catalyzed the oxidation of DOIA under the strictly anaerobic conditions along with consumption of an equimolar amount of SAM to produce 5'-deoxyadenosine, methionine, and 3-amino-2,3-dideoxy-scyllo-inosose (amino-DOI). Kinetic analysis showed substrate inhibition by DOIA but not by SAM, which suggests that the reaction is the Ordered Bi Ter mechanism and that SAM is the first substrate and DOIA is the second. The BtrN reaction with [3-2H]DOIA generated nonlabeled, monodeuterated and dideuterated 5'-deoxyadenosines, while no deuterium was incorporated by incubation of nonlabeled DOIA in the deuterium oxide buffer. These results indicated that the hydrogen atom at C-3 of DOIA was directly transferred to 5'-deoxyadenosine to give the radical intermediate of DOIA. Generation of nonlabeled and dideuterated 5'-deoxyadenosines proved the reversibility of the hydrogen abstraction step. The present study suggests that BtrN is an unusual radical SAM dehydrogenase catalyzing the oxidation of the hydroxyl group by a radical mechanism. This is the first report of the mechanistic study on the oxidation of a hydroxyl group by a radical SAM enzyme. 相似文献
79.
In previous studies, xylem parenchyma cells (XPCs) in the boreal softwood species larch, which has thick and rigid walls similar to those of XPCs in boreal hardwood species, were shown to respond to subfreezing temperature by deep supercooling during summer but change their freezing behavior to extracellular freezing during winter. In this study, we re-examined freezing behavior of XPCs in larch by observation of deep etching of frozen samples as well as observation of re-warmed samples after freezing using a cryo-scanning electron microscope. The results showed that XPCs in larch adapts to subfreezing temperature by deep supercooling throughout all seasons. Such freezing behavior is the same as that of XPCs in boreal hardwood species. 相似文献
80.
Dr. Hiroshi Takikawa Dr. Arata Nishii Dr. Hiromu Takiguchi Hirotoshi Yagishita Masato Tanaka Dr. Keiichi Hirano Prof. Dr. Masanobu Uchiyama Prof. Dr. Ken Ohmori Prof. Dr. Keisuke Suzuki 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(30):12540-12544
An intramolecular benzyne–phenolate [4+2] cycloaddition is reported. Benzyne precursors, having vicinal halogen-sulfonate functionalities, linked with a phenol(ate) by various tether groups undergo efficient intramolecular [4+2] cycloaddition by treatment with either Ph3MgLi or nBuLi for halogen–metal exchange to form various benzobarrelenes. 相似文献