全文获取类型
收费全文 | 1473篇 |
免费 | 55篇 |
国内免费 | 4篇 |
专业分类
化学 | 1161篇 |
晶体学 | 19篇 |
力学 | 13篇 |
数学 | 106篇 |
物理学 | 233篇 |
出版年
2023年 | 12篇 |
2022年 | 11篇 |
2021年 | 16篇 |
2020年 | 27篇 |
2019年 | 36篇 |
2018年 | 24篇 |
2017年 | 10篇 |
2016年 | 23篇 |
2015年 | 35篇 |
2014年 | 42篇 |
2013年 | 71篇 |
2012年 | 96篇 |
2011年 | 104篇 |
2010年 | 50篇 |
2009年 | 60篇 |
2008年 | 123篇 |
2007年 | 88篇 |
2006年 | 91篇 |
2005年 | 77篇 |
2004年 | 84篇 |
2003年 | 70篇 |
2002年 | 66篇 |
2001年 | 33篇 |
2000年 | 28篇 |
1999年 | 10篇 |
1998年 | 12篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 16篇 |
1985年 | 14篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1937年 | 3篇 |
排序方式: 共有1532条查询结果,搜索用时 31 毫秒
21.
Studies on the constituents of Anaxagorea luzonensis A. GRAY 总被引:2,自引:0,他引:2
Five new xanthones, 1,3,6-trihydroxy-5-methoxy-4-prenylxanthone (1), 1,3,5-trihydroxy-6-methoxy-2-prenylxanthone (2), 1,3,5-trihydroxy-4-(3-hydroxy-3-methylbutyl) xanthone (3), 1,3,6-trihydroxy-4-prenylxanthone (4), 3,6-dihydroxy-1,5-dimethoxyxanthone (5) and one new flavonoid, 3,5,7,4'-tetrahydroxy-2'-methoxyflavone (6) along with seven known xanthones and seven known flavonoids were isolated from the bark of Anaxagorea luzonensis A. GRAY and their chemical structures were determined by means of chemical and spectral studies. Almost all flavonoids and one xanthone (13) showed antioxidant activity. 相似文献
22.
Hasegawa E Takizawa S Iwaya K Kurokawa M Chiba N Yamamichi K 《Chemical communications (Cambridge, England)》2002,(17):1966-1967
Photoinduced electron transfer reaction of 2-bromomethyl-2-(3-butenyl)benzocyclic-1-alkanones with amines afforded 5-exo radical cyclization products while electron transfer reaction with samarium diiodide produced cyclopropanols. 相似文献
23.
Kimura T Takahashi S Akiyama S Uzawa T Ishimori K Morishima I 《Journal of the American Chemical Society》2002,124(39):11596-11597
The helix formation dynamics of poly-L-glutamic acids (PGAs) were observed by the microsecond-resolved Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopies. The helix formation of 34-residue PGA from random coil at pH (or pD for FTIR) 8.0 was initiated by a pH jump to 4.9 using the rapid solution mixer whose mixing dead time is 50 micros. The amide I' line in the time-resolved FTIR spectra exhibited the fast (<100 micros) increase of the total helical content. The time-resolved CD spectra of the same process also showed the fast (<150 micros) formation of short helical segments (5 +/- 1 residues), which was followed by the slower (<1 ms) elongation of the short helices to longer helices (>10 residues). Similar dynamics were observed for the same pH jump of approximately 190-residue PGA, although there were additional steps that made the helix formation of approximately 190-residue PGA more complex. The observed multistep helix formation is likely caused by the strong hydrogen-bonding interactions between the protonated side chains of PGAs. 相似文献
24.
Otaka A Watanabe J Yukimasa A Sasaki Y Watanabe H Kinoshita T Oishi S Tamamura H Fujii N 《The Journal of organic chemistry》2004,69(5):1634-1645
A samarium diiodide (SmI(2))-mediated reduction of gamma,gamma-difluoro-alpha,beta-enoates (15, 29, and 34) was successfully applied to the synthesis of (Z)-fluoroalkene dipeptide isosteres (23, 30, and 35), which have served as potential dipeptide mimetics. Reduction of the gamma,gamma-difluoro-alpha,beta-enoates by SmI(2) proceeded via successive two-electron transfers to form dienolate species which upon kinetically controlled trapping with t-BuOH yielded Xaa-Gly-type fluoroalkene isosteres exemplified by 23, 30, and 35. Replacement of the t-BuOH kinetic trapping agent with aldehydes or ketones provided access to alpha-substituted fluoroalkene isosteres (43 and 45) through aldol reactions of Sm-dienolates with the carbonyl compounds. Of particular note, the use of the SmI(2)-HCHO reagent system with chiral enoate 34 provided D-Phe-psi[(Z)-CF[double bond]CH]-D/L-Ser isosteres (45), which could be converted to enantiomerically pure isosteres (49-52) that bore a variety of side chain functionalities at the alpha-position. This was achieved by a sequence of manipulations consisting of beta-lactone formation followed by chromatographic separation and ring-opening with soft nucleophiles. Included in the present work is the first utilization of a Rh-catalyzed Reformatsky reaction of chiral imines for the stereoselective preparation of alpha,alpha-difluoro-beta-amino acid derivatives (28 and 33). The appropriate choice of reagents (carbonyl compounds for kinetic trapping or ring-opening nucleophiles and imines for Reformatsky reactions) allows the presented methodology to yield various fluoroalkene isosteres possessing a wide range of side chain functionalities. 相似文献
25.
Oishi S Kamano T Niida A Odagaki Y Tamamura H Otaka A Hamanaka N Fujii N 《Organic letters》2002,4(7):1051-1054
[reaction: see text] Acyclic psi[(E)-CH=CMe]- and psi[(Z)-CH=CMe]-type dipeptide isosteres were efficiently synthesized. In a key reaction, alpha-alkylation of gamma-mesyloxy-beta-methyl-alpha,beta-unsaturated esters with organocyanocuprates in diethyl ether or tetrahydrofuran preferentially afforded the psi[(E)-CH=CMe]- or psi[(Z)-CH=CMe]-isomer, respectively, via anti-S(N)2' mechanism. 相似文献
26.
Manabe K Iimura S Sun XM Kobayashi S 《Journal of the American Chemical Society》2002,124(40):11971-11978
Dehydration reactions in water have been realized by a surfactant-type catalyst, dodecylbenzenesulfonic acid (DBSA). These reactions include dehydrative esterification, etherification, thioetherification, and dithioacetalization. In these reactions, DBSA and substrates form emulsion droplets whose interior is hydrophobic enough to exclude water molecules generated during the reactions. Detailed studies on the esterification revealed that the yields of esters were affected by temperature, amounts of DBSA used, and the substrates. Esters were obtained in high yields for highly hydrophobic substrates. On the basis of the difference in hydrophobicity of the substrates, unique selective esterification and etherification in water were attained. Furthermore, chemospecific, three-component reactions under DBSA-catalyzed conditions were also found to proceed smoothly. This work not only may lead to environmentally benign systems but also will provide a new aspect of organic chemistry in water. 相似文献
27.
Kuroda N Sako A Ohyama K Akiyama S Nakashima K 《Journal of chromatographic science》2005,43(3):116-120
Modified 3-aminopropylsilyl silica stationary phases with 1,8-naphthalic anhydrides [3-(1,8-naphthalimido)propyl (NAIP), 3-(4-amino-1,8-naphthalimido)propyl (4A-NAIP), 3-(3-nitro-1,8-naphthalimido)propyl (3N-NAIP), and 3-(4-nitro-1,8-naphthalimido)propyl modified silyl silica gel (4N-NAIP)] are prepared in a single reaction. Chromatographic properties of these phases are characterized using various solutes including monosubstituted benzenes, alkyl benzenes, and polycyclic aromatic hydrocarbons (PAHs) in high-performance liquid chromatography. Even though the retention in these NAIPs is mainly based on the reversed-phase mode, 3N- and 4N-NAIP show a remarkable recognition toward aromatic hydrocarbons, which is attributed to their strong pi-pi interaction. Furthermore, the recognition ability is according to the molecular planarity and non-linearity of PAHs. Subsequently, 3N- and 4N-NAIP has a distinct selectivity from octadecyl silica and, therefore, will be a valuable alternative for a separation or a solid-phase extraction of aromatic compounds. 相似文献
28.
Yoshinori Tominaga Hiroto Okuda Shinya Kohra Hisako Mazume 《Journal of heterocyclic chemistry》1991,28(5):1245-1255
Reaction of various types of enaminones, which are prepared by the condensation of 1,3-dicarbonyl compounds with aromatic amines, with carbon disulfide in the presence of sodium hydroxide as the base in dimethyl sulfoxide to give the corresponding enamino dithiocarboxylates, 1,3-thiazines and trithiones. Enamino dithiocarboxylates are cyclized under refluxing in diphenyl ether to give the fused quinoline derivatives. The reaction of 6-arylamino-1,3-dimethyluracils with excess carbon disulfide in the presence of sodium hydroxide and subsequent methylation with dimethyl sulfate gave directly the corresponding 1,3-dimethyl-5-methylthiopyrimido[4,5-b]quinoline-2,4(1H,3H)-diones. 相似文献
29.
Matsuura K Ikoma S Sugiyama M Funauchi M Sinohara H 《Applied biochemistry and biotechnology》2000,83(1-3):107-114
Polyclonal Immunoglobulin (Ig) G from patients with rheumatoid arthritis (RA) and healthy subjects hydrolyzed carbobenzoxy−Val−Gly−Arg
p-nitroanilide and D−Pro−Phe−Arg p-nitroanilide. RA IgG exhibited higher activity against the former substrate, but not the latter. On the other hand, RA IgG
showed reduced activity against D−Pro−Phe−Arg methylcoumarinamide, when compared with those of the healthy controls. These
results suggest that RA IgGs differ from normal IgGs in the substrate specificity of amidase activity. Preliminary studies
have shown that two out of three RA IgG samples cleaved a pentapeptide—Gln−Arg−Arg−Arg−Ala−Ala— which is assumed to be associated
with the risk of developing RA (Gregersen, P. K. et al. (1987), Arthritis Rheum.
30, 1205–1213). By contrast, virtually no cleavage of the same peptide was observed with IgG from healthy controls. A peptide
analog, Gln−Arg−Arg−Trp−Ala, was not cleaved at all by any IgGs examined either from RA patients or healthy controls. 相似文献
30.
Hiroyuki Ishibashi Motofumi Okada Atsuko Akiyama Kazuyuki Nomura Masazumi Ikeda 《Journal of heterocyclic chemistry》1986,23(4):1163-1166
2,3-Dihydro-5H-1,4-benzoxathiepins were prepared by intramolecular Friedel-Crafts reactions of ethyl α-[2-(aryloxy)ethylthio]-α-chloroacetates or by acid-catalyzed cyclizations of ethyl α-[2-(aryloxy)ethylsulfinyl]-acetate. 1,2,3,5-Tetrahydro-4,1-benzothiazepines were similarly prepared. 相似文献