全文获取类型
收费全文 | 2090篇 |
免费 | 32篇 |
国内免费 | 160篇 |
专业分类
化学 | 1061篇 |
晶体学 | 41篇 |
力学 | 19篇 |
综合类 | 31篇 |
数学 | 1055篇 |
物理学 | 75篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 11篇 |
2021年 | 3篇 |
2020年 | 20篇 |
2019年 | 28篇 |
2018年 | 31篇 |
2017年 | 45篇 |
2016年 | 15篇 |
2015年 | 17篇 |
2014年 | 62篇 |
2013年 | 229篇 |
2012年 | 55篇 |
2011年 | 113篇 |
2010年 | 133篇 |
2009年 | 127篇 |
2008年 | 133篇 |
2007年 | 151篇 |
2006年 | 131篇 |
2005年 | 145篇 |
2004年 | 137篇 |
2003年 | 106篇 |
2002年 | 76篇 |
2001年 | 36篇 |
2000年 | 63篇 |
1999年 | 41篇 |
1998年 | 40篇 |
1997年 | 28篇 |
1996年 | 51篇 |
1995年 | 32篇 |
1994年 | 44篇 |
1993年 | 28篇 |
1992年 | 26篇 |
1991年 | 11篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有2282条查询结果,搜索用时 15 毫秒
991.
Rosa M.F. Batista 《Tetrahedron letters》2004,45(13):2825-2828
Bithienyl-1,3-benzothiazole derivatives were synthesised by reacting various 5-formyl-5′-alkoxy- or 5-formyl-5′-N,N-dialkylamino-2,2′-bithiophenes with ortho-aminobenzenethiol in good to excellent yields. Evaluation of the fluorescence properties of these compounds was carried out. They show strong fluorescence in the 450-600 nm region, as well as high quantum yields and large Stokes’ shifts. 相似文献
992.
Flow injection methodology is described for the determination of proline in red and white wines using tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection. Selective conditions were achieved for proline at pH 10, while other amino acids and wine components did not interfere. The precision of the method was less than 1.00% (R.S.D.) for five replicates of a standard (4 × 10−6 M) and the detection limit was 1 × 10−8 M. The level of proline in white and sparkling wines using the developed methodology was equivalent to those achieved using HPLC-FMOC amino acid analysis. SPE removal of phenolic material was required for red wines to minimize Ru(bipy)33+ consumption and its associated effect on accuracy. 相似文献
993.
Michael reaction of 3-(2′-nitrovinyl)indole with eight 3-unsubstituted indoles on TLC-grade silica gel furnished unsymmetrical bis(indolyl)nitroethanes in 7-12 min under microwave irradiation and in 8-14 h at rt. In contrast, the p-TsOH-catalysed reaction of the nitrovinylindole with the 3-unsubstituted and two 3-substituted indoles in solution under reflux furnished both unsymmetrical and symmetrical bis(indolyl)nitroethanes, the latter resulting from novel tandem Michael addition-elimination-Michael addition reactions. The synthesis of a 2′,3″-bis(indolyl)nitroethane, the precursor core structure of two bioactive marine metabolites, and the reduction of 2,2-bis(3′-indolyl)nitroethane to the corresponding ethylamine, isolated as its N-acetyl derivative, have been achieved. Significantly, attempted hydrolysis of three nitronates, derived from the corresponding bis(indolyl)nitroethanes, with buffered aqueous TiCl3 has led to the first isolation of oximes (syn/anti-mixture) as the only products. 相似文献
994.
Oleg V. Gusev Tat’yana A. Peganova Nikolai V. Vologdin Konstantin A. Lyssenko Irina P. Beletskaya 《Journal of organometallic chemistry》2005,690(7):1710-1717
The monoxides [Fe(η5-C5Me4PPh2)(η5-C5Me4P{O}Ph2)] (1) and [Os(η5-C5H4PPh2)(η5-C5H4P{O}Ph2)] (2) have been prepared by treatment of the corresponding diphosphines with CCl4 and methanol.These ligands react with [Pd(PhCN)2Cl2] to give dichloride complexes of different structure.The dimeric complex [{Os(η5-C5H4PPh2)(η5-C5H4P{O}Ph2)}PdCl(μ-Cl)]2 (4) contains the monodentate P-coordinated osmocene ligand with the free P{O}Ph2 group, while the octamethylferrocene ligand gives the chelate k2-P,O complex [{Fe(η5-C5Me4PPh2)(η5-C5Me4P{O}Ph2)}PdCl2] (3).The structures of 3 and 4 have been determined crystallographically.Treatment of 3 and 4 with silver salts in CH2Cl2 or acetonitrile leads to the corresponding dicationic complexes[{M(η5-C5R4PPh2)(η5-C5R4P{O}Ph2)}Pd(MeCN)x]2+ (5, M = Fe, R = Me; 6, M = Os, R = H). Complex 5 decomposes upon isolation, in contrast 6 is rather stable, probably due to Os-Pd bonding. The dichlorides 3 and 4 catalyze catalytic amination of p-bromotoluene with N-(4-tolyl)morpholine with lower activity than (dppf)PdCl2, however they perform comparable to (dppf)PdCl2 activity in coupling of p-bromotoluene with p-methoxyphenyl boronic acid. 相似文献
995.
The solution structure of one of dithymidine monophosphate (TpT) analogues, containing an (N-acetyl)imino backbone linkage (NCOCH3) of 3′-O-(acetylimino)3′-de(phosphinico)-thymidylyl-(3,5′)-deoxythymidine (TNT), has been determined by proton NMR. Two structures, designated as major and minor forms, in a ratio of about 3:2 coexist when the solution temperature is <25°C. Both forms adopt anti conformation with respect to the glycosidic bond, S-type deoxyribofuranose pucker, and have no base stacking. The backbone torsion angles ε′, φON, φNC, and γ′ are trans, gauche+, gauche+, and gauche+ for the major form; and gauche−, gauche−, gauche−, and gauche+ for the minor form. Only major form is found at >25°C. 相似文献
996.
I. I. Chuev O. S. Filipenko S. M. Aldoshin O. A. Kozina 《Russian Chemical Bulletin》1994,43(11):1857-1859
Photochromic 1, 3, 3 -trimethyl-6-trifluoromethylsulfonyl-spiro(indoline-2,2-ben-zo[b]pyran) (1) was studied by X-ray diffraction analysis. In compound1, the Cspiro-O bond (1.49(1) Å (average)), broken on photoexcitation, is the longest of all the indoline spiropyrans studied.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1969–1971, November, 1994. 相似文献
997.
A single step synthesis of 2,3-dialkyl-6-nitro-quinazolin-4(3H)-imines and 3,5-dialkyl-9-nitro-imidazo-[1,2-c]-quinazolin-2(3H)-ones from simple carbonyl compounds, primary amines or amino acid methyl esters and 2-azido-5-nitro-benzonitrile was developed. Key intermediates were N,N′-disubstituted amidines obtained by rearrangement of 4,5-dihydrotriazoles; the new heterocyclic rings were formed by spontaneous intramolecular reaction of the amino and cyano groups which are present in the intermediates. 相似文献
998.
Ahmed E. -S. Abdel-Megied Erik B. Pedersen Carsten M. Nielsen 《Monatshefte für Chemie / Chemical Monthly》1991,122(1-2):59-70
Summary A modified synthesis of protected 2,3-dideoxyribose5 starting fromL-glutamic acid (1) is described. Reaction of5 with silylated 5-hydroxymethyluracil7 a and 5-alkoxymethyluracils7 b–e in the presence of trimethylsilyl triflate afforded an anomeric mixture of 2,3-dideoxyuridine derivatives8 a–e and9 a–e. Deprotection with methanolic ammonia and separation by chromatography gave the corresponding nucleosides10 a–e and11 a–e. Treatment of9 b–e with tri(1H-1,2,4-triazol-1-yl)phosphine oxide and subsequent reaction of12 b–e with ammonia in dioxane afforded the cytosine derivatives13 b–e which on treatment with methanolic ammonia gave the corresponding 2,3-dideoxycytidine derivatives14 b–e and15 b–e. In contrast with the parent compounds, these alkoxymethyl derivatives had no appreciable activity against human immunodeficiency virus (HIV-1).
Synthese von 2,3-Dideoxynucleosiden aus 5-Alkoxymethyluracilen
Zusammenfassung Ausgehend vonL-Glutaminsäure (1) wird eine modifizierte Synthese von geschützter 2,3-Dideoxyribose (5) beschrieben. Reaktion von5 mit silyliertem 5-Alkoxymethyluracilen7 b–e in Gegenwart von Trimethylsilyltriflat ergab anomere Mischungen der 2,3-Dideoxyuridinderivate8 a–e und9 a–e. Abspaltung der Schutzgruppe mit methanolischen Ammoniak und chromatographische Trennung ergab die entsprechenden Nucleoside10 a–e und11 a–e. Behandlung von9 b–e mit Tri(1H-1,2,4-triazol-1-yl)phosphinoxid und nachfolgende Reaktion von12 b–e mit Ammoniak in Dioxan ergab die Cytosinderivate13 b–e, welche nach Behandlung mit methanolischem Ammoniak die entsprechenden 2,3-Dideoxycytidinderivate14 b–e und15 b–e ergaben. Im Gegensatz zur Stammverbindung hatten diese Alkoxymethylderivate keine nennenswerte Wirksamkeit gegen den menschlichen Immunschwächevirus (HIV-1).相似文献
999.
The reaction of zinc enolates synthesized from 1-aryl-2,2-dibromoalkanones and zinc with 6-R-2-oxochromene-3-carboxylic acid N-benzylamide affords 8-R-9c-alkyl-1-aryl-2-benzyl-1-hydroxy-1,2,9b,9c-tetrahydro-5-oxa-2-aza-cyclopenta[2,3]cyclopropa[1,2-a]naphthalene-3,4-diones. Acylation of these compounds is accompanied by an unexpected rearrangement producing a sole geometrical isomer of 4′-alkyl-5′-aryl-1′-benzyl-3,4,2′,3′-tetrahydro-2,2′-dioxospiro[chroman-3,3′-pyrrol]-4-yl acetates. 相似文献
1000.
Naphtho[1,2-b]furans 1a-f, naphtho[2,1-b]furans 2a-f, benzo[1,2-b:5,4-b′]difurans 3a-b, benzo[1,2-b:4,5-b′]difurans 4a-b, and benzo[1,2-b:4,3-b′]difurans 5a-b were synthesized by base-catalyzed cyclization reaction of the corresponding o-alkoxybenzoylarene derivatives. The o-alkoxybenzoylarenes were obtained from the etherification reaction of the o-hydroxybenzoylarenes, which were prepared either by the reaction of methoxyarenes with benzoyl chloride in the presence of aluminum chloride or by photo-Fries rearrangement of aryl benzoates. 相似文献