全文获取类型
收费全文 | 1688篇 |
免费 | 137篇 |
国内免费 | 238篇 |
专业分类
化学 | 2045篇 |
晶体学 | 3篇 |
物理学 | 15篇 |
出版年
2023年 | 18篇 |
2022年 | 19篇 |
2021年 | 32篇 |
2020年 | 62篇 |
2019年 | 44篇 |
2018年 | 50篇 |
2017年 | 39篇 |
2016年 | 53篇 |
2015年 | 55篇 |
2014年 | 84篇 |
2013年 | 177篇 |
2012年 | 73篇 |
2011年 | 89篇 |
2010年 | 88篇 |
2009年 | 95篇 |
2008年 | 77篇 |
2007年 | 110篇 |
2006年 | 104篇 |
2005年 | 90篇 |
2004年 | 117篇 |
2003年 | 106篇 |
2002年 | 147篇 |
2001年 | 57篇 |
2000年 | 63篇 |
1999年 | 35篇 |
1998年 | 37篇 |
1997年 | 29篇 |
1996年 | 22篇 |
1995年 | 22篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 10篇 |
1991年 | 4篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有2063条查询结果,搜索用时 0 毫秒
91.
The method for the synthesis of 4-aryl-2,6,6-trimethyl-5-oxo-5,6-dihydro-4H-thieno[3,2-b]pyrroles from accessible 4-aminothiophene derivatives was developed. 相似文献
92.
The conventional Gross reaction for the formylation of the tetrapropoxythiacalix[4]arene using TiCl4 affords the 18-(chloromethyl)-28-hydroxy-25,26,27-tripropoxythiacalix[4]arene substituted in the meta-position of the macrocycle. The p-tetraformyl-tetrapropoxythiacalix[4]arene, which is an interesting intermediate to the upper-rims functionalization of thiacalixarenes, was prepared with a very good yield using BuLi and N-formylpiperidine. 相似文献
93.
Mine Sulak Ak Hasalettin Deligöz 《Journal of inclusion phenomena and macrocyclic chemistry》2007,59(1-2):115-123
Four new azocalix[4]arenes {5,11,17,23-tetrakis[(2-hydroxy-5-tert-butylphenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (1), 5,11,17,23-tetrakis[(2-hydroxy-5-nitro phenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (2), 5,11,17,23-tetrakis[(2-amino-5-carboxylphenylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (3) and 5,11,17,23-tetrakis[(1-amino-2-hydroxy-4-sulfonicacidnapthylazo)]-25,26,27,28-tetrahydroxycalix[4]arene (4)} have been synthesized from p-tert-butylphenol, p-nitrophenol, p-aminobenzoic acid and 1-amino-2-hydroxy-4-sulphonic acid by diazo coupling reaction with p-aminocalix[4]arene. The resulting ligands (1–4) were treated with three transition metal salts (e.g., CuCl2·2H2O, NiCl2·6H2O or CoCl2·6H2O). Cu(II), Ni(II) and Co(II) complexes of the azocalix[4]arene derivatives were obtained and characterized by UV-vis, IR,
1H-NMR spectroscopic techniques and elemental analysis. All the complexes have a metal:ligand ratio of 2:1. The Cu(II) and
Ni(II) complexes of azocalix[4]arenes are square-planar, while the Co(II) complexes of azocalix[4]arenes are octahedral with
water molecules as axial ligands. The solvent extraction of various transition metal cations from the aqueous phase to the
organic phase was carried out by using azocalix[4]arenes (1–4). It was found that, azocalix[4]arenes 1, 2 and 3 examined selectivity for transition metal cations such as Ag+, Hg+ and Hg2+. In addition, the thermal stability of metal:azocalix[4]arene complexes were also reported.
Dedicated to Prof. Dr. Mustafa Yılmaz on the occasion of his 50th birthday 相似文献
94.
Claudia G. Avila-Ortiz 《Tetrahedron》2005,61(35):8372-8381
β2-(3,4-Dihydroxybenzyl)-β-alanine [β2-Homo-Dopa, 1] is a novel β-amino acid homologue of Dopa, the most successful therapeutic agent in the treatment of Parkinson's disease. Enantioenriched (R)-1 and (S)-1 were obtained via the diastereoselective alkylation of enantiopure pyrimidinone (R)- and (S)-3, chiral derivatives of β-alanine, with veratryl iodide. The major diastereomeric products (2S,5R)-4 and (2R,5S)-4 were hydrolyzed with 57% HBr, and the desired β-amino acids were purified by silica gel chromatography. Alternatively, enantioenriched (R)- and (S)-1 were prepared by means of the highly diastereoselective alkylation (3,4-dimethoxybenzyl iodide) of open-chain β-aminopropionic acid derivatives (R,R,S)-8 and (S,S,R)-8 containing the chiral auxiliary α-phenylethylamine. Finally, nearly enantiopure (R)- and (S)-1 were obtained by resolution of racemic N-benzyloxycarbonyl-2-(3,4-dibenzyloxybenzyl)-3-aminopropionic acid, rac-12, with (R)- or (S)-α-phenylethylamine, followed by catalytic hydrogenolysis. 相似文献
95.
4-Amino-5-(4,6-diphenyl-2-pyrimidinyl)-3,4-dihydro-2H-1,2,4-trazole-3-thione is formed from the reaction of 4,6-diphenylpyrimidinecarboxylic
acid or its ethyl ester with thiocarbonyl hydrazide. Alkylation of the product leads to S-alkyl derivaties or 6-substituted
3-(4,6-diphenyl-2-pyriimidinyl)-7H-1,2,4-triazolo[3,4-b]-1,3,4-thiadiazine. Acetylation of 4-amino-5-(4,6-diphenyl-2-pyrimidinyl)-3,4-dihydro-2H-1,2,4-triazole-3-thione
gave under different conditions monoacetyl-, diacetyl, and triacetyl derivatives at the amino group and the N(2) atom, whereas benzoylation gave a benzoyl group at the amino group and 3-(4,6-diphenyl-2-pyrimidinyl)-6-phenyl-1,2,4-triazolo[3,4-b]-1,3,4-thiadiazole.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, 1088–1094, July, 2007. 相似文献
96.
Condensation of 1-alkyl-, 1-allyl-, and 1-benzyl-1,2,3,9a-tetrahydro-9H-imidazo[1,2-a]indol-2-ones with benzaldehydes in acetic acid and subsequent treatment of the reaction mixture with potassium hydroxide afforded 1-substituted 9a-(2-phenylethenyl)-1,2,3,9a-tetrahydro-9H-imidazo[1,2-a]indol-2-one derivatives. 1-Methyl- and 1-ethyl-9a-[2-(4-dimethylaminophenyl)ethenyl]-1,2,3,9a-tetrahydro-9H-imidazo[1,2-a]indol-2-ones were synthesized by alkylation of 9a-[2-(4-dimethylaminophenyl)ethenyl]-1,2,3,9a-tetrahydro-9H-imidazo[1,2-a]indol-2-one with methyl- and ethyl iodides in DMF in the presence of a strong base. 相似文献
97.
Salts ofN-(β-hydroxyalkyl)-N′-hydroxydiazeneN-oxides, RCH(OH)CH2N(O)=NO− M+ (R=Me, Pri, or But; and M=Li, Na, K, Ag, NH4, or Me4N), were prepared. Their alkylation with alkyl halides R′X (X=Cl, Br, or I) and dimethyl sulfate was studied. Generally, alkylation
afforded mixtures ofN-(β-hydroxyalkyl)-N′-alkoxydiazeneN-oxides RCH(OH)CH2N(O)=NOR′ andO-alkyl-N-(β-hydroxyalkyl)-N-nitrosohydroxylamines RCH(OH)CH2N(NO)OR′.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1996–2001, October, 1998. 相似文献
98.
99.
Therapeutic potential of the D-ring methyl ethers of (–)-epigallocatechin-3-gallate [(–)-EGCG] warrants extensive structure–activity relationship study of various D-ring ethers of (–)-EGCG but, for this purpose, efficient synthetic strategy needs to be developed. In this study, efficient preparation of the 4″-O-alkyl-(–)-EGCGs (4a–e) was demonstrated using KI/K2CO3-promoted deacetylative alkylation of peracetyl (–)-EGCG, which could be broadly utilized for the preparation of various D-ring alkyl ethers of (–)-EGCG and thereby extensive structure–activity relationship study. 相似文献
100.
Owen Zacarias Prof. Ana G. Petrovic Prof. Dr. Rinat Abzalimov Prof. Padmanava Pradhan Prof. Elise Champeil 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(57):14263-14272
Mitomycin C, (MC), an antitumor drug, is a DNA alkylating agent currently used in the clinics. Inert in its native form, MC is reduced to reactive mitosenes, which undergo nucleophilic attack by guanine or adenine bases in DNA to form monoadducts as well as interstrand crosslinks (ICLs). Although ICLs are considered the most cytotoxic lesions, the role of each individual adduct in the drug's cytotoxicity is still not fully understood. Synthetic routes have been developed to access modified oligonucleotides containing dG MC-monoadducts and dG-MC-dG ICL at a single position of their base sequences to investigate the biological effects of these adducts. However, until now, oligonucleotides containing monoadducts formed by MC at the adenine base had not been available, thus preventing the examination of the role played by these lesions in the toxicity of MC. Here, we present a route to access these substrates. Structural proof of the adducted oligonucleotides were provided by enzymatic digestion to nucleosides and high-resolution mass spectral analysis. Additionally, parent oligonucleotides containing a dG monoadduct and a dG-MC-dG ICL were also produced. The stability and physical properties of all substrates were compared via CD spectroscopy and UV melting temperature studies. Finally, virtual models were created to explore the conformational space and structural features of these MC-DNA complexes. 相似文献