全文获取类型
收费全文 | 336篇 |
免费 | 1篇 |
国内免费 | 1篇 |
专业分类
化学 | 284篇 |
晶体学 | 1篇 |
力学 | 5篇 |
数学 | 16篇 |
物理学 | 32篇 |
出版年
2016年 | 2篇 |
2014年 | 1篇 |
2013年 | 5篇 |
2012年 | 4篇 |
2011年 | 5篇 |
2010年 | 5篇 |
2009年 | 3篇 |
2008年 | 3篇 |
2007年 | 13篇 |
2006年 | 11篇 |
2005年 | 10篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 8篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 11篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 16篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 10篇 |
1974年 | 14篇 |
1973年 | 11篇 |
1972年 | 9篇 |
1971年 | 9篇 |
1970年 | 13篇 |
1969年 | 10篇 |
1968年 | 9篇 |
1967年 | 6篇 |
1966年 | 4篇 |
1965年 | 2篇 |
1962年 | 1篇 |
排序方式: 共有338条查询结果,搜索用时 281 毫秒
151.
Richard L. Tolman Glen L. Tolman Roland K. Robins Leroy B. Townsend 《Journal of heterocyclic chemistry》1970,7(4):799-806
The ribosylation of several silylated pyrrolo[2,3-d]pyrimidines by the Wittenberg procedure has produced 1,3 and 7-ribosylpyrrolo[2,3-d]pyrimidine derivatives in high yield. Structure assignments have been made on the basis of the ultraviolet spectra of model compounds and further confirmed by chemical conversion to derivatives of established structure. A convenient ribosylation procedure utilizing silver oxide, a halosugar, and a silylated pyrrolo[2,3-d]pyrimidine derivative in acetonitrile has been described. 相似文献
152.
Nabih S. Girgis Howard B. Cottam Steven B. Larson Roland K. Robins 《Journal of heterocyclic chemistry》1987,24(3):821-827
A number of 2,4-disubstituted pyrrolo[3,2-d]pyrimidine N-5 nucleosides were prepared by the direct glycosylation of the sodium salt of 2,4-dichloro-5H-pyrrolo[3,2-d]pyrimidine (3) using 1-chloro-2-deoxy-3,5-di-O-(p-toluoyl)-α-D -erythropentofuranose (1) and 1-chloro-2,3,5-tri-O-benzyl-α-D-arabinofuranose (11) . The resulting N-5 glycosides, 2,4-dichloro-5-(2-deoxy-3,5-di-O-(p-toluoyl) -β-D-erythropentofuranosyl)-5H-pyrrolo-[3,2-d]pyrimidine (4) and 2,4-dichloro-5-(2,3,5-tri-O-benzyl-β-D-arabinofuranosyl-5H -pyrrolo [3,2-d)pyrimidine (12) , served as versatile key intermediates from which the N-7 glycosyl analogs of the naturally occurring purine nucleosides adenosine, inosine and guanosine were synthesized. Thus, treatment of 4 with methanolic ammonia followed by dehalogenation provided the adenosine analog, 4-amino-5-(2-deoxyerythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidine (6) . Reaction of 4 with sodium hydroxide followed by dehalogenation afforded the inosine analog, 5-(2-deoxy-β-D-erythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidin-4(3H)-one (9) . Treatment of 4 with sodium hydroxide followed by methanolic ammonia gave the guanosine analog, 2-amino-5-(2-deoxy-β-D-erythropentofuranosyl) -5H-pyrrolo[3,2-d]pyrimidin-4(3H)-one (10) . The preparation of the same analogs in the β-D-arabinonucleoside series was achieved by the same general procedures as those employed for the corresponding 2′-deoxy-β-D-ribonucleoside analogs except that, in all but one case, debenzylation of the sugar protecting groups was accomplished with cyclohexene-palladium hydroxide on carbon, providing 4-amino-5-β-D-arabinofuranosyl-5H-pyrrolo [3,2-d]pyrimidin-4(3H)-one (18) . Structural characterization of the 2′-deoxyribonucleoside analogs was based on uv and proton nmr while that of the arabinonucleosides was confirmed by single-crystal X-ray analysis of 15a . The stereospecific attachment of the 2-deoxy-β-D-ribofuranosyl and β-D-arabinofuranosyl moieties appears to be due to a Walden inversion at the C1 carbon by the anionic heterocyclic nitrogen (SN2 mechanism). 相似文献
153.
Timothy A. Riley William J. Hennen N. Kent Dalley Bruce E. Wilson Roland K. Robins 《Journal of heterocyclic chemistry》1986,23(6):1709-1714
The synthesis of the first [1,3,5]triazine carbon linked nucleosides are reported. 4-Amino-6-(β-D-ribofuranosyl)[1,3,5]triazin-2(1H)-one ( 8 ), an analog of 5-azacytidine and pseudoisocytidine was prepared. 2,5-Anhydro-D-allonamidine hydrochloride ( 3 ) was condensed with dimethyl cyanoiminodithiocarbonate ( 4 ) to give 4-methylthio-6-(β-D-ribofuranosyl)[1,3,5]triazin-2-amine ( 5 ). Compound 5 was reacted with m-chloroperbenzoic acid to give 4-methylsulfinyl-6-(β-D-ribofuranosyl)[1,3,5]triazin-2-amine ( 6 ). Displacement of the methyl sulfinyl with the appropriate nucleophile gave 6-(β-D-ribofuranosyl)[1,3,5]triazine-2,4-diamine ( 7 ), 4-amino-6-(β-D-ribofuranosyl)[1,3,5]triazin-2(1H)-one ( 8 ), and 4-amino-6-(β-D-ribofuranosyl)[1,3,5]triazine-2(1H)-thione ( 9 ). Dethiation of compound 5 with Raney nickel gave 4-(β-D-ribofuranosyl)[1,3,5]triazin-2-amine ( 10 ). The crystal structure of 7 was determined by single crystal X-ray. 相似文献
154.
Protection (O5') of 2',3'-anhydroadenosine with tert-butyldiphenylsilyl chloride and epoxide opening with dimethylboron bromide gave the 3'-bromo-3'-deoxy xylo isomer which was treated with benzylisocyanate to give the 2'-O-(N-benzylcarbamoyl) derivative. Ring closure gave the oxazolidinone, and successive deprotection concluded an efficient route to 3'-amino-3'-deoxyadenosine. Analogous treatment of the antibiotic tubercidin [7-deazaadenosine; 4-amino-7-(beta-D-ribofuranosyl)pyrrolo[2,3-d]pyrimidine] gave 3'-amino-3'-deoxytubercidin. Trifluoroacetylation of the 3'-amino function, elaboration of the heterocyclic amino group into a (1,2,4-triazol-4-yl) ring with N,N'-bis[(dimethylamino)methylene]hydrazine, and nucleophilic aromatic substitution with dimethylamine gave puromycin aminonucleoside [9-(3-amino-3-deoxy-beta-D-ribofuranosyl)-6-(dimethylamino)purine] and its 7-deaza analogue. Aminoacylation [BOC-(4-methoxy-L-phenylalanine)] and deprotection gave puromycin and 7-deazapuromycin. Most reactions gave high yields at or below ambient temperature. Equivalent inhibition of protein biosynthesis in a rabbit reticulocyte system and parallel growth inhibition of several bacteria were observed with the 7-aza/deaza pair. Replacement of N7 in the purine ring of puromycin by "CH" has no apparent effect on biological activity. 相似文献
155.
156.
157.
158.
159.
160.