全文获取类型
收费全文 | 106094篇 |
免费 | 17494篇 |
国内免费 | 10099篇 |
专业分类
化学 | 71363篇 |
晶体学 | 1218篇 |
力学 | 6920篇 |
综合类 | 533篇 |
数学 | 11909篇 |
物理学 | 41744篇 |
出版年
2024年 | 369篇 |
2023年 | 2133篇 |
2022年 | 3644篇 |
2021年 | 4052篇 |
2020年 | 4248篇 |
2019年 | 4137篇 |
2018年 | 3551篇 |
2017年 | 3286篇 |
2016年 | 5183篇 |
2015年 | 4947篇 |
2014年 | 6020篇 |
2013年 | 7778篇 |
2012年 | 9424篇 |
2011年 | 9754篇 |
2010年 | 6439篇 |
2009年 | 6155篇 |
2008年 | 6532篇 |
2007年 | 5996篇 |
2006年 | 5477篇 |
2005年 | 4592篇 |
2004年 | 3399篇 |
2003年 | 2667篇 |
2002年 | 2414篇 |
2001年 | 1979篇 |
2000年 | 1762篇 |
1999年 | 2071篇 |
1998年 | 1846篇 |
1997年 | 1720篇 |
1996年 | 1888篇 |
1995年 | 1502篇 |
1994年 | 1459篇 |
1993年 | 1165篇 |
1992年 | 1069篇 |
1991年 | 984篇 |
1990年 | 791篇 |
1989年 | 563篇 |
1988年 | 464篇 |
1987年 | 382篇 |
1986年 | 374篇 |
1985年 | 318篇 |
1984年 | 241篇 |
1983年 | 154篇 |
1982年 | 142篇 |
1981年 | 106篇 |
1980年 | 76篇 |
1979年 | 44篇 |
1978年 | 34篇 |
1976年 | 36篇 |
1975年 | 33篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
One‐pot reactions to produce 2,2′:6′,2″‐terpyridine (tpy) under mild conditions are described under both solventless and solvent‐assisted conditions. Tpy can be obtained in 32% yield in a simple one‐pot reaction, which can readily be scaled‐up to give large quantities of tpy. These new approaches are superior to those previously described because of the fast and efficient synthesis and purification of tpy. 相似文献
862.
A suitable procedure for convenient preparation of 1,3,5-tris(4-hydroxy-5-formylphenyl)benzene (6) was exploited via 5-bromosalicylaldehyde as starting reactant. Among the obtained products, compound 6, 4-methoxy-3-formylphenylboronic acid (9), 1,3,5-tris(4-methoxy-5-formylphenyl)benzene (10), and 4-methoxy-4′-hydroxyl-3,3′-diformyl-1,1′-diphenyl (11) had not been reported previously. Two new chiral Schiff base ligands, L1 and L2, were obtained from the tri- or disalicylaldehydes. 相似文献
863.
A simple, efficient, and general method has been developed for the synthesis of coumarin derivatives through a one‐pot reaction of aromatic aldehydes, 2,2‐dimethyl‐1,3‐dioxane‐4,6‐dione and 5,5‐dimethyl‐1,3‐cyclohexanedione, in the presence of TEBA under solvent‐free conditions using grinding methods. The mild reaction conditions, simple protocol, high yields, and cleaner reaction make this protocol practical and economically attractive. 相似文献
864.
In the presence of 1,8-diazabicycolo[5.4.0]undec-7-ene (DBU) and concentrated H2SO4, 2-naphthol reacted smoothly with α,β-unsaturated trifluoromethyl ketones in CH2Cl2 at room temperature, affording the 3-trifluoromethyl-substituted benzo[f]chromene derivatives in good to excellent yields in a one-pot reaction. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource: Full experimental and spectral details]. 相似文献
865.
A novel and environmentally benign synthetic method for ureas through hydrogen peroxide–promoted desulfurization of thioureas at room tepemperature via grinding is reported. As compared with the traditional method, this one is manipulatively simple and the oxidant is low cost, highly active, and environmentally friendly. 相似文献
866.
An efficient and environmentally benign method for the synthesis of organic carbamates was developed. Amines, CO2, and alkyl halides underwent a three-component reaction with the aid of K2CO3 and polyethylene glycol (PEG, MW = 400), affording the organic carbamates under ambient conditions. PEG could presumably act as a solvent and phase-transfer catalyst (PTC). Notably, the presence of PEG could also depress the alkylation of both the amine and the carbamate, thus resulting in enhanced selectivity toward the target carbamate. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
867.
A convenient one‐step synthesis of 5‐aryl uracils has been developed. The procedure involves heating ethyl 3‐hydroxy‐2‐arylpropenate with urea at 130°C, followed by base‐catalyzed cyclization. The method is simple and high yielding. 相似文献
868.
The C1–8 fragment of autolytimycin was synthesized via a reliable 10-step route capable of delivering 41% overall yield at multi-gram scale. As a key step, a chelation-controlled isopropenylation of α-oxygenated aldehydes was established with a reagent combination of diisopropenylzinc, magnesium halide, and a dichloromethane/toluene mixed solvent. Cram-chelate isopropenylation products dominated for aldehydes with a small α-substituents, such as –OMe and –OBn groups, while the Felkin product could be obtained with a bulky –OTBS group. 相似文献
869.
Yasushi Imada Takahiro Kitagawa Han-Kun Wang Naruyoshi Komiya Takeshi Naota 《Tetrahedron letters》2013,54(7):621-624
An aerobic, organocatalytic, and aqueous method for the oxidation of sulfides is described. Synthetic flavin, 5-ethyl-3,7,8,10-tetramethylisoalloxazinium perchlorate, acts as an efficient catalyst for the oxidation of sulfides in water under an oxygen atmosphere (1 atm) with the assistance of ascorbic acid as a reductant. This is an inexpensive, convenient, and environmentally benign method for the selective oxidative transformation of sulfides into sulfoxides. 相似文献
870.
An efficient and facile route to chiral dihydrodibenzothiazepines has been successfully developed via palladium-catalyzed asymmetric hydrogenation of the dibenzothiazepines with Brønsted acid as activator with up to 94% ee. 相似文献