全文获取类型
收费全文 | 27544篇 |
免费 | 1120篇 |
国内免费 | 1696篇 |
专业分类
化学 | 8582篇 |
晶体学 | 971篇 |
力学 | 647篇 |
综合类 | 220篇 |
数学 | 15409篇 |
物理学 | 4531篇 |
出版年
2023年 | 112篇 |
2022年 | 175篇 |
2021年 | 292篇 |
2020年 | 380篇 |
2019年 | 611篇 |
2018年 | 610篇 |
2017年 | 534篇 |
2016年 | 637篇 |
2015年 | 532篇 |
2014年 | 941篇 |
2013年 | 1984篇 |
2012年 | 894篇 |
2011年 | 2022篇 |
2010年 | 1760篇 |
2009年 | 1762篇 |
2008年 | 1874篇 |
2007年 | 1868篇 |
2006年 | 1527篇 |
2005年 | 1308篇 |
2004年 | 1189篇 |
2003年 | 1015篇 |
2002年 | 980篇 |
2001年 | 676篇 |
2000年 | 649篇 |
1999年 | 568篇 |
1998年 | 556篇 |
1997年 | 489篇 |
1996年 | 499篇 |
1995年 | 479篇 |
1994年 | 437篇 |
1993年 | 345篇 |
1992年 | 327篇 |
1991年 | 249篇 |
1990年 | 235篇 |
1989年 | 211篇 |
1988年 | 152篇 |
1987年 | 138篇 |
1986年 | 101篇 |
1985年 | 166篇 |
1984年 | 134篇 |
1983年 | 78篇 |
1982年 | 103篇 |
1981年 | 139篇 |
1980年 | 120篇 |
1979年 | 64篇 |
1978年 | 115篇 |
1977年 | 94篇 |
1976年 | 93篇 |
1975年 | 39篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
Yu Liang Wen-Xuan Wang Xing Wu Meng Wang Chao-Jun Pu Zheng-Hui Li Tao Feng Juan He Ji-Kai Liu 《Tetrahedron letters》2019,60(28):1838-1840
Panisuffrutin A (1), a highly degraded seco-triterpene derivative, together with the known palbinone, has been isolated from the whole plant Paeonia suffruticosa var. papaveracea (Andr.) Kerner. The structure with absolute configuration of 1 was determined via comprehensive NMR and MS analyses, as well as NMR and ECD calculations. A plausible biosynthetic pathway for 1 was proposed. Compound 1 showed weak cytotoxicity against Hela cancer cell line with an IC50 value of 26.2 μM, while palbinone exhibited a moderate inhibition on NO production with an IC50 of 18.3 μM. 相似文献
82.
Na-Na Jiang Shu-Zong Du Yan-Song Ye Hui Yan Xing-Wei Yang Jian Yang Yin Nian Gang Xu 《Tetrahedron letters》2019,60(45):151220
Hyphenone A (1), a new type of bicyclic polyprenylated acylphloroglucinols (BPAPs) featured with an unprecedented 3,3-diisoprenylated 1,3,5-trione core, were characterized from the roots of Hypericum henryi together with two new congeners (hyphenones B–C, 2–3) and five known biosynthetic related PPAPs. Biogenetically, 1 should be derived from a novel 3,3,5,5-tetraisoprenylated MPAPs precursor. Their structures were elucidated by comprehensive spectroscopic data and X-ray diffraction. Moreover, 1–3 were identified as the first PPAPs type Cav3.1 T-type calcium channel (TTCC) inhibitors, with IC50 values of 7.07, 6.19, and 5.47 μM, respectively. 相似文献
83.
Easy and effective preparation of new 1H-pyrrolizine carboxylates was achieved with high efficiency via KHMDS-induced carbocyclization of N-alkynyl proline carboxylates under substantially mild conditions. Meanwhile, some trans-diiodoallylic N-proline carboxylates were obtained from N-propargyl proline carboxylates using molecular I2 with or without KHMDS. This method is quite feasible in terms of practical and quick access to the pyrrolizines and their derivatives over the formation of carbanions. 相似文献
84.
Ganoderma mushrooms are widely used as effective medicines for treating and preventing chronic diseases. In this study, five new meroterpenoids, dayaolingzhiols A (1) and B (2) featuring a 6/6/6 ring system, dayaolingzhiols C?E (3–5) harboring a γ-lactone motif, along with eight known ones (6–13), were purified from G. lucidum. To clarify their chemical structures, spectroscopic and ECD and OR computational methods were used. In addition, the inhibition of all the new meroterpenoids against AChE was evaluated, disclosing that (+)-4 and (±)-5 possess potent AChE inhibitory activities with respective IC50 values of 8.52?±?1.90?μM and 7.37?±?0.52?μM. 相似文献
85.
Qamar Uddin Ahmed Abdul Hasib Mohd Ali Sayeed Mukhtar Meshari A. Alsharif Humaira Parveen Awis Sukarni Mohmad Sabere Mohamed Sufian Mohd. Nawi Alfi Khatib Mohammad Jamshed Siddiqui Abdulrashid Umar Alhassan Muhammad Alhassan 《Molecules (Basel, Switzerland)》2020,25(23)
In recent years, there is emerging evidence that isoflavonoids, either dietary or obtained from traditional medicinal plants, could play an important role as a supplementary drug in the management of type 2 diabetes mellitus (T2DM) due to their reported pronounced biological effects in relation to multiple metabolic factors associated with diabetes. Hence, in this regard, we have comprehensively reviewed the potential biological effects of isoflavonoids, particularly biochanin A, genistein, daidzein, glycitein, and formononetin on metabolic disorders and long-term complications induced by T2DM in order to understand whether they can be future candidates as a safe antidiabetic agent. Based on in-depth in vitro and in vivo studies evaluations, isoflavonoids have been found to activate gene expression through the stimulation of peroxisome proliferator-activated receptors (PPARs) (α, γ), modulate carbohydrate metabolism, regulate hyperglycemia, induce dyslipidemia, lessen insulin resistance, and modify adipocyte differentiation and tissue metabolism. Moreover, these natural compounds have also been found to attenuate oxidative stress through the oxidative signaling process and inflammatory mechanism. Hence, isoflavonoids have been envisioned to be able to prevent and slow down the progression of long-term diabetes complications including cardiovascular disease, nephropathy, neuropathy, and retinopathy. Further thoroughgoing investigations in human clinical studies are strongly recommended to obtain the optimum and specific dose and regimen required for supplementation with isoflavonoids and derivatives in diabetic patients. 相似文献
86.
87.
88.
89.
90.