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991.
Reduction of a bis-radical precursor, 6-phenyl-1,1-bis[3-(phenylselanyl)propyl]-3a,4-dihydro-1H,3H-cyclopenta[c]furan-5-one ( 6 ), with 3 equiv. of Ph3SnH provides mixtures of cis,cis- or cis,trans-angular triquinane products (3aα,5aβ,8β,8aR*)- and (3aα,5aα,8β,8aR*)-hexahydro-3a-propyl-8-phenyl-5H-dicyclopenta[b,c]furan-7(8H)-one (cis,cis- 12 /cis,trans- 12 ), in yields that vary from 50%/50% to 91%/6% depending on the reaction concentration. A mechanistic model for this process is proposed that involves a non-selective phenylselenium-group abstraction step followed by successive kinetic resolutions of diastereoisomeric radical intermediates. This reaction shows how yields in group-selective reactions can be compounded to levels above that ostensibly permitted by the level of the group-selective step. 相似文献
992.
Cluster models of SnO2(110) face and oxygen vacancies and oxygen adsorption on its surface have been calculated by EHMO method. The results show that a tin atom with a coordination number of four is the adsorption center, because the total energy of cluster model becomes lower when an oxygen atom adsorpts on the tin atom with a coordination number of four. The tin atom with this coordination number gains and loses electrons more easily than tin atoms with a coordination number of five. All tin atoms in the cluster of SnO2(110) face are Sn4+. 相似文献
993.
This study presents a novel miniaturized Hadamard transform fluorescence imaging microscope, by combining a conventional fluorescence microscope with Hadamard transform multiplexing encoding using a one-dimensional movable mask to realize spatial resolution and a linear CCD for multichannel detection. The microscope can provide high-resolution automatically-generated 0–255 gray level HT images for morphological analysis and visualization of a single cell, and normalized HT images for cellular quantitative measurements. The microscopes imaging capability was applied to measure the DNA content in human lymphocyte, chicken erythrocyte and eel erythrocyte, and a comparative study was performed. The results show that the calibrated DNA content in a chicken erythrocyte is 2.32 pg when human lymphocyte is used as the standard, and eel erythrocyte may be a potentially reliable and novel standard for determining DNA contents in other species because it has a stable DNA value of 2.06 pg, with a CV of 4.3% when 20 eel erythrocytes are measured. The results also demonstrate that the HT imaging microscope should be valuable in the fields of medicine and cell biology. 相似文献
994.
995.
996.
Dysamide U,a new tricblorinated diketopiperanne(1)along with seven known ones(2-8)were isolated from the sponge of Dysidea sp.collected from the Pacific ocean.The structure of dysamide U(1)was elucidated by high resolution FARMS and 1H NMR data induding COSY and RCT-COSY. 相似文献
997.
Jian Gong SHI Han Qing Wang Min WANG Guang Xiong ZHOU. Yong Chun YANG Wen Yan HU 《中国化学快报》2000,11(3):225-228
In a previous paper' we reported two optically active pyridine-2, 6 (l H. 3H)-dionealkatoids speranskalines A and B from SPeranskia luberculala (Bge.) (Euphorbiaceae)We report here the isolation and structural elucidation of two novel poly-oxygenbipyridine alkaloids named speranculatines A (l) and B (2) from this plant.The acetone (or methanol) extract of the air-dried whole plants of .q. IllhelT.lllaluwas subjected to column chromatography on St gel to afford a fraction. whichdemonstrate… 相似文献
998.
A new way to Prepare ts-2 with Silica sol as Silicon Source 总被引:3,自引:0,他引:3
The addition of suitable amounts of aqueous ammonia into precursor gel with silica sol as silicon source, TS-2 with small particle size and high titanium content has been successfully synthesized. It displays the same excellent performance in hydroxylation of phenol as the sample prepared with TEOS as silicon source. 相似文献
999.
叙述了细胞凋亡概念及神经细胞凋亡的基本特征。细胞凋亡是迟发性神经元死亡的基本形式,细胞凋亡在短暂性局灶性脑缺血、短暂性全脑缺血及永久性脑缺血中随缺血时间再灌注时间的不同,细胞凋亡发生部位及时间、范围亦不相同。 相似文献
1000.
马红燕 《延安大学学报(自然科学版)》1999,18(1):48-51
本文用荧光光度法对酸性介质中甲氧氯普胺和十二烷基硫酸钠形成的胶束配合物的性能进行了系统研究。利用胶束的增敏增稳作用,提出了测定甲氧氯普胺的荧光光度新方法。本法线性范围宽,为1.3×10-8~8.0×10-6mol·L-1,灵敏度高,检出限为1.2×10-8mol·L-1。应用本法于样品中甲氧氯普胺的测定,平均回收率为98.8%,结果令人满意。 相似文献