Tasumatrols P – T,Five New Taxoids from Taxus sumatrana

The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells.     

Mathematical Correction for Polyatomic Isobaric Spectral Interferences in Determination of Lanthanides by Inductively Coupled Plasma Mass Spectrometry

The determination of lanthanides by Inductively Coupled Plasma Mass Spectrometry (ICP‐MS) is complicated by several spectral overlaps from M⁺, MO⁺ or MOH⁺ ions formed in the ICP. Especially, it is essential to avoid the spectral interferences from lighter lanthanide and Ba polyatomic ions on middle or heavier lanthanides. To tackle this problem, we have developed a mathematical correction method, which reduces all the spectral overlaps from oxide species of Pr, Nd, Ce and Sm over Gd, Tb, Dy and Ho, and Gd, Tb over Yb and Lu. It can also successfully correct the oxide and hydroxide interference of Ba over Eu. The effectiveness of the proposed the mathematical correction scheme is demonstrated for the USGS Standard Rock samples AGV‐1 and G‐2. The results show that the experimental data obtained by applying the mathematical correction scheme for lanthanides is in good agreement with the reported values, using pneumatic and ultrasonic nebulisation methods, for their ICP‐MS analysis.     

Two new compounds derived from bufalin

The biotransformation of bufalin by cell suspension cultures of Platycodon grandiflorus was investigated and two new biotransformed products were obtained,which was 3-epi-telocinobufagin and 3-epi-bufalin-3-O-β-D-glucoside.     

New azabenzoquinones by ring‐expansion reactions

Ozonolysis of the pyrrolidinediones 4 afforded the pyrrolidinetriones 5 , which in the presence of Lewis acids were converted into maleimide 6 . Analogously, ozonolysis of the pyrrolidinones 7 gave the pyrrolidinediones 8 , which were converted into the pyridinetriones 11a, b via Lewis acid catalyzed isomerization to yield the trihydroxypyridones 10 and ensuing air oxidation. In solution two tautomeric forms of the pyridinetriones 11 may exist both of which represent hydroxy‐azabenzoquinones. In two steps compounds 11 were transformed into the azaquinone derivatives 19 . Representatives of another type of azaquinones are compounds 28a, b. These were generated in two steps from the pyridones 25 . The azaquinone 28a reacted easily with acidic compounds yielding the adducts 26, 27 and 29 or with 2‐butenal forming the cycloadduct 30 .     

某些子群介于正规与反正规之间的有限群北大核心CSCD

有限群G的子群H称为G的BNA子群,若对任意的x∈G有H^(x)=H或x∈.若有限群G的所有素数阶和4阶循环子群都是G的BNA子群,则称G为CBNA群.本文主要刻画CBNA群的结构,并且给出所有真子群都是CBNA群的完全分类.     

主动冷却点阵夹层防热结构温度响应计算模型北大核心CSCD

针对点阵夹层结构主动热防护问题,建立了夹层结构面板和芯体导热与冷却剂对流耦合的非稳态传热理论模型,利用有限体积法离散控制方程并在MATLAB中进行了迭代求解.模型首次考虑了面板与夹芯杆之间的收缩热阻,并利用分离变量法得到了收缩热阻的近似解析解.基于单胞模型和周期性边界条件,模拟得到了模型所需的表面对流传热系数h_(b)和h_(fin).最后,选取多单胞计算工况进行数值模拟和理论模型对比,并讨论了收缩热阻对模型预测精度的影响.结果表明:理论模型能够准确预测夹层结构及内部流体的温度变化,理论与仿真之间的最大误差不超过1%;随着外加热流密度不断增大,忽略收缩热阻使得计算结果造成的误差不断增大;与数值模拟相比,理论模型可显著地减少计算时间并节省计算资源,尤其适用于非均匀、非稳态复杂热载荷下点阵夹层结构的温度响应计算.     

导数振荡与应变能极小的平面分段三次参数Hermite插值

由于分段三次参数Hermite插值的切矢往往被作为变量,故可对其进行优化以使得构造的插值曲线满足特定的要求.为了构造兼具保形性与光顺性的平面分段三次参数Hermite插值曲线,给出了一种通过同时极小化导数振荡和应变能来确定切矢的方法.首先以导数振荡函数和应变能函数为双目标建立了切矢满足的方程系统;然后证明了方程系统存在唯一解,并给出了解的具体表达式;最后给出了误差分析,并通过数值算例表明方法的有效性.结果表明,相对于导数振荡极小化方法和应变能极小化方法,所提出的导数振荡和应变能极小化方法同时兼顾了平面分段三次参数Hermite插值曲线的保形性和光顺性.     

Upper bounds of Schubert polynomials

Science China Mathematics - Let w be a permutation of {1, 2, …, n}, and let D(w) be the Rothe diagram of w. The Schubert polynomial ${\mathfrak{S}_w}\left(x \right)$ can be realized as the...     

Asymptotic Behavior of a Stochastic Predator-prey Model with Beddington-DeAngelis Functional Response and L\'{e}vy Jumps

A stochastic two-prey-one-predator model with Beddington-DeAngelis functional response and L\''{e}vy jumps is proposed and investigated in this paper. First of all, we prove the existence and uniqueness of the global positive solution, and stochastic ultimate boundedness of the solution. Next, under a simple assumption, by using It\^{o} formula and other important inequalities, some sufficient conditions are established to ensure the extinction and persistence in the mean of the system. The results show that neither strong white noise nor L\''{e}vy noise is conducive to the persistence of the population. Finally, the theoretical results are verified by numerical simulations.     

Scanning Kelvin Microscope: a new method for surface investigations

Based on the well known Kelvin probe for work function measurements a new microstructure analysis system – the Scanning Kelvin Microscope – has been developed. It allows to measure simultaneously with high lateral resolution the distribution of the contact potential difference (CPD) between a conductive sample and a reference probe together with the topographical structure of the sample surface. The measurement is contact free and non-destructive and can be carried out in natural environments. At present the lateral resolution of the measurement approaches 5 m. The results can be displayed on a computer in three dimensional colour pictures.