Lead-oriented synthesis: a new opportunity for synthetic chemistry

The pharmaceutical industry remains solely reliant on synthetic chemistry methodology to prepare compounds for small-molecule drug discovery programmes. The importance of the physicochemical properties of these molecules in determining their success in drug development is now well understood but we present here data suggesting that much synthetic methodology is unintentionally predisposed to producing molecules with poorer drug-like properties. This bias may have ramifications to the early hit- and lead-finding phases of the drug discovery process when larger numbers of compounds from array techniques are prepared. To address this issue we describe for the first time the concept of lead-oriented synthesis and the opportunity for its adoption to increase the range and quality of molecules used to develop new medicines.     

A planning strategy for diversity-oriented synthesis

In contrast to target-oriented synthesis (TOS) and medicinal or combinatorial chemistry, which aim to access precise or dense regions of chemistry space, diversity-oriented synthesis (DOS) populates chemical space broadly with small-molecules having diverse structures. The goals of DOS include the development of pathways leading to the efficient (three- to five-step) synthesis of collections of small molecules having skeletal and stereochemical diversity with defined coordinates in chemical space. Ideally, these pathways also yield compounds having the potential to attach appendages site- and stereoselectively to a variety of attachment sites during a post-screening, maturation stage. The diverse skeletons and stereochemistries ensure that the appendages can be positioned in multiple orientations about the surface of the molecules. TOS as well as medicinal and combinatorial chemistries have been advanced by the development of retrosynthetic analysis. Although the distinct goals of DOS do not permit the application of retrosynthetic concepts and thinking, these foundations are being built on, by using parallel logic, to develop a complementary procedure known as forward-synthetic analysis. This analysis facilitates synthetic planning, communication, and teaching in this evolving discipline.     

基于有限反馈的数据流数自适应SDMA

针对目前绝大部分有限反馈自适应SDMA由于使用固定满波束发送策略而导致在稀疏网络中严重的用户间干扰的问题,该文提出了一种基于有限反馈的数据流数自适应SDMA,该方案能通过自适应地选择并行发送波束的数目来达到有效控制用户间干扰的目的。具体实现如下：通过用户的有限反馈信息,以及信道的二阶统计信息,基站端使用约束最大似然估计的方法估计出使用不同数目发送波束的发送模式所支持的最大和吞吐量,从而自适应地选择最优的发送模式。仿真结果表明：该文提出的方案能在增加很少反馈量的前提下显著改善现有SDMA方案的性能。     

无线通信中编码协作技术研究

编码协作是无线通信中一种新的用户协作技术,它通过信道编码方式达到协作的目的。用户发送的码字被分为2部分,分别通过用户和其协作者的天线进行传输。通过协作通信中编码协作机制下的多节点协作伙伴的选择,分析了4种情况下的中断概率,并阐述了编码协作技术的研究方向。     

长白山土壤腹足类物种多样性初步研究

自1993年5月至10月对吉林长白山森林生态系统定位站的土壤腹足类进行了多样性调查，据初步调查的结果显示，长白山地区土壤腹足类种类少而单调，采样区仅发现15种，隶属于腹足纲9科13属，采样区土壤腹足类物种多样性指数和丰富度指数都是5月份最低，8月份达最大值，之后逐渐减小；均匀度指数5月份最高，7月份最低；单纯度指数也是5月份最高，最低值出现在10月份，土壤表层的陆生贝类物种多样性指数高于上层。     

基于随机相位误差Tikhonov分布的MC-DS-CDMA系统性能分析

本文针对工作于慢变频率选择性Rayleigh衰落信道中的部分相干的MC-DS-CDMA系统，基于随机相位误差的Tikhonov密度分布提出了误码率上边界的一种分析方法。仿真结果表明，随着环路信噪比的增加，相位误差损耗所产生的系统误码率性能损失变得越来越小，当环路信噪比高于系统信噪比(Eb/N0)20dB时，系统误码率性能损失小于0．2dB。在MC-DS-CDMA系统中，由于能用频率分集取代路径分集，因而接收机结构更易实现。     

差分吸收相干激光雷达中偏振分集技术研究

为减小差分吸收相干激光雷达(DIAL)中偏振衰落对 回波信号功率反演精度影响,研究了偏振分集技术(PDT)在相干DIAL中 的作用,构建了利用STM32控制的双光束切换DIAL实验装置。依托构 建的装置,利用平衡探 测,分别采用PDT和一般外差探测完成多组次实验,应用单频检测方法得到20m光程下吸收 气体不同浓度下相干中频信号的振 幅值,并由此求解出on光和off光回波信号功率。对回波信号功率值差分信息与由HITRAN得 到的参考差分信息比较 发现:应用PDT能使探测分辨率提高4倍,对应到2km距离内,可以探测到3.5×10－6ppm的CO₂浓度变化,满足了 气候研究所需的优于1％的精度要求,验证了PDT消除偏振衰落以及提高相干DIAL分辨精度的 作用。     

Synthesis of β3‐Homophenylalanine‐Derived Amino Acids and Peptides by Suzuki Coupling in Solution and on Solid Support

β‐Peptides and, to a certain extent, also mixed α,β‐peptides, are resistant to degradation by a variety of proteolytic enzymes that rapidly degrade natural α‐peptides. This is one of many characteristics that make β‐peptides an attractive class of compounds for drug‐discovery studies. On the other hand, modern organometallic reactions such as the Suzuki–Miyaura cross‐coupling have become standard tools in industry laboratories to derivatize side chains of α‐peptidic compounds to build up libraries of unnatural peptides. Combining both features, we prepared (4‐bromo)‐β³‐homophenylalanine derivatives 3 – 5 and 12 as precursors for Suzuki–Miyaura couplings. From these bromo compounds, we synthesized biaryl‐substituted β‐homoamino acids 6 , and analogs 13 and 15 of the anti‐AIDS drug Saquinavir.     

全固态立体声调频广播发射机原理和维修（三）

本文较全面地分析了调频发射机各部分的原理、电路、测量、安装、使用及维修技术,既有理论分析,更注重电路实践。文中还收集了不同电路方案,有关国家标准,故障分析,判断及排除经验。     

全固态立体声调频广播发射机原理和维修（一）

本文较全面地分析了调频发射机各部分的原理、电路、测量、安装、使用及维修技术,既有理论分析,更注重电路实践。文中还收集了不同电路方案,有关国家标准、故障分析、判断及排除经验。