A new chiral polymer-metal complex, wool-osmium tetroxide(wool-OsO4) complex was prepared by a very simple method. This complex was found to be able to catalyze the asymmetric dihydroxylation of allylamine to get (R)-...A new chiral polymer-metal complex, wool-osmium tetroxide(wool-OsO4) complex was prepared by a very simple method. This complex was found to be able to catalyze the asymmetric dihydroxylation of allylamine to get (R)-(+)-3-amino-l, 2-propanediol. The experimental results showed that OsO4 content in the complex, reaction time, allylamine/OsO4 molar ratio all have great effects on the chemical and optical yields of product. Additionally, wool-OsO4 complex catalyst could be reused without remarkable change in optical catalytic activity.展开更多
The osmium tetroxide catalyzed asymmetric dihydroxylation of the(22E)- steroidal sidechain is described and an unexpected 8:1 ratio of(22R,23R)and (22S,23S)was obtained from the(22E,24S)-24-ethyl substituted sidechain.
Aim To synthesize naturally occurring (-) methyl2,2-dimethyl-3-hydroxychromanyl-6-formate. Methods Sharpless' asymmetric dihydroxylation wasemployed to construct the 3-hydroxychroman. Results First asymmetric synt...Aim To synthesize naturally occurring (-) methyl2,2-dimethyl-3-hydroxychromanyl-6-formate. Methods Sharpless' asymmetric dihydroxylation wasemployed to construct the 3-hydroxychroman. Results First asymmetric synthesis of (-) methyl 2,2-dimethyl-3-hydroxychromanyl-6-formate (1) was accomplished starting from methyl 4-hydroxy-benzoate(2), and the absolute configuration was established. Conclusion A useful method for constructingchiral 3-hydroxychroman by employing Sharpless' asymmetric dihydroxylation is achieved.展开更多
(3R,5R)-3-((tert-Butyldimethylsily)oxy)-5-((Z)-2-bromovinyl)-tetrahydro-furan-2-one, an intermediate for the synthesis of Fostriecin was achieved by intramolecular asymmetric induction in propene addition of (-)-8-phe...(3R,5R)-3-((tert-Butyldimethylsily)oxy)-5-((Z)-2-bromovinyl)-tetrahydro-furan-2-one, an intermediate for the synthesis of Fostriecin was achieved by intramolecular asymmetric induction in propene addition of (-)-8-phenylmenthyl glyoxylate followed by inversion of C3-hydroxyl group and Sharpless asymmetric dihydroxylation with simultaneous cyclization to give lactone 5. Then protection of C3-hydroxyl group and oxidation of the C6-primary hydroxyl group which reacted with Wittig reagent to yield the target compound 4.展开更多
针对Mobile Ad Hoc网络提出了一个改进的匿名认证协议,主要目的是提供一个隐藏通信节点真实身份的机制,提高对恶意攻击的抵御能力。端到端的匿名认证进行基于对称和非对称密钥加密的三次握手,在成功完成认证过程后,安全的复合匿名数据...针对Mobile Ad Hoc网络提出了一个改进的匿名认证协议,主要目的是提供一个隐藏通信节点真实身份的机制,提高对恶意攻击的抵御能力。端到端的匿名认证进行基于对称和非对称密钥加密的三次握手,在成功完成认证过程后,安全的复合匿名数据通道才能被建立。匿名由拥有随机选择伪身份的用户来担保,网络节点公开认证并且独立于用户的伪身份。文章通过一个网络模型介绍了该协议的执行过程,并对其安全性进行了简要分析。展开更多
Accurate link quality estimation is a fundamental building block in quality aware multi hop routing. In an inherently lossy, unreliable and dynamic medium such as wireless, the task of accurate estimation becomes very...Accurate link quality estimation is a fundamental building block in quality aware multi hop routing. In an inherently lossy, unreliable and dynamic medium such as wireless, the task of accurate estimation becomes very challenging. Over the years ETX has been widely used as a reliable link quality estimation metric. However, more recently it has been established that under heavy traffic loads ETX performance gets significantly worse. We examine the ETX metric＇s behavior in detail with respect to the MAC layer and UDP data; and identify the causes of its unreliability. Motivated by the observations made in our analysis, we present the design and implementation of our link quality measurement metric xDDR - a variation of ETX. This article extends xDDR to support network mobility. Our experiments show that xDDR substantially outperforms minimum hop count, ETX and HETX in terms of end-to-end packet delivery ratio in static as well as mobile scenarios.展开更多
优化链路状态路由(Optimized Link State Routing,OLSR)协议是一种先验式路由协议,网络中的所有节点通过周期性地发送控制消息来计算全网路由信息。在短波自组织网络中,节点周期性地发送控制消息会占据大量的信道资源,大幅增加网络的控...优化链路状态路由(Optimized Link State Routing,OLSR)协议是一种先验式路由协议,网络中的所有节点通过周期性地发送控制消息来计算全网路由信息。在短波自组织网络中,节点周期性地发送控制消息会占据大量的信道资源,大幅增加网络的控制开销,浪费短波有限的带宽资源,导致网络通信性能急剧下降。其次,受到地形地貌、天线方向和接收性能的个体差异等影响,造成无线链路不稳定,导致网络中存在非对称链路,增加了通信端到端时延。为此,提出了一种低时延的短波自组网OLSR协议。该协议在执行MPR(Multipoint Relay)选择算法时综合考虑了节点的连接度和链路可靠性,在优化MPR节点个数的同时选择链路可靠性较大的节点作为MPR节点,在进行路由选择时能够利用网络中的非对称链路。仿真结果表明,该协议能优化数据包投递成功率、吞吐量、端到端时延和网络控制开销等性能指标。展开更多
文摘A new chiral polymer-metal complex, wool-osmium tetroxide(wool-OsO4) complex was prepared by a very simple method. This complex was found to be able to catalyze the asymmetric dihydroxylation of allylamine to get (R)-(+)-3-amino-l, 2-propanediol. The experimental results showed that OsO4 content in the complex, reaction time, allylamine/OsO4 molar ratio all have great effects on the chemical and optical yields of product. Additionally, wool-OsO4 complex catalyst could be reused without remarkable change in optical catalytic activity.
文摘The osmium tetroxide catalyzed asymmetric dihydroxylation of the(22E)- steroidal sidechain is described and an unexpected 8:1 ratio of(22R,23R)and (22S,23S)was obtained from the(22E,24S)-24-ethyl substituted sidechain.
基金This research work was financially supported by the National Natural Science Foundation of China(Grant NO.20272020).
文摘Aim To synthesize naturally occurring (-) methyl2,2-dimethyl-3-hydroxychromanyl-6-formate. Methods Sharpless' asymmetric dihydroxylation wasemployed to construct the 3-hydroxychroman. Results First asymmetric synthesis of (-) methyl 2,2-dimethyl-3-hydroxychromanyl-6-formate (1) was accomplished starting from methyl 4-hydroxy-benzoate(2), and the absolute configuration was established. Conclusion A useful method for constructingchiral 3-hydroxychroman by employing Sharpless' asymmetric dihydroxylation is achieved.
文摘(3R,5R)-3-((tert-Butyldimethylsily)oxy)-5-((Z)-2-bromovinyl)-tetrahydro-furan-2-one, an intermediate for the synthesis of Fostriecin was achieved by intramolecular asymmetric induction in propene addition of (-)-8-phenylmenthyl glyoxylate followed by inversion of C3-hydroxyl group and Sharpless asymmetric dihydroxylation with simultaneous cyclization to give lactone 5. Then protection of C3-hydroxyl group and oxidation of the C6-primary hydroxyl group which reacted with Wittig reagent to yield the target compound 4.
文摘针对Mobile Ad Hoc网络提出了一个改进的匿名认证协议,主要目的是提供一个隐藏通信节点真实身份的机制,提高对恶意攻击的抵御能力。端到端的匿名认证进行基于对称和非对称密钥加密的三次握手,在成功完成认证过程后,安全的复合匿名数据通道才能被建立。匿名由拥有随机选择伪身份的用户来担保,网络节点公开认证并且独立于用户的伪身份。文章通过一个网络模型介绍了该协议的执行过程,并对其安全性进行了简要分析。
文摘Accurate link quality estimation is a fundamental building block in quality aware multi hop routing. In an inherently lossy, unreliable and dynamic medium such as wireless, the task of accurate estimation becomes very challenging. Over the years ETX has been widely used as a reliable link quality estimation metric. However, more recently it has been established that under heavy traffic loads ETX performance gets significantly worse. We examine the ETX metric＇s behavior in detail with respect to the MAC layer and UDP data; and identify the causes of its unreliability. Motivated by the observations made in our analysis, we present the design and implementation of our link quality measurement metric xDDR - a variation of ETX. This article extends xDDR to support network mobility. Our experiments show that xDDR substantially outperforms minimum hop count, ETX and HETX in terms of end-to-end packet delivery ratio in static as well as mobile scenarios.
文摘优化链路状态路由(Optimized Link State Routing,OLSR)协议是一种先验式路由协议,网络中的所有节点通过周期性地发送控制消息来计算全网路由信息。在短波自组织网络中,节点周期性地发送控制消息会占据大量的信道资源,大幅增加网络的控制开销,浪费短波有限的带宽资源,导致网络通信性能急剧下降。其次,受到地形地貌、天线方向和接收性能的个体差异等影响,造成无线链路不稳定,导致网络中存在非对称链路,增加了通信端到端时延。为此,提出了一种低时延的短波自组网OLSR协议。该协议在执行MPR(Multipoint Relay)选择算法时综合考虑了节点的连接度和链路可靠性,在优化MPR节点个数的同时选择链路可靠性较大的节点作为MPR节点,在进行路由选择时能够利用网络中的非对称链路。仿真结果表明,该协议能优化数据包投递成功率、吞吐量、端到端时延和网络控制开销等性能指标。
