关键词： 不对称合成   双中心协同催化作用   手性配体   双中心协同催化剂   模拟酶  

摘要： 介绍了双中心协同催化作用的概念以及A型和B型两类催化剂 .综述了A型催化剂在不对称硝基 -类羟醛反应、迈克尔加成反应、亚胺和醛的氢膦酰化反应、曼尼希反应、不对称醛酮羟醛反应以及环氧开环反应中的应用 ;

关键词： Crystals   Crystallization   Nuclear radiation   Molecules   Sodium  

摘要： The readily available sodium chlorate is an interesting ionic compound because, although achiral, it forms optically active crystals. When unperturbed, an aqueous solution of sodium chlorate yields a random distribution of + and - crystals on evaporation. Some chiral perturbations profoundly alter this distribution, however. These include seeding, the addition of the optically active cosolutes D-mannitol and D-sorbitol, and the energetic and chiral beta particles and positrons. The chiral weak interaction inherent in all atomic nuclei and thus atoms and molecules does not alter the distribution of + and - crystals. Stirring during evaporation, an achiral perturbation, affects the distribution of crystals in a given crystallization;when crystals from a large number of experiments are considered, however, the distribution of + and - crystals is random.

关键词： Positron emission tomography  

摘要： Procedures were developed for synthesis of 3,4-methylenedioxy-6-[18F]fluorobenzyl bromide I and 2-[18F]fluoro-4-methoxybenzyl bromide Ia, which are intermediates in asymmetric synthesis of fluorinated α-amino acids used in positron emission tomography (PET). The bromination procedures involving two brominating agents, an ethereal solution of HBr or triphenyldibromophosphorane in various solvents, as well as purification procedures, were compared. An optimized procedure was suggested for synthesis of I and Ia using an Anatech robotic system;the total synthesis time is 45-48 min. The radiochemical yield of I and Ia, corrected for the 18F decay, was 35-40 and 60-65%, respectively. The suggested scheme can be adapted to modern automated modules for production of radiopharmaceuticals. © MAIK "Nauka/Interperiodica" 2002.

关键词： Ethers   Organic compounds   Molecular structure   Alkyls   Cyclization  

摘要： [GRAPHICS] We report a method for the synthesis of chiral cyclopentanes using tin-lithium exchange and cycloalkylation reactions. The sec-butyllithium/(-)-sparteine-mediated deprotonation of an alkyl carbamate and subsequent substitution furnishes a highly enantioenriched stannane as a stable carbanion equivalent. It was transformed into suitable cyclization precursors, which underwent tin-lithium exchange and stereoselective cycloalkylation when reacted with n-butyllithium, giving highly enantioenriched cyclopentanes in very good yields. A kinetic resolution was observed with a higher substituted stannane.

关键词： PALLADIUM   AMINATION   ACETATES  

摘要： 4'-Substituted nucleoside analogues have been synthesized using palladium-catalyzed asymmetric allylic amination conditions. A kinetic discrimination between the diastereomeric lactol acetates (3) produced the desired aminated products (6a-d) and recovered acetate (alpha-3) in high yields and <97:3 diastereoselectivity. Epimerization of the recovered lactol acetate (alpha-3) produced a 60:40 alpha/beta mixture of (3), which could be resubjected, in principle, to the palladium-catalyzed asymmetric allylic amination conditions.

关键词： AMINO acids   HYDRIDES  

摘要： Esters of three types of silylated alpha-amino acids have been prepared from appropriate zirconaaziridines. Slow addition (syringe pump) of the (R,R) carbonate of transstilbene gave metallacycles with the maximum possible diastereomeric excess (as determined by the diastereomeric excess produced by the Hoffmann test-the same reaction but with racemic carbonate). Methanolysis gave esters (RO2C)CH(R')(NHPh) (R' = Me3Si, Me3SiCH2, and p-Me-3-SiC6H4) with the same optical purity at the alpha carbon.

关键词： Amino Alcohols   Stereoisomerism   Support   Non-U.S. Gov't   Support   U.S. Gov't   Non-P.H.S.  

摘要： The first application of metalloenamines derived from N-sulfinyl imines is reported for the highly diastereoselective addition to aldehydes. Reduction of the beta-hydroxy-N-sulfinyl imine products with catecholborane and LiBHEt3 provides syn- and anti-1,3-amino alcohol derivatives, respectively, with very high diastereomeric ratios.

关键词： Anti-Bacterial Agents   Catalysis   Ethylenes   Imines   Ketones   Lactams   Models   Molecular   Support   Non-U.S. Gov't   Support   U.S. Gov't   Non-P.H.S.   Support   U.S. Gov't   P.H.S.  

摘要： We report practical methodology for the catalytic, asymmetric synthesis of beta-lactams resulting from the development of a catalyzed reaction of ketenes (or their derived zwitterionic enolates) and mines. The products of these asymmetric reactions can serve as precursors to a number of enzyme inhibitors and drug candidates as well as valuable synthetic intermediates. We present a detailed study of the mechanism of the beta-lactam forming reaction with proton sponge as the stoichiometric base, including kinetics and isotopic labeling studies. Stereochemical models based on molecular mechanics (MM) calculations are also presented to account for the observed stereoregular sense of induction in our reactions and to provide a guidepost for the design of other catalyst systems.

关键词： 手性   2,4-二羟甲基吡咯   衍生物   不对称合成   1,3-偶极环加成  

摘要： 在碱Na2 CO3或CH3ONa存在下 ,芳香醛或脂肪醛与芳香取代的甘氨酸甲酯反应 ,生成Schiff碱 ;Schiff碱在有机强碱DBU和金属离子作用下 ,生成 1,3 偶极试剂 ,并与手性丙烯酸薄荷醇酯原位发生不对称 1,3 偶极环加成反应 ,得手性 2 ,4 二酯基四氢吡咯衍生物 ;然后 ,在无水THF中用氢化铝锂还原 ,得纯的手性 2 ,4 二羟甲基四氢吡咯衍生物 .所有二酯基、二羟甲基四氢吡咯衍生物的结构都经过红外光谱、核磁共振、质谱鉴定 .

关键词： 内酯   缩合   烷基化   催化转移氢化   催化不对称合成  

摘要： 内酯结构广泛存在于天然产物和各种各样的生物活性分子中.目前己报道的关于内酯烷基化的方法主要有1） LDA作用下的直接烷基化法;2）通过手性噁唑琳法.但是前者缺少一定的通用性且有难以克服γ-丁内酯自缩合副反应,后者则实验步骤烦琐,结果不够理想.由于内酯烷基化现有方法的欠缺和条件苛刻,通过对文献方法的考察我们自行设计新的合成路线,对α-烷基取代的-γ-丁内酯类化合物的合成进行了研究.