On the Lewis acid-induced [1,3]-dipolar cycloaddition of allylic and homoallylic alcohols to N-methyl-C-phenyl nitrone
作者:Andrew D. Jones、David W. Knight、Steven R. Thornton
DOI:10.1039/a906573c
日期:——
Both the rate and stereoselectivity of [1,3]-dipolar cycloadditions between monosubstituted allylic and homoallylic alcohols 7, 17, 25, 28 and 34 and the benzaldehyde-derived nitrone 8 are increased enormously when the reactions are carried out in the presence of one equivalent of magnesium bromide–diethyl ether. Stereoselectivities in the range of 5∶1 up to >95∶5 are achieved in 2–5 hours at 80 °C, in contrast to purely thermal reactions which are almost stereorandom and which require over 48 hours at this temperature. The major products 9, 18, 26, 29 and 35 are endo adducts with the opposite trans-stereochemistry across the isoxazolidine ring relative to the exo adducts 42 obtained from similar catalysed reactions using C-acyl nitrones 4. Models which account for this difference are proposed.
当反应在等量的溴化镁-乙醚存在下进行时,单取代烯丙基和同烯丙基醇7、17、25、28和34与苯甲醛衍生物硝酮8之间的[1,3]-偶极环加成反应速率和立体选择性均显著增加。在80 °C下,2-5小时内即可达到5∶1至>95∶5的立体选择性,而纯热反应几乎为立体随机,且在此温度下需要超过48小时。主要产物9、18、26、29和35是内加合物,与外加合物42相比,异恶唑烷环具有相反的跨立体化学结构,外加合物42是通过类似的催化反应使用C-酰基硝酮4获得的。提出了解释这种差异的模型。