Polar Hetero-Diels−Alder Reactions of 4-Alkenylthiazoles with 1,2,4-Triazoline-3,5-diones: An Experimental and Computational Study
作者:Mateo Alajarín、José Cabrera、Aurelia Pastor、Pilar Sánchez-Andrada、Delia Bautista
DOI:10.1021/jo7021668
日期:2008.2.1
cycloadducts in moderate diastereomeric excesses. The stereochemical course is dominated by the steric interactions at the two diastereomeric transition states. A computational study of these processes with structurally simpler reagents has been carried out. A concerted pathway via a highly asynchronous transition state is preferred for 2-unsubstituted 4-vinyl and 4-styrylthiazoles. However, two alternative
4-链烯基噻唑与4-苯基-1,2,4-三唑啉-3,5-二酮(PTAD)的杂Diels-Alder反应可通过独特的表面方法以优异的收率和高水平的立体控制产生新的杂多环系统。立体异构中心位于烯基取代基上的4-烯基噻唑与PTAD反应,得到适度非对映异构体过量的相应手性环加合物。立体化学过程由两个非对映异构过渡态的空间相互作用决定。用结构上较简单的试剂对这些过程进行了计算研究。对于2-未取代的4-乙烯基和4-苯乙烯基噻唑,优选通过高度异步过渡态的协同途径。但是,有两种替代的,同样可能的途径,即协调一致的和循序渐进的,已发现随后是2-甲基或2-苯基取代的4-苯乙烯基噻唑。协调的路径具有高度异步的过渡状态。对于逐步路径,速率确定步骤是第一个步骤,因为实际上不存在第二个步骤的能垒。