School of Chemistry, Monash University, Clayton 3800, Victoria, Australia.
J Org Chem. 2012 Jan 20;77(2):1113-24. doi: 10.1021/jo202500k. Epub 2012 Jan 6.
The N-heterocyclic carbene catalyzed (4 + 2) cycloaddition between α,β-unsaturated acid fluorides and TMS dienol ethers provides cyclohexene fused β-lactone intermediates stable below -20 °C. These can be intercepted reductively or with organolithium reagents to produce diastereomerically pure cyclohexenes (>20:1 dr) with up to four contiguous stereocenters. The mechanism has been investigated using theoretical calculations and by examining secondary kinetic isotope effects. Together these studies implicate the formation of a diastereomerically pure β-lactone intermediate by a stepwise (4 + 2) cycloaddition involving Michael addition, aldol cyclization, and lactonization.
N-杂环卡宾催化的（4 + 2）环加成反应，使α,β-不饱和氟代酸与 TMS 二烯醇醚反应，提供了在-20°C 以下稳定的环己烯稠合β-内酰胺中间体。这些中间体可以通过还原或与有机锂试剂进行捕获，生成具有四个连续立体中心的非对映异构体纯的环己烯（>20:1 dr）。该机理已通过理论计算和考察二级动力学同位素效应进行了研究。这些研究共同表明，通过涉及迈克尔加成、醛醇环化和内酯化的逐步（4 + 2）环加成反应，形成了非对映异构体纯的β-内酰胺中间体。
