Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
Nat Chem. 2010 Nov;2(11):972-6. doi: 10.1038/nchem.801. Epub 2010 Aug 29.
Asymmetric reactions that transform racemic mixtures into enantio-enriched products are in high demand, but classical kinetic resolution produces enantiopure compounds in <50% yield even in an ideal case. Many deracemization processes have thus been developed including dynamic kinetic resolution and dynamic kinetic asymmetric transformation, which can provide enantio-enriched products even after complete conversion of the racemic starting materials. However, these dynamic processes require racemization or symmetrization of the substrates or intermediates. We demonstrate a direct chemical enantio-convergent transformation without a racemization or symmetrization process. Copper(I)-catalysed asymmetric allylic substitution of a racemic allylic ether afforded a single enantiomer of an α-chiral allylboronate with complete conversion and high enantioselectivity (up to 98% enantiomeric excess). One enantiomer of the substrate undergoes an anti-S(N)2'-type reaction whereas the other enantiomer reacts via a syn-S(N)2' pathway. The products, which cannot be prepared by dynamic procedures, have been used to construct all-carbon quaternary stereocentres.
不对称反应可将外消旋混合物转化为对映体富集产物，需求很大，但经典动力学拆分即使在理想情况下也只能以 <50%的产率得到对映纯化合物。因此，已经开发了许多外消旋化过程，包括动态动力学拆分和动态动力学不对称转化，即使在外消旋起始原料完全转化的情况下，也能提供对映体富集产物。然而，这些动态过程需要底物或中间体的外消旋化或对称化。我们证明了一种无需外消旋化或对称化过程的直接化学对映体转化。铜（I）催化的外消旋烯丙基醚的不对称烯丙基取代反应以完全转化和高对映选择性（高达 98%对映过量）得到了单一对映体的α-手性烯丙基硼酸酯。底物的一个对映体经历反 S(N)2'-型反应，而另一个对映体则通过顺 S(N)2'途径反应。不能通过动态过程制备的产物已被用于构建全碳季碳立体中心。
