Department of Organic Chemistry, University of Geneva, Geneva, Switzerland.
Chemistry. 2020 Dec 1;26(67):15471-15476. doi: 10.1002/chem.202003426. Epub 2020 Oct 29.
Pnictogen-bonding catalysis and supramolecular σ-hole catalysis in general is currently being introduced as the non-covalent counterpart of covalent Lewis acid catalysis. With access to anti-Baldwin cyclizations identified as unique characteristic, pnictogen-bonding catalysis appeared promising to elucidate one of the hidden enigmas of brevetoxin-type epoxide opening polyether cascade cyclizations, that is the cyclization of certain trans epoxides into cis-fused rings. In principle, a shift from S 2- to S 1-type mechanisms could suffice to rationalize this inversion of configuration. However, the same inversion could be explained by a completely different mechanism: Ring opening with C-C bond cleavage into a branched hydroxy-5-enal and the corresponding cyclic hemiacetal, followed by cascade cyclization under conformational control, including stereoselective C-C bond formation. In this report, a pnictogen-bonding supramolecular Sb catalyst is used to demonstrate that this unorthodox polyether cascade cyclization mechanism occurs.
目前，人们将磷属元素键合催化和超分子 σ-hole 催化引入为共价路易斯酸催化的非共价对应物。由于反鲍尔温和环化反应被确定为独特的特征，磷属元素键合催化似乎很有希望阐明隐溴毒素型环氧化物开环聚醚级联环化反应中的一个谜团，即某些反式环氧化物成环为顺式稠合环。原则上，从 S 2- 到 S 1- 型机制的转变足以合理地解释这种构型的反转。然而，同样的反转可以通过完全不同的机制来解释：开环形成 C-C 键断裂成支化的羟基-5-烯醛和相应的环状半缩醛，然后在构象控制下进行级联环化，包括立体选择性的 C-C 键形成。在本报告中，使用磷属元素键合超分子 Sb 催化剂来证明这种非传统的聚醚级联环化机制的存在。
