Formal [3 + 2] Cycloadditions via Indole Activation: A Route to Pyrroloindolines and Furoindolines

Here,
we describe a novel [3 + 2] cycloaddition of 3-substituted
indoles with vinyl aziridines and vinyl epoxides that provides a straightforward
approach to pyrroloindolines and furoindolines bearing vinyl groups
(up to 96% yield and 9:1 dr). In contrary to previous reports involving
Lewis acid activation, this work reports successful reactions based
on the activation of indole using t-BuOK and BEt3 (triethylborane), thereby preserving the free N–H
group on indoles. In addition, a gram-scale reaction and a ring-closing
metatheis reaction are performed to provide good demonstrations of
the synthetic utility of this approach.