Carbon-Atom Scavengers Enable Divergent, Selective Carbon Deletion of Azaarenes

Abstract

Divergent synthesis is a powerful strategy that provides simultaneous access to multiple derivatives of a given substrate. However, the emerging developments in skeletal editing have largely delivered methods that lack this potential for diversification. Herein, we report the serendipitous discovery of reagent-controlled selective deletion of C3 or C2 carbon atoms of quinolines, affording indoles. An initial observation that an impurity in commercial samples of DBU promoted cyclization of a benzoxazepine-derived imidate led to the identification of indoline and aminoethanol as C3- and C2-selective carbon-atom scavengers, respectively. These two methods successfully convert a broad scope of quinolines and related azaarenes to the corresponding indoles and azaindoles, enabling divergent carbon deletion. In-depth mechanistic studies support the HFIP-promoted ring opening of 3,1-benzoxazepines to amidine intermediates as a rate-determining step, while providing insights into the selectivity afforded by indoline. These methods and their associated mechanisms offer a blueprint for the rational design of reagent-controlled, divergent skeletal edits.