Synthesis of indoline-2,3-fused tetrahydroquinolines bearing two free N–H using a protecting-group-free approach.

Abstract

We report the first general synthesis of tetrahydro-5H-indolo[2,3-b]quinolines via a transition-metal- and protecting-group-free two-step approach. The strategy combines NaOtBu/Et3B-promoted alkylative dearomatization of indoles with a B2(OH)4-mediated cascade nitro reduction and amine–imine cyclization. The method is mild, diastereoselective, high-yielding, scalable, and has broad substrate scope. Importantly, it furnishes products with two free N–H groups, enabling further downstream functionalization.