Skeletal Reorganization of Cycloalkylamines Into Angular Azatetracyclic Scaffolds

The efficient, single-step construction of angular azatetracyclic frameworks bearing vicinal quaternary stereocenters remains a formidable challenge in synthetic chemistry. Herein, we disclose the first borane-catalyzed cascade reorganization of N-cycloalkylidenyl (CAD)-substituted aryl cycloalkylamines that enables rapid access to these architectures. Initiated by B(C₆F₅)₃-mediated α-hydride abstraction, the reaction proceeds through an aza-6π electrocyclization and two successive ring expansions, followed by hydride rebound, to deliver densely functionalized angular/fused azatetracycles with high diastereoselectivity. This designed cascade forges multiple C-C bonds and stereocenters in a single operation and exhibits broad substrate scope and excellent efficiency. Detailed mechanistic investigations, combining experimental results and DFT calculations, support the proposed sequence of hydride abstraction, cyclization, double ring expansion, and hydride rebound.