Acid/Base‐Switchable Site‐Divergent Annulation of Oxetane‐Tethered Indoles: Switchable Access to Indole‐Fused Benzooxepin and Benzoxazepine

A divergent intramolecular annulation of oxetane‐tethered indoles that converts a single precursor class into two distinct seven‐membered fused scaffolds under simple condition switching has been developed. Under Sc(OTf) ₃ catalysis (10 mol%) in toluene at 80°C, a hydrogen‐bond‐gated activation mode selectively suppresses indole N1 nucleophilicity and enforces C3 capture, affording indole‐fused oxepines in 30 examples and 44%–90% isolated yields, whereas a base switches the ambident indole nucleophile to N1‐cyclization, delivering indole‐fused benzoxazepines in 55%–89% yields. Gram‐scale synthesis is demonstrated for both manifolds, and both manifolds exhibit broad functional group tolerance and provide rapid entry to previously unexplored fused heterocycles. Mechanistically, we propose a polarity‐gated ambident annulation model in which a Sc(OTf) ₃ ‐controlled electrophile polarization and NH hydrogen bonding gates regioselective C3 closure, offering a general blueprint for strain‐release‐driven divergent synthesis.