Visible Light‐Mediated Dearomative Hydrogen Atom Abstraction/ Cyclization Cascade of Indoles

Abstract The photochemical synthesis of yet unknown 2‐oxospiro[azetidine‐3,3′‐indolines] (17 examples, 80–95 % yield), 2,4‐dioxospiro[azetidine‐3,3′‐indolines] (eight 87–97 and 1‐oxo‐1,3‐dihydrospiro[indene‐2,3′‐indolines] 85–97 yield) is described. Starting from readily accessible 3‐substituted indoles, a dearomatization the indole core was accomplished upon irradiation at λ=420 nm in presence thioxanthen‐9‐one (10 mol%) as sensitizer. Based on mechanistic evidence (triplet energy determination, deuteration experiments, by‐product analysis) it proposed that reaction proceeds by transfer via 1,4‐ or 1,5‐diradical intermediate. latter intermediates are formed excited state hydrogen atom suitable alkyl groups within C3 substituent to C2 carbon atom. Subsequent ring closure with pronounced diastereoselectivity generate 4‐ 5‐membered spirocyclic dearomatized product several options for further functionalization.