Access to 4‐Membered Heterocycles via Visible‐Light Triggered Intramolecular Cyclization from Alkynes: Bypassing Unfavorable 4‐endo‐dig Cyclization

Abstract We describe a catalyst, oxidant, and coupling‐reagent free strategy to access 4‐membered heterocycles, representing a unique example of visible‐light triggered intramolecular cyclization of propargylic alcohols and amines to access oxetanones and azetidinones respectively. Despite the direct 4‐endo‐dig cyclization from these starting materials has proven to be unfavorable, the formation of key p ‐quinone methide intermediacy allows an efficient bypass for regioselective 4‐exo‐trig cyclization, resulting in the formation of the desired 4‐membered heterocycles. This mild and operationally simple protocol facilitates the synthesis of products with distinct substitution patterns such as quaternary α‐carbons, and enables late‐stage functionalization.