Visible‐Light‐Mediated Cyclization of 1,3‐Diones and Chalcogenoalkynes: An Eco‐Friendly Protocol for the Regioselective Formation of Polysubstituted Chalcogenofurans

Abstract Here, we present the synthesis of polysubstituted chalcogenofurans involving an oxidative cycloaddition between 1,3‐diones and chalcogenoalkynes. The method utilizes blue LEDs as a clean energy source, methylene blue as a photocatalyst – an inexpensive and non‐toxic dye – and ammonium persulfate as an oxidant, all conducted at room temperature. This benign protocol enables access to a wide range of chalcogenofurans with good to excellent yields and outstanding regioselectivity. Only one regioisomer was obtained, indicating strong radical stabilization by the chalcogen atom. Various functionalities in both alkynes and 1,3‐diones demonstrated amenability to the developed reaction. A mechanistic understanding, derived from control experiments and the detection of radical intermediates, suggests that a radical forms at the carbon‐2 position of the 1,3‐dione, which subsequently adds to the chalcogenoalkyne, resulting in the formation of an olefinic intermediate. Following this, an intramolecular radical addition occurs, which, upon photoredox cycle closure, leads to the formation of the desired products.