Domino Synthesis of Thioflavones and Thioflavothiones by Regioselective Ring Opening of Donor-Acceptor Cyclopropane Using In-Situ-Generated Thiolate Anions

A copper-catalyzed intramolecular ring opening of donor-acceptor cyclopropane is developed for the synthesis of 3-alkyl-carbonated thioflavones and further extended to 3-alkyl-carbonated thioflavothione, using xanthate as a sulfur surrogate. This reaction proceeds through thiolate formation/ring opening/Krapcho decarboxylation, followed by hydrogen abstraction, to give thioflavanone, which is further oxidized by in-situ-generated iodine from waste byproduct KI. Experimental studies prove that the charge-transfer complex is responsible for the conversion of thioketone to ketone.