Lewis Acid Catalyzed Reductive Cyclization of 2-Aryloxybenzaldehydes and 2-(Arylthio)benzaldehydes to Unsubstituted 9H-Xanthenes and Thioxanthenes in Diisopropyl Ether

Abstract

Readily accessible 2-aryloxybenzaldehydes and 2-(arylthio)benzaldehydes undergo a sequence of reactions leading to a wide variety of unsubstituted 9H-xanthenes and thioxanthenes in high yields when heated with a Lewis acid in diisopropyl ether. This reductive cyclization method is compatible with several important functional groups. The method is also applicable for the selective reductive cyclization of the more electron-rich aryl ring of a 2,6-bis(aryloxy)benzaldehyde. The key feature of this transformation is the chemoselective reduction of a transient xanthylium ion in the presence of aldehydic group via intermolecular hydride transfer from diisopropyl ether (solvent). (Figure presented.). © 2020 Wiley-VCH GmbH