Metal-Free Oxidative Coupling: Xanthone Formation via Direct Annulation of 2-Aryloxybenzaldehyde using Tetrabutylammonium Bromide as a Promoter in Aqueous Medium

A metal-free intramolecular annulation of 2-aryloxybenzaldehydes to xanthones is disclosed, which proceeds through the direct oxidative coupling of an aldehyde CH bond and aromatic CH bonds using tetrabutylammonium bromide (TBAB) as a promoter in aqueous medium. This strategy works smoothly in the presence of both electron-donating and electron-withdrawing groups, and displays good tolerance towards catalytically reactive substituents, thus promising further functionalizations of xanthone products.