Covalently anchored sulfonic acid on silica gel(SiO_2-R-SO_3H)as an efficient and reusable heterogeneous catalyst for the one-pot synthesis of 1,8-dioxooctahydroxanthenes under solvent-free conditions

A highly efficient one-pot synthesis of 1,8-dioxooctahydroxanthenes under solvent-free conditions catalyzed by sulfonic acid covalently anchored onto the surface of silica gel is reported. All types of aldehydes, including aromatic, unsaturated, and heterocyclic, are used. The silica gel/sulfonic acid catalyst （SiO2-R-SO3H） is completely heterogeneous and can be recycled.