The reaction between aromatic aldehydes, dimedone (5,5-dimethyl-1,3-cyclohexane dione) and 2-naphthol catalysed by nanoparticle silica supported sulfuric acid (NPs SiO2-H2SO4) in CH2Cl2 as a solvent at room temperature provided a simple and efficient one-pot route for the synthesis of benzo[a]xanthene-11-one derivatives in high yield. Benzoxanthenes are important heterocycles because of their antiviral, anti-inflammatory and antibacterial activities.