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A highly efficient and clean procedure is described for the synthesis of thioethers with excellent yields under solvent-free conditions at room temperature using a catalytic amount of HClO 4 -SiO 2 . The catalyst is easily prepared, stable, and reusable without much loss of catalytic activity, and is efficient under the reaction conditions.

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Fluoroboric acid supported on silica gel (HBF 4 -SiO 2 ) catalyzes efficiently the three component condensation of an aldehyde, a 1,3-dicarbonyl compound and urea or thiourea to afford the corresponding 3,4-dihydropyrimidin-2(1H)-ones and thiones in high yields. The ambient conditions, fast reaction rates, and excellent product yields are the important characteristics of this reaction.