Sulfolene is formed by the cheletropic reaction between butadiene and sulfur dioxide.  The reaction is typically conducted in an autoclave. Small amounts of  hydroquinone or pyrogallol are added to inhibit polymerization of the diene.  The reaction proceeds at room temperature over the course of days. At 130 °C, only 30 minutes are required.  An analogous procedure gives the isoprene-derived sulfone.