We report a two-step approach to bicyclic and monocyclic 5-(1-alkoxyalkylidene)tetronates starting from lactones/esters. The method features the use of thionolactones and thionoesters as activated forms of lactones/esters that allows the direct condensation with tetronates via one-pot enolate formation, nucleophilic addition, S-methylation, and DBU-promoted elimination. The value of the method was demonstrated by the stereoselective syntheses of two natural products: 5,6-Z-fadyenolide (Z/E ratio = 6:1) and 9,10-methylenedioxy-5,6-Z-fadyenolide (Z/E ratio = 9:1).