We began our study with the readily available benzofuran-3(2H)-ones 1a and β-phenylethenesulfonyl fluorides 2a as the model substrates (Table ). To our delight, in the presence of 0.5 equivalent of Cs 2 CO 3 , the annulation reaction proceeded smoothly in acetonitrile at room temperature to afford the desired δ-sultone fused benzofuran 3a in 81% yield (Table , entry 1). Increasing the loading of Cs 2 CO 3 to 1.0 equivalent, the yield of 3a was increased to 92% yield (Table ). Further increasing the amount of Cs 2 CO 3 cannot give a better yield (Table and). The strong basic sodium hydroxide mediated the reaction in 80% yield (Table ). Owing to the lowered basicity, Na 2 CO 3 mediated the reaction in low yield (Table ). Organic bases such as triethylamine, DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), DBN (1,5-Diazabicyclo[4.3.0]non-5-ene) and KO t Bu mediated the tandem reaction in moderate yields (Table , entries 7 to 10). The brief evaluation of reaction media showed that THF and DMSO gave the desired product in high yield (Table and), while dichloromethane, toluene, 1,4-dioxane and ethyl acetate showed slightly decreased efficiency (Table , entries 13 to 16). When the reaction was performed in methanol, only 25% yield of 3a was obtained (Table , entry 17). Increasing the amount of 2a to 1.5 equivalent, 3a was formed in quantitative yield (Table , entry 18). Pleasingly, when the combination of 20 mol% DBU and molecular sieves 4Å was used instead of stoichiometric base, the desired δ-sultone fused benzofuran can also be obtained in 87% With the optimized reaction conditions in hand, we next studied the substrates scope. As shown in Table , both electron-withdrawing anddonating substituents substituted β-arylethenesulfonyl fluorides reacted with benzofuran-3(2H)-one 1a smoothly to furnish the corresponding δsultone fused benzofurans in high yields (3aa-3ah). Different positions of the substituents have no apparent impact on the reaction yields (3ai-3ap).