The mixture was stirred overnight at room temperature. After the reaction was completed, the mixture was neutralized with 1N HCl. EtOAc (30 mL) and water (30 mL) was added. The mixture was extracted 3 times with EtOAc. The organic layer was dried with MgSO4. After the solvent was removed under reduced pressure, the crude product was purified by silica gel chromatography to provide S8.