Toluene-p-sulphonic anhydride → Allyl 2-(4-fluoro-1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-3-oxo-butyrate

Allyl 3-(4-fluoro-1,3-dioxo-1,3-dihydro-isoindol-2-yl)-2-(2-methyl-[1,3]dioxolan-2-yl)propionate (331 mg, 0.91 mmol) was dissolved in a a solvent mixture of acetone/water (v/v=5/1), and catalyst amount of p-toluenesulfonic acid anhydride was added to the obtained solution. The resulting mixture was refluxed at 80° C. for 18 hours. After the reaction was completed, the reaction mixture was washed with water, and then acetone was removed by distilling under a reduced pressure. The residue was extracted with dichloromethane. The combined organic layer was dried with anhydrous sulfate and distilled under a reduced pressure. The desired product was obtained (131.2 mg, 31%) by performing column chromatography (eluent: n-hexane/ethyl acetate=2/1).