Methanesulfonic Acid → Phthalimide

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To a stirred solution of acetic anhydride (5.0 mL, 53.0 mmol) and 7.7 wt % phosphorus pentoxide in methanesulfonic acid (Eaton's Reagent) (5.0 mL, 4.60 mmol) was added 2-(3-oxobutyl)isoindoline-1,3-dione (1.0 g, 4.60 mmol). The reaction was heated at 70° C. and for 8 h. The reaction was allowed to cool to room temperature and maintained overnight. The reaction was diluted with EtOAc, washed with ice cold water, dried (Na2SO4), filtered and concentrated under vacuum. The residue was purified by silica gel chromatography (Analogix, SF40-115 g, 20-100% EtOAc in hexanes). The pure fractions were combined and evaporated to dryness and triturated with 10% EtOAc in hexanes, filtered and evaporated to dryness to give 2-(2,6-dimethyl-4-oxo-4H-pyran-3-yl)methyl)isoindoline-1,3-dione (290 mg, 1.024 mmol, 22.24% yield) as a yellow solid. 1H NMR (400 MHz, DMSO-d6) δ 7.91-7.74 (m, 4 H), 6.06 (s, 1 H), 4.54 (s, 2 H), 2.43 (s, 3 H), 2.22 (s, 3 H). MS(ES) [M+H]+ 284.0.

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