Step A: Synthesis of 6-nitrobenzofuran-1(3H)-one
1. dissolve phthalide (40.0 g, 298 mmol) in sulfuric acid (50.0 mL) under ice bath conditions and stir until homogeneous.
2. dissolve potassium nitrate (30.4 g, 300 mmol) in sulfuric acid (80 mL) and add to the above sulfuric acid solution of phthalide by dropping.
3. Keeping the reaction mixture under stirring, the reaction mixture was slowly warmed up to room temperature, which took about 5 hours.
4. Upon completion of the reaction, the mixture was slowly poured into ice water to precipitate.
5. The precipitate was collected and recrystallized from ethanol (3 L) to give 6-nitrobenzofuran-1(3H)-one (50.4 g, 94% yield) as an off-white solid.
6. The structure and purity (98%) of the product were confirmed by nuclear magnetic resonance hydrogen spectroscopy (1H NMR, 500 MHz, CDCl3) with the following characteristic peaks: δ 8.77 (d, J = 1.5 Hz, 1H), 8.58 (dd, J = 8.5, 2.0 Hz, 1H), 7.72 (d, J = 8.5 Hz, 1H), 5.45 (s, 2H).
