Step 2: 5-Fluoro-7-nitro-1,3-dihydroindol-2-one (20a, 4.0 g, 20 mmol) was dissolved in 200 mL of acetic acid and palladium/activated carbon catalyst (1.0 g, 5%) was added with stirring at room temperature. The reaction mixture was stirred continuously under hydrogen atmosphere. The reaction progress was monitored by thin layer chromatography (TLC) and stopped when the feedstock completely disappeared. The reaction mixture was filtered to remove the catalyst and the filtrate was concentrated under reduced pressure to afford the white solid product 7-amino-5-fluoro-1,3-dihydroindol-2-one (20b, 3.2 g, 97.5% yield).
