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A stirred solution of 23 (1.0 g, 3.74 mmol) in acetic acid (10 mL) was refluxed for 16 h. After cooling, the solution was evaporated to dryness. The crude product was adsorbed on silica gel and purified by flash chromatography, eluting with hexane/EtOAc (4:1) to give 24 (160 mg, 24%) as a white solid. 1H NMR (500 MHz, CDCl3): δ: 9.80 (1H, s, CHO), 7.49-7.45 (2H, m, ArH), 7.12-7.09 (2H, m, ArH), 3.85 (3H, s, OCH3).

1. temperatureAfter cooling
2. customthe solution was evaporated to dryness
3. custompurified by flash chromatography
4. washeluting with hexane/EtOAc (4:1)