Compound 15 (CAS: 19164-42-2, 10 g, 0.061 mol) was used as starting material and dissolved in dichloromethane (50 mL). A solution of triphenylphosphine (16 g, 0.061 mol) in dichloromethane (50 mL) was slowly added dropwise at 0 °C in an ice bath. After the dropwise addition was completed, the reaction mixture was continued to be stirred at 0 °C for 30 min. Upon completion of the reaction, the solvent was removed by rotary evaporator under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography using ethyl acetate/petroleum ether (1:20, v/v) as eluent. The target fraction was collected and concentrated to give benzo[c][1,2,5]oxadiazole-5-carbaldehyde (product 16, 6.3 g, 70% yield) as white needle-like crystals. The melting point is 55-56°C (literature values are consistent).
