At -78 °C, 5-vinylbenzo[d]oxazole (LX) (6.1 g, 42.1 mmol, 1.0 eq.) was dissolved in dichloromethane (DCM, 100 mL) and ozone was passed through the solution until the solution took on a blue color. Subsequently, the solution was purged with oxygen (O2) and then nitrogen (N2) for 5 minutes. Triethylamine (TEA, 12.8 g, 126.3 mmol, 3.0 eq.) was added and the reaction mixture was stirred at 25 °C for 1 hour. Upon completion of the reaction, the solution was poured into water (200 mL) and extracted with ethyl acetate (EtOAc, 3 x 300 mL). The organic phases were combined, dried over anhydrous sodium sulfate and filtered, and the filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (eluent: petroleum ether (PE):ethyl acetate (EtOAc) = 10:1→1:1) to afford benzo[d]oxazole-5-carboxaldehyde (LXI) (2.1 g, 14.3 mmol, 33.9% yield) as a colorless oil.1H NMR (CDCl3, 400 MHz) δ ppm: 7.75 (d, J = 8.4Hz, 1H), 8.02 (dd, J = 8.4Hz, J = 1.2Hz, 1H), 8.22 (s, 1H), 8.33 (s, 1H), 10.13 (s, 1H); ESIMS m/z: 148.0 ([M + H]+, C8H5NO2).
