Step A: 2-Amino-3-hydroxybenzoic acid hydrobromide (300 mg, 1.28 mmol), triethyl orthoformate (0.85 mL, 5.1 mmol), pyridinium p-toluenesulfonate (64 mg, 0.26 mmol), and xylene (15 mL) were added to the reaction flask, and the mixture reacted for 4 hr under reflux conditions. Upon completion of the reaction, the reaction mixture was cooled to room temperature and subsequently concentrated under reduced pressure to afford benzo[D]oxazole-4-carboxylic acid (210 mg, quantitative yield), the product was a light yellow solid. The product was characterized by 1H NMR (300 MHz, DMSO-d6): δ 13.12 (s, 1H), 8.86 (s, 1H), 8.03 (dd, J = 8.1, 0.9 Hz, 1H), 7.92 (dd, J = 8.1, 0.9 Hz, 1H), 7.54 (t, J = 8.1 Hz, 1H); the mass spectrometry (ESI+) showed m/ z 164 ([M + H]+).
