General procedure for the synthesis of 2-methyl-1,3-benzoxazole-6-carboxylic acid from methyl 2-methylbenzo[d]oxazole-6-carboxylate: To a mixed solution of tetrahydrofuran (THF) (300 mL) and water (H2O) (100 mL) of methyl 2-methyl-1,3-benzoxazole-6-carboxylate (8) (12.2 g, 0.064 mol), lithium hydroxide monohydrate (LiOH-H2O) (5.4 g, 0.128 mol) and the reaction mixture was stirred vigorously at room temperature. After 2 h of reaction, THF was removed by rotary evaporator under reduced pressure.The remaining aqueous phase was acidified with acetic acid (HOAc) to pH ≈ 3 and then extracted several times with dichloromethane (CH2Cl2). The organic phases were combined, dried with anhydrous sodium sulfate (Na2SO4), filtered and concentrated to give a beige solid product (10.2 g, 90% yield): melting point 245-246 °C (literature value 245-246 °C); thin-layer chromatography (TLC) (silica gel plate, unfolding agent ethyl acetate/methanol (90:10), detected under a UV lamp) showed a single spot with an Rf value of 0.38. Nuclear magnetic resonance hydrogen spectrum (1H NMR) (300 MHz, DMSO-d6): δ 2.66 (s, 3H, NCH3), 7.75 (d, J = 8.3 Hz, 1H, H-5), 7.96 (dd, J = 8.3, 1.4 Hz, 1H, H-4), 8.16 (d, J = 0.7 Hz, 1H, H-7), 13.12 (bs , 1H, OH).
