[Synthesis]
General procedure for the synthesis of methyl benzo[d]oxazole-7-carboxylate from methyl 3-amino-2-hydroxybenzoate and triethyl orthoformate: Methyl 3-amino-2-hydroxybenzoate (1.69 g, 10.13 mmol) was suspended in toluene (2 mL) in a microwave reaction flask. Triethyl orthoformate (1.80 g, 12.15 mmol) and ytterbium trifluoromethanesulfonate (Yb(OTf)3, 0.003 g, 0.005 mmol) were subsequently added. The reaction mixture was microwaved at 100 °C for 1 hour. After completion of the reaction, the solvent was removed by evaporation and the crude product was purified by fast column chromatography (eluent: petroleum ether/ethyl acetate, 4:1, v/v) to afford methyl benzo[d]oxazole-7-carboxylate (1.63 g, 91% yield) as light yellow crystals. The melting point was 116-118 °C (literature value: 9.77 °C).1H NMR (300 MHz, CDCl3) δ 8.21 (s, 1H), 8.04 (dd, J = 7.8, 1.2 Hz, 1H), 7.99 (dd, J = 8.0, 1.2 Hz, 1H), 7.44 (dd, J = 8.0, 7.8 Hz, 1H). 3.76 (s, 3H).13C NMR (75 MHz, CDCl3) δ 180.9, 164.4, 153.4, 141.4, 128.0, 125.6, 124.5, 115.4, 52.6. |