4-Methoxy-2-benzofurancarboxylic acid, Ethyl ester
Procedure for the Synthesis of Benzofuran 15a
To a suspension of CsF (304 mg, 2.0 mmol) in DMF (4.0 mL) were added 3-methoxy-2-(trimethylsilyl)phenyl triflate (1, 105 µL, 0.40 mmol) and ethyl iodoacetate 14 (95 µL, 0.80 mmol) under argon atmosphere at 100 °C. After being stirred at the same temperature for 12 h, the reaction mixture was diluted with saturated NaHCO3 and then extracted with CH2Cl2. The organic phase was dried over Na2SO4 and concentrated at reduced pressure. Purification of the residue by flash silica gel column chromatography (EtOAc/hexane = 1:20–1:4) afforded the product 15a (35 mg, 40%). Product 16 was also formed.
4-Methoxy-2-benzofurancarboxylic acid, Ethyl ester (15a). Colorless oil.
IR (KBr) 2981, 1726, 1609, 1570, 1500 cm−1.
1H-NMR (CDCl3) δ 7.62 (1H, d, J = 1.0 Hz), 7.35 (1H, t, J = 8.2 Hz), 7.18 (1H, d, J = 8.2 Hz), 6.67 (1H, d, J = 8.2 Hz), 4.43 (2H, q, J = 7.1 Hz), 3.94 (3H, s), 1.41 (3H, t, J = 7.1 Hz).
13C-NMR (CDCl3) δ 159.5, 156.9, 154.6, 144.4, 128.5, 117.8, 111.6, 105.1, 103.5, 61.4, 55.6, 14.3.
HRMS (ESI+) calcd for C12H13O4 (M+H+): 221.0808, Found: 221.0806.
Procedure for Transformation of Dihydrobenzofuran 8a into Benzofuran 15a
To a solution of dihydrobenzofuran 8a (50 mg, 0.16 mmol) in THF (3.2 mL) was added KHMDS (0.50 M in toluene, 320 µL, 0.16 mmol) under argon atmosphere at −40 °C. After being stirred at the same temperature for 12 h, the reaction mixture was diluted with saturated NaHCO3 and then extracted with CH2Cl2. The organic phase was dried over Na2SO4 and concentrated at reduced pressure. Purification of the residue by PTLC (EtOAc/hexane = 1:4 with 2% CH2Cl2) afforded benzofuran 15a (33 mg, 96%).
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