Compound 9a
(5aR*,10bS*)-methyl 4-hydroxy-6H-5a,10b-butanobenzofuro[2,3-b]indole-3-carboxylate
From: Unified biomimetic assembly of voacalgine A and bipleiophylline via divergent oxidative couplings
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Compound data: CIF
Synthetic procedure: See article for the definitive version of this procedure and for full experimental details.
General procedure A for the synthesis of benzofuro[2,3-b]indolines (9) from methyl 2,3-dihydroxybenzoate (7): to a solution of indole 6 (1 equiv.), methyl 2,3-dihydroxybenzoate 7 (2 equiv.) in dichloromethane is added formic acid (2 equiv.) and then silver oxide (5 equiv.) at rt. After stirring, the reaction mixture is filtered through a pad of silica gel with ethyl acetate and concentrated to dryness. The residue is purified by preparative thin layer chromatography over silica gel affording the titled benzofuro[2,3-b]indoline .
According to general procedure A, indole 6a (30.5 mg, 0.178 mmol), methyl 2,3-dihydroxybenzoate 7 (59.8 mg, 0.356 mmol), formic acid (14 µL, 0.356 mmol) and silver oxide (206.2 mg, 0.89 mmol) in dichloromethane (1.8 mL) delivered after 40 min. the titled compound 9a (23.0 mg, 38%) and 8a (5 mg, 8%) after preparative TLC (petroleum ether/ethyl acetate 80:20). 9a: yellow solid, melting point: 194-198 °C, Rf = 0.51 (pentane/AcOEt 70:30). IR (neat): ν max = 3382, 2942, 1676, 1468, 1439, 1294, 1145, 995, 738 cm–1. HRMS ESI: m/z [MH]+: 338.1396; calculated for C20H20NO4+: 338.1392. 1H NMR (300 MHz, chloroform-d), δ (ppm): 10.54 (s, 1H), 7.44 (d, J = 8.1 Hz, 1H), 7.04 (t, J = 7.4 Hz, 1H), 7.01 (d, J = 7.4 Hz, 1H), 6.90 (d, J = 8.1 Hz, 1H), 6.75 (d, J = 7.4 Hz, 1H), 6.71 (t, J = 7.4 Hz, 1H), 4.90 (br s, 1H), 3.89 (s, 3H), 2.45 (dt, J = 13.9; 4 Hz, 1H), 2.31 (dt, J = 13.9; 4 Hz, 1H), 2.02-1.88 (m, 1H), 1.73-1.21 (m, 5H). 13C NMR (90 MHz, chloroform-d), δ (ppm): 170.8, 147.0, 146.8, 146.7, 139.1, 133.9, 128.3, 122.9, 122.7, 120.4, 113.1, 112.8, 110.1, 57.1, 52.5, 32.4, 32.2, 21.0, 20.2.
