Compound 10a
(6aR*,11bR*)-3,4-dihydroxy-6a,11b-butanoisochromeno[3,4-b]indol-5(7H)-one
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 B for the synthesis of isochromeno[3,4-b]indolines (10) from 1,2-bishydroxybenzoic acid (3): to a solution of indole 6 (1 equiv.) and 2,3-dihydroxybenzoic acid 3 (2 equiv.) in acetonitrile is added formic acid (2 equiv.) and then silver oxide (5 equiv.). After 5-12 hr. stirring, the reaction mixture is filtered through a pad of silica gel with ethyl acetate and concentrated to dryness. The residue is purified by flash chromatography over silica gel affording the titled isochromeno[3,4-b]indoline .
According to general procedure B, indole 6a (31.5 mg, 0.184 mmol), 2,3-dihydroxybenzoic acid 3 (58.0 mg, 0.376 mmol), formic acid (14 µL, 0.368 mmol) and silver oxide (212 mg, 0.920 mmol) in acetonitrile (1.8 mL) delivered after 12 hr. the titled compound 10a (37 mg, 62 %) after flash chromatography over silica gel (pentane/diethyl ether 50:50). Off white solid. Melting point: 225-229 °C Rf = 0.35 (pentane/diethyl ether 50:50). IR (neat): ν max = 3360, 2934, 1705, 1661, 1487, 1449, 1272, 1276, 1254,1167, 919, 873, 803, 759, 739 cm–1. HRMS ESI: m/z [MH]+: 324.1224; calculated for C19H17NO4+: 324.1230. 1H NMR (360 MHz, acetone-d6), δ (ppm): 11.81 (s, 1H), 8.13 (s, 1H), 7.30(d, J =8.3 Hz, 1H), 7.26 (d, J = 8.2 Hz, 1H), 7.07 (t, J = 7.5 Hz, 1H), 6.84 (d, J = 7.5 Hz, 1H), 6.77 (d, J = 7.5 Hz, 1H), 6.71 (t, J = 7.5 Hz, 1H), 6.55 (s, 1H), 2.65 (d, J = 13.5 Hz, 1H), 2.39 (d, J = 13.5 Hz, 1H), 1.85-1.59 (m, 3H), 1.54-1.42 (m, 2H), 1.24-1.12 (m, 1H). 13C NMR (90 MHz, acetone-d6), δ (ppm): 170.4, 152.1, 147.3, 146.0, 137.1, 130.2, 128.8, 123.1, 122.4, 120.6, 117.6, 111.2, 109.4, 108.2, 50.6, 36.7, 32.5, 24.3, 21.5.
