Compound 18a
(5aR*,9aR*,14bS*,15bS*)-15-hydroxy-5a,9a,14b,15b-tetramethyl-5,5a,7,9a,14b,15b-hexahydro-indolo[3'',2'':4',5']furo[3',2':6,7]isochromeno[3,4-b]indol-7-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.
According to general procedure B, as outlined in the synthesis of 10a, indole 6j (139.0 mg, 0.962 mmol), 2,3-dihydroxybenzoic acid 3 (76.0 mg, 0.481 mmol), formic acid (37 µL, 0.962 mmol) and silver oxide (555 mg, 2.43 mmol) in acetonitrile (4.8 mL) delivered after 12 hr. a 1:1 mixture (61 mg, 29 %) of the diastereomeric titled compounds 18a and 18b after flash chromatography over silica gel (pentane/ diethyl ether 50:50). Preparative thin layer chromatography over silica gel (pentane/ethyl acetate 70:30) allows to obtain separately 18a and 18b.
Second anchoring procedure: indole 6j (29.3 mg, 0.202 mmol), 3,4-dihydroxy-6a,11b-dimethyl-6a,7-dihydroisochromeno[3,4-b]indol-5(11bH)-one 10j (40.0 mg, 0.135 mmol), formic acid (12 µL, 0.404 mmol) and silver oxide (156 mg, 0.673 mmol) in dichloromethane (2.0 mL) delivered after 96 hr. a 1:1.6 mixture of the diastereomeric compounds 18a and 18b (29.6 mg, 50 %). Preparative HPLC purification (XBridge® column, 5% to 100% MeCN in H2O (0.2% HCOOH) over 20 min at 42 mL/min, RT = 18.70 and 18.95 min) allows to obtain separately 18a and 18b. 18a: off white solid; melting point: 240 °C (decomp.); Rf = 0.30 (pentane/ ethyl acetate 70:30). IR (neat): ν max = 3367, 1661, 1699, 1468, 1439, 1321, 1296, 1234, 1164, 1062, 1039, 996, 867, 744 cm–1. HRMS ESI: m/z [MNa]+: 463.1614; calculated for C27H24N2NaO4+: 463.1628 1H NMR (360 MHz, acetone-d6), δ (ppm): 11.53 (s, 1H), 7.60 (s, 1H), 7.43 (d, J = 7.7 Hz, 1H), 7.10-7.01 (m, 2H), 6.91 (d, J = 7.7 Hz, 1H), 6.79-6.65 (m, 4H), 6.58 (s, 1H), 3.80 (s, 1H), 1.78 (s, 3H), 1.72 (s, 3H), 1.69 (s, 3H), 1.63 (s, 3H). 13C NMR (90 MHz, acetone-d6), δ (ppm): 170.2, 148.5, 147.8, 147.6, 145.9, 142.9, 136.7, 134.8, 133.9, 129.1, 129.0, 124.1, 123.6, 120.7, 119.7, 112.8, 110.6, 109.9, 109.3, 106.7, 58.7, 50.5, 24.2, 21.3, 20.9, 20.0.
