Reaction Steps
Concentrated HCl (50 mL, 0.600 mmol, 5.5 equiv.) was added to a solution of ampelopsin D (2, 5.0 mg, 0.110 mmol, 1.0 equiv) in MeOH (0.5 mL) at 25° C., and the resultant mixture was stirred at 80° C. for 12 h. Upon completion, the reaction mixture was quenched with water (3 mL) and extracted with EtOAc (3×10 mL). The combined organic layers were then washed with water (5 mL) and brine (5 mL), dried (MgSO4), and concentrated. The resulted light yellow product was purified by flash column chromatography (silica gel, CH2Cl2/MeOH, 9:1) to give isoampelopsin D 17 (4.8 mg, 96%) as a colorless oil. 17: Rf=0.13 (silica gel, CH2Cl2/MeOH, 9:1); IR (film) νmax 3411, 2810, 1680, 1628, 1511, 1443, 1371, 1333, 1206, 1149, 1055, 1006, 833; 1H NMR (300 MHz, methanol-d4) δ 7.11 (d, J=8.4 Hz, 2H), 7.07 (d, J=8.7 Hz, 2H), 6.73 (d, J=8.7 Hz, 2H), 6.66 (d, J=8.7 Hz, 2H), 6.17 (d, J=2.1 Hz, 1H), 6.06 (d, J=1.5 Hz, 1H), 6.06 (d, J=2.1 Hz, 2H), 5.99 (t, J=2.1 Hz, 1H), 4.80 (s, 1H), 3.84 (s, 2H); 13C NMR (75 MHz, methanol-d4) δ 158.9, 158.7, 157.5, 156.5, 154.0, 150.4, 149.9, 144.0, 136.6, 132.1, 131.1, 130.2, 128.9, 125.4, 116.3, 115.8, 108.1, 101.4, 100.7, 56.7, 32.2; HRMS (MALDI-FTMS) calcd for C28H22O6+[M+]454.1416. found 454.1428. All spectroscopic data for this synthetic material match those reported by Niwa and co-workers for natural isoampelopsin (15) [22].
