[Synthesis]
Compound B-42 (1.0 g, 6.6 mmol) was mixed with concentrated sulfuric acid (1.4 mL) and the reaction was stirred at 85 °C for 5 min. Upon completion of the reaction, the mixture was cooled to room temperature, slowly poured into ice water (40 mL) and neutralized with saturated aqueous sodium bicarbonate to pH=7-8. Subsequently, the mixture was diluted by addition of dichloromethane (100 mL) and stirred for 12 hr at 20 °C. Upon completion of the reaction, the precipitated product was collected by filtration. The resulting solid was further purified by silica gel column with eluent ratio of dichloromethane: methanol = 10:1, resulting in compound B-43 (0.9 g, 63% yield) as a yellow solid.1H-NMR (CDCl3, 400 MHz) data were as follows: δ 8.67 (s, 1H), 7.90 (q, J=8.0 Hz, 1H), 7.23 (dd, J =4.0 Hz, J=12Hz, 1H), 7.07-7.02 (m, 1H), 6.82 (s, 1H). |