mL) was added. After stirring at 0 °C for 30 min, pH ≈ 3 or 4 was adjusted by addition of 1 M aqueous HCl solution. The solvent was removed under vacuum and the residue was purified by flash column chromatography on a reversed-phase silica gel column C18 (4 g, 40 μm) with a 0–15% l