NH2OTHP (0.08 g, 0.72 mmol) was added to a solution of 3 (0.18 g, 0.60 mmol), PyBOP (0.33 g, 0.63 mmol), and triethylamine (0.20 ml, 1.43 mmol) in DMF (1.5 mL). The reaction mixture was stirred at room temperature for 2 h before it was quenched with water and extracted with EtOAc (15 mL×3). The combined organic layer was dried over anhydrous MgSO4 and concentrated under reduced pressure. The residue was purified by silica gel chromatography (CH2Cl2:CH3OH=30:1:1% NH3(aq)) to give a white solid, which was treated with TFA (1.70 ml, 22.89 mmol) in the presence of CH3OH (31 mL). The reaction mixture was stirred overnight at room temperature before it was concentrated under reduced pressure to give a white residue. The residue was recrystallized with CH3OH to afford Compound 1 (0.10 g). 1H NMR (500 MHz, CD3OD): δ 6.80 (d, J=3.65 Hz, 1H), 7.49-7.52 (m, 2H), 7.59-7.62 (m, 1H), 7.68 (d, J=8.53 Hz, 1H), 7.75 (d, J=3.72 Hz, 1H), 7.93 (d, J=7.52 Hz, 2H), 7.94-7.97 (m, 1H), 8.04 (d, J=8.53 Hz, 1H); HRMS (EI) for C15H12N2O4S (M+): calcd, 316.0518. found, 316.0518.
