The crude hydrochloride salt was taken into DMF (3 mL), and the solution was cooled to 0 °C before adding 4-methoxy-indole-2-carboxylic acid (73 mg, 0.38 mmol), collidine (125 μL, 0.95 mmol), and HATU (144 mg, 0.38 mmol) in order, and the resulting suspension was stirred at 0 °C for 6 h.