To a solution of the crude compound obtained in step 2 (219 mg, 0.50 mmol), 1-tert-butoxycarbonyl-piperazine (98 mg, 0.55 mmol), N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride (105 mg, 0.55 mmol) and 1-hydroxybenzotriazole (88 mg, 0.58 mmol) in DMF (5 ml) was added N-methyl-morpholine (330 μl, 3.00 mmol), and the mixture was stirred at room temperature overnight. The mixture was quenched with water and extracted with EA. The organic layer was separated, dried over sodium sulfate, filtered and evaporated. The residue was purified by silica gel chromatography (HEP/EA). 300 mg of the title compound were obtained.
