A solution of 3 (15 mg, 0.014 mmol) in DMF (2 mL) was treated with 5% piperidine in DMF (2 mL) and the reaction mixture was stirred for 10 min at 25° C. The crude product was precipitated with diethyl ether and used for the next step without further purification. The crude product was dissolved in DMF (3 mL) followed by the addition of 4 (25 mg, 0.025 mmol) in DMF (2 mL) and DIEA (20 uL). The reaction mixture was stirred for 2 hrs at room temperature. TFA was added to the reaction mixture and the reaction mixture was stirred at room temperature for 45 min to yield compound 5 as indicated by analytical HPLC. Crude product was purified by Prep HPLC (SymmetrPrep C18, 7 μm, 19×150 mm column), eluted at 10 ml/min (0.01% TFA in water/acetonitrile) with a gradient: 10% acetonitrile in 5 min, 10% to 50% acetonitrile in 15 min, maintaining 50% acetonitrile in 5 min, 50% to 100% acetonitrile in 5 min, to obtain 5 (3 mg, 94%). MS: (MH)+ 889.
