The synthesis of HATU is as follows:The resulting residue treated with 2-Methoxycarbonylamino-3-methyl-butyric acid (60 mg, 0.343 mmol) and HATU (130 mg, 0.343 mmol), suspended in DMF (3 mL) and cooled to 0° C. DIPEA (0.272 mL, 1.56 mmol) was added dropwise. After stirring for 4 h, NaOH (5M in H2O, 0.300 mL, 1.5 mmol) was added. This mixture was stirred for 3 h then diluted with EtOAc and washed with 1 M LiOH (2*) then brine. The organic phase was dried over MgSO4, filtered and concentrated. The crude residue was then purified by HPLC to afford the title compound (53 mg, 44%).