Compound 134 (1.70 g, 6.8 mmol) was dissolved in anhydrous THF (20 mL) under argon protection. To this solution was sequentially added diisopropylethylamine (1.30 mL, 7.5 mmol, 1.1 eq.) and di-tert-butyl dicarbonate (1.78 g, 8.2 mmol, 1.20 eq.). The reaction mixture was stirred at room temperature for 16 hours. After completion of the reaction, the mixture was dissolved in EtOAc (100 mL) and washed sequentially with 0.3 M HCl solution (20 mL), half-saturated NaHCO3 solution, 14% NH4OH solution (30 mL) and brine (50 mL). The organic phase was dried with Na2SO4, filtered and concentrated under reduced pressure to give the Boc-protected diamino ester crude product. The crude product was purified by CombiFlash system (eluent: EtOAc in DCM, gradient 0-5%) to give a viscous oily product (2.10 g, 88% yield). The product was characterized by 1H NMR (300 MHz, CDCl3) and 13C NMR (75 MHz, CDCl3) and the data were consistent with the target structure.
