4-Methylmorpholine (1.89 mL, 17.2 mmol, 1 equiv) and ethyl chloroformate (1.65 mL, 17.3 mmol, 1 equiv) were added to a mixture of commercially available Cbz-Gln(Trt)-OH (9.00 g, 17.2 mmol, 1 equiv) in THF (23 mL) at −10° C. After stirring 20 min, the reaction mixture was filtered and the filtrate was added dropwise to a suspension of NaBH4 (1.47 g, 38.9 mmol, 2.25 equiv) in H2O (10 mL) at 0° C. The resulting mixture was allowed to warm to 23° C. and stirred for 5 h. It was then cooled again to 0° C., quenched by the careful addition of 1 N HCl (30 mL) and then partitioned between MTBE (500 mL) and brine (2×100 mL). The organic phase was dried over Na2SO4 and evaporated to provide the title intermediate which was used without further purification.
