Diisopropylethylamine → Diethylbenzyl({[(1R)-1-phenylethyl]carbamoyl}amino)propanedioate

To the product from Step B (7.79 g, 20.54 mmol) in N,N-dimethylormamide (100 mL) was added diisopropylethylamine (10.76 mL, 61.6 mmol) at 0° C. the mixture was stirred for 5 min then (R)-(+)-1-phenylethyl isocyanate (3.76 mL, 26.7 mmol) was added dropwise. The mixture was warmed to ambient temperature over 30 min and stirred for 16 h. Water (150 mL) was added and a white solid crashed out, the mixture was filtered and the solid washed with water. The solid was diluted in ethyl acetate (250 mL) and half saturated aqueous sodium hydrogen carbonate (200 mL) was added and the mixture was extracted with ethyl acetate (4×50 mL). The combined organic fractions were washed with brine (saturated, 1×100 mL), dried with anhydrous sodium sulfate, filtered and the solvent removed in vacuo to afford the title compound as a colorless amorphous solid. LC/MS 413.1 (M+1).