To a solution of the objective compound of step (9) (87.8 g, 0.308 mol), 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (480 mg 3.07 mmol) and sodium bromide (31.7 g, 0.308 mol) in ethyl acetate (900 mL)-toluene (900 mL) were added 6% sodium hypochlorite aqueous solution (PURELOX, 410 mL, 0.34 mol) and a solution of sodium hydrogencarbonate (75 g, 0.89 mol) in water (540 mL) with dropwise over 1.5 hours under ice-cooling, and the mixture was stirred for 1 hour. This reaction mixture was extracted with ethyl acetate and the extract was washed successively with potassium iodide (2.5 g, 15 mmol)-containing 10% potassium hydrogensulfate aqueous solution (400 mL), 10% sodium thiosulfate aqueous solution (2×200 mL), 0.2 M phosphate buffer (pH 7, 500 mL) and saturated brine and dried over magnesium sulfate. The solvent was then distilled off under reduced pressure. To the residue were added ethyl acetate (150 mL) and hexane (500 mL), and the mixture was stirred at room temperature ˜0° C. for 5 hours. The resulting crystal crop was collected by filtration, washed with hexane-ethyl acetate (3:1, 300 mL) and dried in vacuo to give 63.28 g (73%) of N-benzyloxycarbonyl-DL-phenylalaninal as white crystals.
