To a solution of 4.53 ml (35.65 mmol) of oxalyl chloride in 76 ml dichloromethane at −50 to −60° C. was added a solution of 5.81 ml (74.40 mmol) dimethylsulfoxide in 16 ml of dichloromethane within 20 min. The solution was stirred for 5 min and a solution of 10.03 g (31.0 mmol) 3-[benzyloxycarbonyl-(2-hydroxy-ethyl)-amino]-propionic acid tert-butyl ester in 67 ml dichloromethane was added within 20 min. The mixture was stirred for 15 min and 21.6 ml (155 mmol) of triethylamine were added within 25 min. The suspension was stirred for 2 h and slowly warmed to 0° C. The reaction was neutralized with cold 10% aq. potassium dihydrogenphosphate solution (adjusted with solid potassium dihydrogenphosphate to pH 4-5) and extracted with diethyl ether (3×). The organic phases were washed with 10% aq. potassium dihydrogenphosphate solution, sat. aq. sodium hydrogencarbonate solution, 10% aq. sodium chloride solution, dried over Na2SO4 evaporated to yield 9.94 g (99.8%) of the titled compound as yellow oil. MS: 322.1 (MH+).
