To a solution of (S)-2-tert-butoxycarbonylamino-3-thiophen-2-yl-propionic acid (1.1 g, 4.06 mmol) and 2-fluoro-4-iodoaniline (800 mg, 3.38 mmol) in pyridine (15 mL) at −10° C. was slowly added phosphorus oxychloride (0.35 mL, 3.72 mmol) under an atmosphere of dry nitrogen. The mixture was stirred for 2 hours at −10° C. After removal of the solvent and the excess reagent by rotary evaporator, ice water was added. The mixture was extracted with dichloromethane and the organic layer washed with 1 M aqueous citric acid, brine, saturated aqueous sodium carbonate, brine and dried over sodium sulfate. The solvents were removed to give [(S)-1-(2-fluoro-4-iodo-phenylcarbamoyl)-2-thiophen-2-yl-ethyl]-carbamic acid tert-butyl ester as a yellow viscous oil for use in the next step (1.52 g, 92%).
