11.49 g of ethyl (S)-3-amino-3-phenylpropionate hydrochloride were suspended in 100 ml of tetrahydrofuran and cooled to 0-10° C. in an icebath. After adding 14 ml of triethylamine, a solution of 11.52 g of 3,5-dinitrobenzoyl chloride in 40 ml of tetrahydrofuran was added with stirring and cooling over a period of 30 min. The cooling bath was removed, and the reaction mixture stirred for a further 120 min, in the course of which it warmed to room temperature. The reaction mixture was filtered, and the filtrate concentrated to dryness (product fraction 1). The filtercake—consisting of triethylamine hydrochloride and further product—was taken up in 100 ml of ethyl acetate and digested at 35° C. for 20 min. The suspension was filtered, and the filtercake discarded. The filtrate was concentrated to dryness under reduced pressure at 40° C. (product fraction 2). The product fractions were combined. Yield: 16.4 g; HPLC purity >97 area %. For further purification, the crude product was digested in 400 ml of water at room temperature for 2 h and then filtered. The moist yield was 15.4 g; HPLC purity 99.8 area %. The product was reacted further without further purification.
