To a solution of 6.80 g of 4-(1-ethyl-propylamino)-3-nitro-benzoic acid in 210 ml of dry DMF 1.82 g of HOBT, 7.23 g of EDC and 6.6 ml of DIPEA were added at 0° C. After 30 min 7.24 g of L-leucin-tert.-butylester-hydrochloride and 4.7 ml of DIPEA were added and the reaction was stirred at rt for 4 h. The reaction was then concentrated to about a fifth of its volume and poured into water. It was then extracted with ethyl acetate three times. The combined organic phases were washed with saturated aqueous sodium carbonate solution and brine, dried over magnesium sulphate and concentrated to yield 11.50 g (100%) of (S)-2-[4-(1-ethyl-propylamino)-3-nitro-benzoylamino]-4-methyl-pentanoic acid tert-butyl ester.
