To a solution of 1 g of 4-Hydroxy-quinoline-2-carboxylic acid and 2.6 g of 4-((S)-2-Amino-4-benzyloxy-butyryl)-piperazine-1-carboxylic acid butyl ester trifluoroacetate in 12 ml of DMF, 0.8 g of HOBT, 1.0 g of EDC and 1.2 g of NEM was added and the reaction mixture was stirred for 16 h at RT. Then, the reaction mixture was diluted with water and extracted with ethyl acetate. The organic phase was dried over MgSO4 and the solvents were removed under reduced pressure. The crude product was precipitated from a mixture of n-heptane and ethyl acetate. The precipitated product was collected by filtration and dried under reduced pressure. The isolated product was pure enough for the next reaction step. Yield: 900 mg.
