3.51 g (13.25 mmol) of Cbz-L-Leucine, 2.41 g (1 eq.) of (S)-2-amino-4-butyrolactone hydrobromide, 1.97 g of HOBT (1.1 eq.) and 5.59 g (2.2 eq.) of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) is dissolved in 60 ml of anhydrous DMF then 7.64 ml (3.3 eq.) of N,N-diisopropylethylamine is added. The reaction mixture is stirred for 15 hours at 20° C. before being poured into 200 ml of a 1/1 mixture of ethyl acetate/water. After stirring and decantation, the organic solution is washed successively with 100 ml of a saturated solution of NaHCO3, 50 ml of water, 100 ml of a 1M solution of citric acid and finally 100 ml of a solution of salt water. The organic phase is dried over sodium sulphate, filtered and concentrated to dryness under vacuum. The oil obtained is washed using isopentane and then crystallized from a dichloromethane/isopentane mixture. A white solid is obtained with a yield of 68%. Melting point: 130-131° C.
