Cl.[NH2:2][C@H:3]([CH2:8][NH:9][C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[C:4]([O:6][CH3:7])=[O:5].Cl[CH2:18][CH2:19][N:20]([CH2:25][CH2:26]Cl)[S:21]([CH3:24])(=[O:23])=[O:22].O>C(N(CC)C(C)C)(C)C>[C:13]([O:12][C:10]([NH:9][CH2:8][C@@H:3]([N:2]1[CH2:26][CH2:25][N:20]([S:21]([CH3:24])(=[O:23])=[O:22])[CH2:19][CH2:18]1)[C:4]([O:6][CH3:7])=[O:5])=[O:11])([CH3:16])([CH3:15])[CH3:14]A solution of 6 g (23.6 mmol) of methyl (R)-2-amino-3-tert-butoxycarbonylaminopropanoate hydrochloride and 5.2 g (23.6 mmol) of N,N-bis(2-chloroethyl)methanesulphonamide in 80 ml of N,N-diisopropylethylamine is heated at 125° C. with vigorous stirring for 18 h. After addition of water, the product is extracted with ethyl acetate. The organic phases are combined, washed with water, dried over magnesium sulphate, filtered and concentrated under vacuum. The crude product obtained is purified by chromatography over silica gel eluted with a 5/5 heptane/ethyl acetate mixture. 3.8 g (44%) of methyl (R)-3-tert-butoxycarbonylamino-2-(4-methanesulphonylpiperazin-1-yl)propanoate are obtained in the form of a white solid.
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