Dicyclohexylcarbodiimide → Z-Ala-Ala-OSu

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The active ester Z-Ala-Ala-OSu (1) was prepared according to a procedure described in Bodansky, M., Bodansky, A., The Practice of Peptide Synthesis 2nd Edition, Springer-Verlag. To a stirred solution of Z-Ala-Ala-OH (4.0 g, 13.6 mmol, obtained from Bachem) in 25 ml dry THF N-hydroxysuccinimide (1.56 g, 13.6 mmol) was added at 0° C. and the mixture was stirred for 10 minutes. A solution of dicyclohexylcarbodiimide (2.81 g, 13.6 mmol) in 3 ml dry THF was added. The mixture was stirred for 14 h, during which time it was allowed to warm to room temperature. The separated N,N′-dicyclohexylurea was removed by filtration and the solvent was evaporated in vacuo. The residue was twice recrystallized from isopropanol to give the active ester as a white solid (4.47 g, 84%) of m.p. 140° C.—TLC(MeOH/CHCl3, 1:50): Rf=0.5.—1H NMR (400 MHz, DMSO-d6): δ=1.20 (d, 3 H, J=7.0 Hz, CH3), 1.44 (d, 3 H, J=7.4 Hz, CH3), 2.79 (s, 4 H, 2×CH2), 4.04-4.10 (m, 1 H, CH), 4.63-4.69 (m, 1 H, CH), 4.96-5.04 (m, 2 H, CH2), 7.28-7.38 (m, 5 H, aryl-H), 7.45 (d, 1 H, J=7.8 Hz, NH), 8.58 (d, 1 H, J=7.0 Hz, NH).—MS (EI) m/z (%): 392 [M+H+], 414 [M+Na+], 430 [M+K+].

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