VAL-CIT-PAB.

The general steps for synthesizing compound (CAS:159857-79-1) from compound (CAS:159858-22-7) are as follows: First, Fmoc-val-cit-PAB-OH (14.61 g, 24.3 mmol, 1.0 equivalent; see US Patent No. 6,214,345 by Firestone et al.) is diluted with DMF (120 mL, 0.2 M). Diethylamine (60 mL) is added to this solution. The reaction progress is monitored by HPLC, confirming that the reaction is complete within 2 hours. The reaction mixture is then concentrated, and the resulting residue is precipitated using ethyl acetate (about 100 mL) under ultrasonic treatment for 10 minutes. After adding ether (200 mL), the precipitate is further ultrasonicated for 5 minutes. Let the solution stand for 30 minutes without stirring, then filter and dry under high vacuum to obtain Val-cit-PAB-OH, which is used in the next step without further purification. Yield: 8.84 g (96%). Next, Val-cit-PAB-OH (8.0 g, 21 mmol) is diluted with DMF (110 mL) and treated with MC-OSu (6.5 g, 21 mmol, 1.0 equivalent; see Willner et al., 1993, Bioconjugate Chem. 4:521). After 2 hours, the reaction is confirmed complete by HPLC. The reaction mixture is concentrated and the resulting oily substance is precipitated with ethyl acetate (50 mL). After 15 minutes of ultrasonic treatment, ether (400 mL) is added and the mixture is further ultrasonicated until all large particles are broken. The solution is then filtered and the solid is dried, obtaining a gray-white solid intermediate.

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