The general procedure for the synthesis of (R)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-((4-(tert-butoxy)-4-oxobutyl)thio)propanoic acid from methyl chloroformate-9-fluorenyl and S-(4-(tert-butoxy)-4-oxobutyl)-L-cysteine was as follows. cysteine (2 g, 7.60 mmol) was dissolved in 10% aqueous Na2CO3 solution, and a THF (7 mL) solution of methyl 9-fluorenyl chloroformate (2.94 g, 11.4 mmol) was slowly added. The reaction mixture was stirred at room temperature overnight. After completion of the reaction, the pH was adjusted to 4~5 with 10% aqueous citric acid solution. THF was removed by distillation under reduced pressure and the residual aqueous solution was extracted with CH2Cl2. The organic phase was washed sequentially with saturated NaCl solution, dried with anhydrous Na2SO4 and concentrated. The crude product was purified by silica gel column chromatography to afford the target compound (550 mg, 15% yield). The structure of the product was confirmed by 1H NMR and 13C NMR: 1H NMR (400 MHz, DMSO-d6) δ 7.85 (d, J = 7.5 Hz, 2H), 7.77-7.61 (m, 3H), 7.37 (t, J = 7.5 Hz, 2H), 7.28 (t, J = 7.4 Hz, 2H), 4.32-4.14 (m, 3H), 4.08 (m, 3H). 3H), 4.08 (qd, J = 8.8, 4.6 Hz, 1H), 2.86 (dt, J = 24.4, 10.2 Hz, 1H), 2.70 (ddd, J = 15.2, 9.6, 5.6 Hz, 1H), 2.55-2.41 (m, 2H), 2.23 (t, J = 7.3 Hz, 2H), 1.68 (dd, J = 8.4, 6.1 Hz, 2H), 1.33 (s, 9H); 13C NMR (100 MHz, DMSO-d6): δ 172.86, 172.31, 156.54, 144.32, 141.26, 141.24, 128.17, 127.59, 125.84, 125.80, 120.64, 80.13, 66.26, 54.58, 47.14, 34.11, 33.05, 33.04, 31.15, 28.24, 25.03.
