[C:1]([O:5][C:6]([NH:8][C@H:9]([C:13]([OH:15])=[O:14])[CH2:10][S:11][CH3:12])=[O:7])([CH3:4])([CH3:3])[CH3:2].[CH2:16]([O:18][C:19](Cl)=[O:20])[CH3:17].C(N(CC)CC)C>C1COCC1.O>[C:1]([O:5][C:6]([NH:8][C@H:9]([C:13]([O:15][C:19]([O:18][CH2:16][CH3:17])=[O:20])=[O:14])[CH2:10][S:11][CH3:12])=[O:7])([CH3:4])([CH3:2])[CH3:3]To a solution of N-(tert-butoxycarbonyl)-S-methylcysteine (23.5 g, 0.100 mol) in THF (400 mL) was added dropwise ethylchloroformate (9.6 mL, 0.10 mol) and triethylamine (10.1 g, 0.100 mol) slowly at −10° C. The reaction mixture was stirred at −10° C. for 30 minutes, and then the reaction mixture was diluted with water and extracted with ethyl acetate. The combined organic layers were washed with brine, dried over sodium sulfate, filtered and concentrated under reduced pressure to give ethoxycarbonyl N-(tert-butoxycarbonyl)-S-methylcysteinate which was used without further purification.
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