C[Si](N([Si](C)(C)C)[C:6](=[O:11])[C:7](F)(F)F)(C)C.[NH2:16][C@H:17]([C:22]([OH:24])=[O:23])[C@H:18]([CH2:20][CH3:21])[CH3:19].[F:25][C:26]([F:36])([F:35])[CH:27]([CH2:31]C(=O)C)[C:28](Cl)=[S:29].[OH2:37]>C(#N)C.CO>[C:6]([S:29][CH2:28][CH:27]([C:26]([F:25])([F:35])[F:36])[C:31]([NH:16][C@H:17]([C:22]([OH:24])=[O:23])[C@H:18]([CH2:20][CH3:21])[CH3:19])=[O:37])(=[O:11])[CH3:7]Bis(trimethylsilyl)trifluoroacetamide (16 mL, 60 mmol.) was added to a suspension of L-isoleucine (2.62 g., 20 mmol.) in 40 mL of acetonitrile at 0° C. The suspension was allowed to warm to room temperature and stirred for 2 hours. At this time approximately one third of the solid had dissolved. An additional amount of bis(trimethylsilyl)trifluoroacetamide (4 mL, 15 mmol.) was added and the mixture was stirred for 45 minutes. The mixture was cooled to 0° C. and 2-trifluoromethyl-3-acetylthiopropionyl chloride (4.7 g., 20 mmol.) was added dropwise over 15 minutes as a solution in 10 mL of acetonitrile. After stirring for 18 hours at room temperature, the reaction mixture was a clear light yellow solution. Approximately 150 mL of water was added and the resulting mixture was extracted with ethyl acetate (2×150 mL). The combined organic layers were washed with brine (100 mL), dried (MgSO4), and concentrated to afford an oily residue which was preadsorbed onto Celite for chromatography. After several chromatographies, triturations and recrystallizations 1.41 g. of (R)-N-[2-[(acetylthio)methyl]-3,3,3-trifluoro-1-oxopropyl]-L-isoleucine; m.p. 81°-85° C., [α]D =-105.5° (c=0.38, methanol) were isolated and 1.56 g. of (S)-N-[2-[(acetylthio)methyl]-3,3,3-trifluoro-1-oxopropyl]-L-isoleucine, m.p. 111°-112° C., [α]D =+122.6° (c=0.38, methanol) were isolated.
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