Trifluoroacetic acid → methyl (3R,4S)-4-allyl-3-[(trifluoroacetyl)amino]pyrrolidine-3-carboxylate trifluoroacetate

While under nitrogen, a solution of racemic 1-tert-butyl 3-methyl (3R,4S)-4-allyl-3-[(trifluoroacetyl)amino]pyrrolidine-1,3-dicarboxylate (Step 3, 0.207 g, 0.544 mmol) in methylene chloride (11 mL) was treated with trifluoroacetic acid (838 uL, 10.9 mmol). After stirring for 2 h, the mixture was concentrated and dried under high vacuum overnight to give methyl (3R,4S)-4-allyl-3-[(trifluoroacetyl)amino]pyrrolidine-3-carboxylate trifluoroacetate as a faint yellow foam that was used without further purification. A stirred solution of this intermediate in dry methylene chloride (11 mL) under nitrogen was treated with Et3N (0.341 mL, 2.45 mmol) followed by 4-fluorobenzoyl chloride (0.0979 mL, 0.816 mmol). After stirring at room temperature for 1.5 h, the solution was diluted with methylene chloride (15 mL), washed successively with water (10 mL), saturated aqueous NaHCO3 (10 mL) and saturated aqueous sodium chloride (5 mL). The resulting organic phase was dried over Na2SO4 and concentrated, and purification by radial chromatography (2000 micron silica gel rotor, 40-75% ethyl acetate in hexanes) to give methyl (3R,4S)-4-allyl-1-(4-fluorobenzoyl)-3-[(trifluoroacetyl)amino]-pyrrolidine-3-carboxylate (racemic) (220 mg, 100%) as a faint yellow film. Rf=0.33 (50% Ethyl acetate in hexanes). For intermediate pyrrolidine trifluoroacetate, LC-MS ESI+ MS found for C11H15F3N2O3 m/z 281.1 (M+H). For amide product, LC-MS ESI− MS found for C18H18F4N2O4 m/z 401.1 (M−H).