To a solution of (3RS,4SR)-1-benzyl-4-(3,4-difluoro-phenyl)-pyrrolidine-3-carboxylic acid (25 g, 78.8 mmol) in THF (350 mL) at -20° C. were added triethylamine (27.45 mL, 197 mmol) and pivaloyl chloride (24.2 mL, 197 mmol) subsequently. Stirring was continued for two hours at the same temperature, and to this mixture were added (S)-4-Benzyl-oxazolidin-2-one (14.7 g, 82.7 mmol) and lithium chloride (3.67 g, 86.7 mmol). The reaction mixture was gradually warmed to room temperature and stirred overnight. The reaction mixture was diluted with ethyl acetate and washed with water two times and saturated sodiumbicarbonate. The separated organic phase was dried on Na2SO4 and concentrated under vacuo. The two diastereoisomeres were separated by column chromatography (SiO2, EtOAc/H, 15:85) to yield (S)-4-benzyl-3-[(3R,4S)-1-benzyl-4-(3,4-difluoro-phenyl)-pyrrolidine-3-carbonyl]-oxazolidin-2-one 18 g (48%) as awhite solid. MS m/e: 477.1 (M+H+).
