A mixture of 2.49 g (6.21 mmol) rac-tert-butyl (3R,4S)-1-benzyl-4-(4-chlorophenyl)-3-methylpyrrolidin-3-ylcarbamate, 0.836 g (7.45 mmol) potassium tert.-butoxide and 0.98 g (7.76 mmol) dimethyl sulfate in 80 mL DMSO was stirred at room temperature for 1 h. The mixture was poured onto ice-water and extracted with TBME. The combined organic layers were washed with brine, died with Na2SO4, filtered off and evaporated to dryness. The residue was dissolved in 40 mL DCM and 7.68 g (67.3 mmol) TFA was added. The mixture was stirred at room temperature over night and ice-water was added. The pH was adjusted to 12 and the organic layer was washed with brine. The aqueous layer was extracted with DCM and the combined organic layers were dried with Na2SO4, filtered off and evaporated to dryness. The residue was dissolved in 40 mL DCM and 1.04 g (8.08 mmol) DIPEA and at −15° C. 1.29 g (7.41 mmol) 4-fluorophenyl chloroformate was added. The mixture was stirred to 0° C. during 2 h and concentrated under vacuum the residue was purified by column chromatography on silica eluting with a gradient formed from heptane and ethyl acetate to yield after evaporation of the product containing fractions 1.68 g (59%) of the title compound as light brown viscous oil. MS m/e: 453.1 [M+H]+.
