To methyl 3-(R)-n-butyl-3-[2-(S)-tert-butoxycarbonylpyrrolidin-1-ylcarbonyl)-2-(R)-hydroxypropionate H-3 (8.7 mmol) in DCM (10 mL) was added pyridine (25 mmol), the reaction was cooled to −20° C., then triflic anhydride (12.5 mmol) was added. The solution was stirred for 1 hour, then worked up as described above. The intermediate triflate was resuspended in DCM and cooled to −50° C. Tris(dimethylamino)sulfur (trimethylsilyl)difluoride (TAS-F) was added (8.7 mmol) and the solution allowed to warm to rt. The reaction mixture was washed with aqueous sodium bicarbonate and brine, dried (Na2SO4) and concentrated, then purified on silica gel (ethylacetate/hexanes) to afford 4.3 mmol (50%) of methyl 3-(S)-n-butyl-3-[2-(S)-tert-butoxycarbonylpyrrolidin-1-yl-carbonyl)-2-(S)-fluoropropionate H-4.
