4-Fluoro-3-(trifluoromethylsulfonyl)benzenesulfonamide (INTERMEDIATE 6, 21.67 g, 71.22 mmol) and DIPEA (129 ml, 738 mmol) were added to a solution of (R)-methyl 3-amino-4-(phenylthio)butanoate, trifluoroacetic acid salt (INTERMEDIATE 7, STEP 2) in DMF (20 ml). The resulting reaction mixture was heated at 50° C. for 1.5 hours, allowed to cool to room temperature, diluted with ethyl acetate (500 ml), and washed with water (250 ml) and brine (250 ml). The organic layer was dried over sodium sulfate, filtered, and concentrated under reduced pressure. The concentrate was purified by column chromatography (ISCO, eluting with 0→30% EtOAc/DCM over 54 minutes) to give the title product (20.5 g, yield: 56%).
