A slurry of nitrosonium tetrafluoroborate (215 mg, 1.84 mmol) in methylene chloride (6 mL) was cooled to 0° C. and then treated dropwise with a solution of 2-(3-amino-4-methanesulfonyl-phenyl)-3-cyclopentyl-propionic acid methyl ester (500 mg, 1.54 mmol) in a small amount of methylene chloride. The resulting reaction mixture was stirred at 0° C. for 1 h. The reaction mixture was then allowed to warm to 25° C. and was treated with 1,2-dichlorobenzene (6 mL). The resulting reaction mixture was heated at 100° C. for 1 h, during which time, the methylene chloride was distilled off. After 1 h at 100° C., the reaction mixture was allowed to cool to 25° C. The crude reaction mixture was directly purified by flash chromatography (Merck Silica gel 60, 230-400 mesh, 9/1 hexanes/ethyl acetate to elute 1,2-dichlorobenzene then 2/1 hexanes/ethyl acetate) to afford impure 3-cyclopentyl-2-(3-fluoro-4-methanesulfonyl-phenyl)-propionic acid methyl ester as a yellow oil. Repurification by flash chromatography (Merck Silica gel 60, 230-400 mesh, 3/1 hexanes/ethyl acetate) afforded pure 3-cyclopentyl-2-(3-fluoro-4-methanesulfonyl-phenyl)-propionic acid methyl ester (214 mg, 42%) as a pale yellow oil which solidified upon sitting at 25° C. to a pale yellow solid: mp 66-68° C.; EI-HRMS m/e calcd for C16H21FO4S (M+) 328.1144, found 328.1148.
