A slurry of copper bromide (I) dimethyl sulfide complex (5.132 g, 24.967 mmol) in THF (60 mL) and dimethyl sulfide (30 mL) as a co-solvent was cooled to −40° C. and n-butyl magnesium chloride (25 mL, 49.93 mmol) was added dropwise for 10 min and stirred for 20 min while warning to −15° C. The black slurry was cooled to −40° C. and (4R)-4-benzyl-3-[(E)-2-heptenoyl]-1,3-oxazolidin-2-one (6 g, 20.80 mmol) was added dropwise over 10 min as a solution in THF (20 mL) at −40° C. The reaction was let warm up to 25° C. overnight (20 h). N-Bromosuccinimide (7.407 g, 41.61 mmol) was added portionwise to a cold −78° C. solution of the black slurry. It was allowed to warm to 0° C. and was stirred for an additional 3 h. The reaction was quenched with a 1:1 solution of saturated ammonium carbonate and 0.5 N potassium bisulfate. The black slurry became greenish to blue. A precipitate formed (light blue). It was filtered. The mother liquor was diluted with ethyl acetate (150 mL) and the organic was dried over MgSO4, filtered, concentrated to obtain (4R)-4-benzyl-3-[(2R)-2-bromo-3-butylheptanoyl]-1,3-oxazolidin-2-one as a crude semi-solid (green) (8.49 g, 96.15%). Mass Spectrum (−ESI): 423 (M−H)−.
