1-(2-(Trimethylsilyl)ethyl) 1′-tert-butyl spiro[indoline-3,3′-pyrrolidine]-1,1′-dicarboxylate (15.2 g, 36.5 mmol) was dissolved in t-butanol (240 mL) and water (40 mL). Thereafter, N-bromosuccinimide (7.9 g, 44.4 mmol) was added to the above obtained solution at room temperature, and the thus obtained mixture was then stirred at 60° C. for 15 hours. Thereafter, an aqueous solution of sodium thiosulfate was added to the reaction solution, and the mixed solution was then extracted with ethyl acetate. The organic layer was washed with brine, and was then dried over anhydrous sodium sulfate, followed by concentration in vacuo. The obtained residue was purified by silica gel column chromatography (hexane:ethyl acetate=3:1) to obtain 1-(2-(trimethylsilyl)ethyl) 1′-tert-butyl 5-bromospiro[indoline-3,3′-pyrrolidine]-1,1′-dicarboxylate (12.9 g, 71%) in the form of a white amorphous product.
