tert-Butyl bromoacetate → (S)-1-((9H-fluoren-9-yl)methyl) 2-(2-(tert-butoxy)-2-oxoethyl) pyrrolidine-1,2-dicarboxylate

A solution of (S)-1-(((9H-fluoren-9-yl)methoxy)carbonyl)pyrrolidine-2-carboxylic acid (Fmoc-Pro-OH) (5.00 g, 14.8 mmol) in methylene chloride (59 mL) was mixed with N-ethyldiisopropylamine (DIPEA) (7.77 mL, 44.5 mmol) and tert-butyl 2-bromoacetate (4.34 g, 22.2 mmol) with stirring at room temperature, and the reaction mixture was stirred at room temperature for 48 hours. The reaction mixture was washed with a saturated aqueous ammonium chloride solution, and the organic layer was extracted with methylene chloride. The resulting organic layer was washed with a saturated aqueous sodium chloride solution and dried over sodium sulfate. The organic layer was concentrated under reduced pressure, and the resulting residue was purified by silica gel column chromatography (hexane/ethyl acetate) to afford (S)-1-((9H-fluoren-9-yl)methyl) 2-(2-(tert-butoxy)-2-oxoethyl) pyrrolidine-1,2-dicarboxylate (Fmoc-Pro-HOGly-OtBu) (Compound SP631) (6.40 g, 14.2 mmol, 96%).