[C:1]([O:5][C:6]([N:8]1[CH2:12][CH2:11][C@H:10]([CH2:13][OH:14])[CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2].[Br-].[K+].C(=O)(O)[O-].[Na+].Cl[O-].[Na+]>C(Cl)Cl.O.CC1(C)N([O])C(C)(C)CCC1>[C:1]([O:5][C:6]([N:8]1[CH2:12][CH2:11][C@H:10]([CH:13]=[O:14])[CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2]
A solution of (S)-3-hydroxymethyl-pyrrolidine-1-carboxylic acid t-butyl ester (25.0 g, 124 mmol, 1.0 eq.) in DCM (200 mL) was cooled with stirring to 0° C. A solution of potassium bromide (1.5 g, 12.4 mmol, 0.1 eq.) and sodium bicarbonate (1.5 g, 17.4 mmol, 0.14 eq.) dissolved in water (100 mL) was added. After 15 minutes of stirring at 0° C., TEMPO (195.3 mg, 1.2 mmol, 0.01 eq.) was added, followed by the slow addition of sodium hypochlorite (77.3 mL, 136.6 mmol, 1.1 eq.) dropwise keeping the internal temperature in the 6-8° C. range. The mixture was placed in an ice bath until the layers separated. The organic layer was separated and the aqueous layer was extracted with DCM (200 mL). The combined organic layers were washed with saturated aqueous NaCl (200 mL), dried over Na2SO4, filtered, and concentrated to yield crude (S)-3-formylpyrrolidine-1-carboxylic acid t-butyl ester (21.5 g).
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