[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].CC1(C)N([O])C(C)(C)CCC1.[Br-].[K+].Cl[O-].[Na+]>C(Cl)Cl.O.C([O-])(O)=O.[Na+]>[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]To a solution of (S)-3-hydroxymethyl-pyrrolidine-1-carboxylic acid t-butyl ester (4.0 g, 19.9 mmol, 1.0 eq.) in DCM was added TEMPO (62 mg, 0.4 mmol, 0.02 eq.) and potassium bromide (120 mg, 1.0 mmol, 0.05 eq.). The resulting mixture was cooled over ice (and a small amount of salt, −4° C.). A 1:1 mixture of 0.7M sodium hypochlorite in water and saturated NaHCO3 (56 mL total) was added dropwise. The resulting mixture was allowed to stand in the ice bath until layers separated (˜5 minutes). The layers were separated and extracted with DCM (3×30 mL). The organic layer was washed with water (30 mL) and saturated aqueous NaCl (30 mL), then dried over anhydrous Na2SO4, filtered and concentrated to yield (S)-3-formylpyrrolidine-1-carboxylic acid t-butyl ester (3.0 g), which was used without further purification.
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