[CH:1](=O)[CH3:2].C([BH3-])#N.[Na+].C(O)(=O)C.[NH2:12][C@H:13]1[CH2:17][CH2:16][N:15]([C:18]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])=[O:19])[CH2:14]1.C(=O)([O-])O.[Na+]>CO>[C:21]([O:20][C:18]([N:15]1[CH2:16][CH2:17][C@H:13]([NH:12][CH2:1][CH3:2])[CH2:14]1)=[O:19])([CH3:24])([CH3:23])[CH3:22]367 μL (6.5 mmol) of acetaldehyde, 817 mg (13 mmol) of sodium cyanoborohydride and 3.75 mL (65 mmol) of acetic acid are added to a solution of 1.47 mL (8.4 mmol) of (S)-3-amino-1-tert-butoxycarbonylpyrrolidine in 30 mL of anhydrous methanol. The solution is stirred at room temperature for 24 hours then a saturated aqueous solution of sodium hydrogencarbonate is added to adjust the pH to 7. The compound is extracted with ethyl acetate. The organic phases are dried over magnesium sulfate, filtered then evaporated and the residue is purified by chromatography on silica (eluent dichloromethane/methanol 94/6) to give 970 mg (70%) of (S)-1-tert-butoxycarbonyl-3-ethylamino-pyrrolidine in the form of a yellow oil.
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