[BH4-].[Na+].[C:3]([O:7][C:8]([N:10]1[C:15](=[O:16])[CH:14]2[CH2:17][CH:11]1[CH2:12][CH2:13]2)=[O:9])([CH3:6])([CH3:5])[CH3:4]>C1COCC1.O>[C:3]([O:7][C:8](=[O:9])[NH:10][C@H:11]1[CH2:12][CH2:13][C@@H:14]([CH2:15][OH:16])[CH2:17]1)([CH3:6])([CH3:4])[CH3:5]In a round-bottomed flask, 3-oxo-2-aza-bicyclo[2.2.1]heptane-2-carboxylic acid tert-butyl ester (crude from Step 2, 1.87 g) was dissolved in THF (20 ml) and water (2 ml). The reaction mixture was cooled to 0° C. and sodium borohydride (301 mg, 7.97 mmol) was added. The reaction was stirred at 0° C. for 30 min then a second portion of sodium borohydride (301 mg, 7.97 mmol) was added. The reaction mixture was stirred at 0° C. for 7.5 h then a third portion of sodium borohydride (602 mg, 15.94 mmol) was added. The reaction mixture was stirred at room temperature overnight then a fourth portion of sodium borohydride (602 mg, 15.94 mmol) was added. The reaction mixture was stirred at room temperature overnight then cooled to 0° C. and carefully quenched with 1M HCl. When gas evolution had ceased (pH=˜6) the mixture was extracted with dichloromethane (3×). The organic layers were combined, dried over sodium sulfate, filtered and concentrated. The residue was chromatographed over silica gel with EtOAc/hexanes (gradient 0-60% EtOAc) to afford 1.73 g (95%, 2 steps) of (cis-3-hydroxymethyl-cyclopentyl)-carbamic acid tert-butyl ester as an off-white solid.
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