A stirred solution of (3R,4S)-3-acetamido-N-tert-butyl-4-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)propyl)pyrrolidine-3-carboxamide (Example 8, step 4) (198 mg, 0.5 mmol) in anhydrous 1,2-dichloroethane (5 mL) was treated with anhydrous sodium sulfate (1 g) and isobutyraldehyde (0.060 mL, 0.65 mmol), stirred for 2.5 h, then treated with sodium triacetoxyborohydride (212 mg, 1.0 mmol) and glacial acetic acid (2 drops) and stirred for 16 h. Aqueous sodium carbonate (10%, 5 mL) was added and the mixture stirred for a few minutes and extracted with ethyl acetate (30 mL, then 2×10 mL). The combined organic solution was washed with water and saturated aqueous sodium chloride (20 mL each), dried (MgSO4), and concentrated. The crude material was deprotected and isolated using the method described for example 8, step 5 to afford (3R,4S)-3-amino-4-(3-boronopropyl)-1-isobutylpyrrolidine-3-carboxylic acid (44 mg, 26%) as a white semisolid. NMR (D2O) δ 3.90 (m, 1 H), 3.50 (m, 1 H), 2.90-3.20 (m, 4 H), 2.50 (m, 1 H), 1.90 (m, 1 H), 1.50 (m, 1 H), 0.80-1.30 (m, 9 H), 0.65 (m, 2 H). MS (m+1): 273.2; MS (m−H2O+1): 255.2; MS (m−2 H2O+1): 237.2.
