C(=O)[CH2:2][CH:3]([CH3:5])[CH3:4].[CH2:7]([O:9][C:10]([C@H:12]1[C@@H:17]([NH2:18])[C@H:16]2[CH2:19][C@@H:13]1[CH2:14][CH2:15]2)=[O:11])[CH3:8].C([BH3-])#N.[Na+]>C(O)(=O)C.CO>[CH2:7]([O:9][C:10]([C@H:12]1[C@@H:17]([NH:18][CH2:2][CH:3]([CH3:5])[CH3:4])[C@H:16]2[CH2:19][C@@H:13]1[CH2:14][CH2:15]2)=[O:11])[CH3:8]Isovaleraldehyde (0.374 mL, 4.10 mmol) and 10 drops glacial acetic acid were added sequentially to a solution of (1S,2R,3S,4R)-3-amino-bicyclo[2.2.1]heptane-2-carboxylic acid ethyl ester (prepared as described in Example 6k, 0.750 g, 4.09 mmol) in methanol (12 mL) at 25° C. Sodium cyanoborohydride (0.643 g, 10 2 mmol) was added, and the reaction mixture was stirred at 25° C. for 2 h, and then was partitioned between half-saturated aqueous sodium bicarbonate solution (150 mL) and ethyl acetate (2×150 mL). The organic layers were dried over sodium sulfate, filtered and concentrated in vacuo to afford the desired product, (1S,2R,3S,4R)-3-isobutylamino-bicyclo[2.2.1]heptane-2-carboxylic acid ethyl ester, as a clear oil, which was used in the next step without any further purification.
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