[C:1]([O:5][C:6]([NH:8][CH2:9][C:10]1([CH2:16][C:17]([OH:19])=O)[CH2:15][CH2:14][CH2:13][CH2:12][CH2:11]1)=[O:7])([CH3:4])([CH3:3])[CH3:2].C([N:22](CC)CC)C.C(=O)C(C)C>C1COCC1>[C:1]([O:5][C:6](=[O:7])[NH:8][CH2:9][C:10]1([CH2:16][C:17](=[O:19])[NH2:22])[CH2:15][CH2:14][CH2:13][CH2:12][CH2:11]1)([CH3:4])([CH3:3])[CH3:2][1-(tert-Butoxycarbonylamino-methyl)-cyclohexyl]-acetic acid (2) (152 g, 0.56 mdl) was taken up in 1 L THF and triethylamine (66.2 g, 0.65 mol) and cooled to −10° C. Over a 1-hour period, isobutyraldehyde was added (84.7 g, 0.62 mol), and the heterogeneous mixture was stirred at 0° C. for 15 minutes. Ammonia gas was bubbled into the cold reaction mixture for 30 minutes, and the mixture was allowed to warm to room temperature. After 16 hours stirring, the reaction mixture was evaporated to dryness on a rotary evaporator, and the residue was taken up in water, extracted 3× EtOAc, washed 2× brine and dried over MgSO4. Evaporation yielded an oil which was crystallized from pentane to yield 116.5 g (77%) white crystals, mp 123-125° C.
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