N[C@H](C(O)=O)CC1N=CNC=1.[OH-].[Na+].C(=O)C(C)C.Cl.[CH:20]([CH:23]1[C:28]2[N:29]=[CH:30][NH:31][C:27]=2[CH2:26][CH:25]([C:32]([OH:34])=[O:33])[NH:24]1)([CH3:22])[CH3:21]>O.CO>[CH:20]([C@H:23]1[C:28]2[N:29]=[CH:30][NH:31][C:27]=2[CH2:26][C@@H:25]([C:32]([OH:34])=[O:33])[NH:24]1)([CH3:22])[CH3:21]L-Histidine (10.0 g, 43.8 mmol) was dissolved in a solution of NaOH (7.73 g, 193 mmol) in water (25 mL) and MeOH (100 mL), and isobutyraldehyde (11.8 mL, 129 mmol) was added. The reaction mixture was heated under reflux at 80° C. for 24 h. The pH was adjusted to 7 with 1 M aq HCl solution and the solvents were removed in vacuo. The residue was dissolved in hot EtOH and cooled to room temperature. The precipitate was removed by filtration and the mother liquor concentrated in vacuo, washed with acetone (100 mL) and dried to give 4-isopropyl-4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid as a pale yellow solid (18.6 g, crude, 6:1 mixture of (4S,6S):(4R,6S) diastereoisomers).
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