[CH3:1][C:2]1[CH:3]([CH2:9][CH2:10][CH3:11])[CH2:4][C:5](=[O:8])[NH:6][N:7]=1.BrBr>CC(O)=O>[CH3:1][C:2]1[C:3]([CH2:9][CH2:10][CH3:11])=[CH:4][C:5](=[O:8])[NH:6][N:7]=1To a solution of 6-methyl-5-propyl-4,5-dihydropyridazin-3-one (16.7 g, 108 mmol) in HOAc (200 ml) heated to 85° C. is added Br2 (5.5 ml, 108 mmol) dropwise. After the addition, the mixture is stirred at 85° C. for 1 hour. The solvent is removed in vacuo and the residue is dissolved in EtOAc (250 ml) and washed with NaHCO3 (200 ml), followed by Na2S2O3 saturated solution (50 ml) and brine (200 ml). The organic phase is dried (Na2SO4) and evaporated. The resulting 6-methyl-5-propyl-pyridazin-3-one is obtained as a yellow solid, which is used in the next step without further purification.
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