[OH:1][C:2]1[CH:7]=[CH:6][C:5]([C:8]2[CH:9]([CH3:15])[CH2:10][C:11](=[O:14])[NH:12][N:13]=2)=[CH:4][CH:3]=1.[C:16](Cl)(=[O:20])[CH2:17][CH2:18][CH3:19].Cl>N1C=CC=CC=1>[C:16]([O:1][C:2]1[CH:7]=[CH:6][C:5]([C:8]2[CH:9]([CH3:15])[CH2:10][C:11](=[O:14])[NH:12][N:13]=2)=[CH:4][CH:3]=1)(=[O:20])[CH2:17][CH2:18][CH3:19]A 100 ml round-bottomed flash was charged with pyridine (30 ml) and 7.10 g (34.8 mmol) of (±)-4,5-dihydro-6-(4'-hydroxyphenyl)-5-methyl-3(2H)-pyridazinone from Example I, then cooled to 0° C. under an atmosphere of nitrogen. Butanoyl chloride (4.10 g, 38.3 mmol) was added dropwise to the solution via syringe. The reaction mixture was stirred for 30 min at 0° C., warmed to 23° C. over 15 min then poured into 1N aqueous hydrochloric acid (400 ml). The aqueous solution was extracted with EtOAc (2×200 ml). The combined EtOAc extracts were successively washed with 5% aqueous sodium bicarbonate solution (300 ml), saturated aqueous sodium chloride (200 ml), dried over anhydrous magnesium sulfate (5 g), filtered and concentrated in vacuo to provide 9.8 g of crude product as a solid. Recrystallization from ethyl acetate/hexanes (2:1, 35 ml) yielded 7.61 g (80%) of pure (±)-6-(4'-butanoyloxyphenyl)-4,5-dihydro-5-methyl-3(2H)-pyridazinone as colorless needles, m.p. 112°-113° C.
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