Cl[C:2]([O:4][CH2:5][CH3:6])=[O:3].[C:7]1([C:13]2[CH:21]=[C:16]3[CH:17]=[CH:18][CH:19]=[CH:20][N:15]3[N:14]=2)[CH:12]=[CH:11][CH:10]=[CH:9][CH:8]=1.[N:22]1[CH:27]=[CH:26][CH:25]=[CH:24][N:23]=1>C(Cl)Cl>[CH2:5]([O:4][C:2]([N:22]1[CH:27]=[CH:26][CH:25]([C:21]2[C:13]([C:7]3[CH:8]=[CH:9][CH:10]=[CH:11][CH:12]=3)=[N:14][N:15]3[CH:20]=[CH:19][CH:18]=[CH:17][C:16]=23)[CH:24]=[N:23]1)=[O:3])[CH3:6]Ethyl chloroformate (3.38 g) was added dropwise with stirring to a solution of 2-phenylpyrazolo[1,5-a]pyridine (2.54 g) and pyridazine (5.00 g) in methylene chloride (5.0 ml) at 10° C. After being stirred at 10° C. for 1 hour and then at room temperature for 2 hours, the reaction mixture was poured onto ice-water (100 ml), and extracted with ethyl acetate. The combined extracts were washed with saturated sodium chloride aqueous solution (100 ml), dried over magnesium sulfate and evaporated in vacuo. The residue was chromatographed on silica gel (150 g) with a mixture of chloroform and n-hexane (1:1) as an eluant. The fractions containing main-product [compound (A)] were combined and evaporated in vacuo to give 3-(1-ethoxycarbonyl-1,4-dihydropyridazin-4-yl)-2-phenylpyrazolo[1,5-a]pyridine (2.42 g).
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