C([O:3][C:4]([CH2:6][C:7]([N:9]1[CH:14]([C:15]([O:17][C:18]([CH3:21])([CH3:20])[CH3:19])=[O:16])[CH2:13][CH2:12][CH2:11][NH:10]1)=[O:8])=O)C>C(O)(=O)C>[O:3]=[C:4]1[N:10]2[CH2:11][CH2:12][CH2:13][CH:14]([C:15]([O:17][C:18]([CH3:21])([CH3:20])[CH3:19])=[O:16])[N:9]2[C:7](=[O:8])[CH2:6]1A solution of 16.9 g of tert.butyl 2-(2-ethoxycarbonylacetyl)hexahydropyridazine-3-carboxylate in 350ml of glacial acetic acid was heated at 100° C. for 1.5 hours. Evaporation gave an oil which was chromatographed on silica gel to yield 9.45 g (66%)of tert.butyl hexahydro-1,3-dioxo-1H-pyrazolo[1,2-a]pyridazine-5-carboxylate in the form of a pale yellow solid having a melting point of 121°-122° C. (from chloroform/hexane).
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