C=[O:2].S(=O)(=O)(O)O.O.[Cl:9][CH2:10][CH2:11][CH2:12][CH2:13][CH2:14][C:15](=NO)[C:16]([O:18][CH2:19][CH3:20])=[O:17]>>[Cl:9][CH2:10][CH2:11][CH2:12][CH2:13][CH2:14][C:15](=[O:2])[C:16]([O:18][CH2:19][CH3:20])=[O:17]The solution of ethyl 7-chloro-2-hydroxyiminoheptanoate [III-1] in toluene obtained in Example 9 (without being concentrated) is added dropwise into 36% aqueous formalin solution, and thereto is added sulfuric acid. The mixture is warmed, and further thereto is added water. The organic layer is separated, neutralized, washed with water, and concentrated under reduced pressure to give ethyl 7-chloro-2-oxoheptanoate [I-1].
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