[Cl:1][C:2]1[CH:3]=[C:4]([CH:23]=[CH:24][C:25]=1[Br:26])[CH2:5][N:6]1[N:15]=[C:14]([CH2:16][C:17]([O:19]CC)=[O:18])[C:13]2[C:8](=[CH:9][CH:10]=[CH:11][CH:12]=2)[C:7]1=[O:22].[OH-].[K+].O>C(O)C>[Cl:1][C:2]1[CH:3]=[C:4]([CH:23]=[CH:24][C:25]=1[Br:26])[CH2:5][N:6]1[N:15]=[C:14]([CH2:16][C:17]([OH:19])=[O:18])[C:13]2[C:8](=[CH:9][CH:10]=[CH:11][CH:12]=2)[C:7]1=[O:22]A solution of ethyl 2-(3-chloro-4-bromobenzyl)-1,2-dihydro-1-oxophthalazin-4-ylacetate (7.0 g.) in ethanol (70 ml.) containing potassium hydroxide (7.0 g.) was heated under reflux for 30 minutes. The solution was then poured into water (250 ml.) and the aqueous solution was extracted with ether (2×150 ml.). The aqueous phase was acidified to pH 2 with concentrated hydrochloric acid. The solid which was thus precipitated was separated, washed with water, dried in vacuo and then recrystallised from a 6:2:3 v/v/v mixture of toluene, propan-2-ol and petrol (60-80) to give 2-(3-chloro-4-bromobenzyl)-1,2-dihydro-1-oxophthalazin-4-ylacetic acid (3.9 g.), m.p. 186° C.
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