S(=O)(=O)(O)O.N[C:7]1[CH:16]=[C:15]2[C:10]([C:11]([Br:21])=[N:12][N:13]([CH:18]([CH3:20])[CH3:19])[C:14]2=[O:17])=[CH:9][CH:8]=1.N([O-])=O.[Na+].NC(N)=O.O(CC)C([S-])=[S:32].[K+].[OH-].[Na+].Cl>CC(O)=O.O.C(Cl)Cl>[SH:32][C:7]1[CH:16]=[C:15]2[C:10]([C:11]([Br:21])=[N:12][N:13]([CH:18]([CH3:20])[CH3:19])[C:14]2=[O:17])=[CH:9][CH:8]=1Concentrated sulfuric acid (5 ml) was added dropwise to a solution of 7-amino-2-isopropyl-4-bromo-2H-phthalazin-1-one (1.5 g, 5.3 mmol) in HOAc (15 ml) and the solution was cooled to 0° C. A solution of sodium nitrite (0.5 g, 7.4 mmol) in water (2.5 ml) was added dropwise and the reaction mixture was stirred at 0° C. for 20 min, after which time urea (0.17 g, 2.8 mmol) was added in one portion. The reaction mixture was then added dropwise to a solution of potassium ethyl xanthate (6 g, 37.7 mmol) in water (7.5 ml) and the mixture was heated to 80° C. for 30 min. After this time, the reaction mixture was cooled to RT and DCM (100 ml) was added. The organic layer was separated, dried (MgSO4), filtered and concentrated under vacuum. The residue was taken up in THF (10 ml), NaOH (4.95 g, 0.12 mmol) was added in one portion and the mixture was heated to reflux for 24 hours. The mixture was then cooled to RT and the suspension was acidified to pH 2 with concentrated HCl. DCM (100 ml) was added, the organic layer was separated and was subsequently washed with HCl (1M, 20 ml) and water (20 ml). The organic layer was extracted with NaOH (1M, 200 ml), the aqueous layer was separated and acidified to pH 1 with concentrated HCl. The mixture was extracted with DCM (2×50 ml), the organic layers were combined, dried (MgSO4), filtered and concentrated under vacuum to give the title compound (0.77 g, 48% yield) as a light brown solid which was taken on directly without further purification.
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