[C:1]([CH2:4][CH2:5][N:6]1[C:15](=[O:16])[N:9]2[N:10]=[C:11](Cl)[CH:12]=[CH:13][C:8]2=[N:7]1)([OH:3])=[O:2].O.O.[SH-:19].[Na+].Cl.[OH-].[Na+]>O>[C:1]([CH2:4][CH2:5][N:6]1[C:15](=[O:16])[N:9]2[N:10]=[C:11]([SH:19])[CH:12]=[CH:13][C:8]2=[N:7]1)([OH:3])=[O:2]A mixture of 2-(2-carboxyethyl)-6-chloro-2,3-dihydro-s-triazolo[4,3-b]pyridazin-3-on (567 mg., 2.34 m.moles) and 70% sodium hydrosulfide dihydrate (924 mg., 7.02 m.mole) in water (10 ml.) was stirred at room temperature for two hours. The reaction mixture was adjusted successively to pH 1 with c. HCl, to pH 10 with NaOH and then to pH 1 with c. HCl. The resulting precipitate of 2-(carboxyethyl)-2,3-dihydro-6-mercapto-s-triazolo[4,3-b]pyridazin-3-on was collected by filtration and washed with water. Yield: 418 mg. (74%). M.p. 174°-176° C.
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