N([O-])=O.[Na+].N[C:6]1[CH:15]=[CH:14][CH:13]=[C:12]2[C:7]=1[C:8](=[O:21])[C:9]([O:19][CH3:20])=[C:10]([O:17][CH3:18])[C:11]2=[O:16].C(O)(=O)C.[ClH:26]>O>[Cl:26][C:6]1[CH:15]=[CH:14][CH:13]=[C:12]2[C:7]=1[C:8](=[O:21])[C:9]([O:19][CH3:20])=[C:10]([O:17][CH3:18])[C:11]2=[O:16]A solution of sodium nitrite (0.69 g, 10 mmol) in water (5 ml) was added at 0°-5° C. to a solution of 5-amino-2,3-dimethoxy-1,4-naphthoquinone (1.17 g, 5 mmol) in 5:1 acetic acid:water (25 ml) containing concentrated hydrochloric acid (1.7 ml). A further quantity of sodium nitrite (0.69 g) was then added to the reaction mixture after cooling to -5° C., followed by a solution of cuprous chloride (0.6 g) in concentrated hydrochloric acid (5 ml). The mixture was allowed to warm to 22° C. and solid cuprous chloride was added portionwise until the mixture assumed a green color. Water was then added to the reaction mixture and the precipitated yellow solid filtered off, washed with water and recrystallized from methanol:water (2:1) giving 1.01 g of 5-chloro-2,3-dimethoxy-1,4-naphthoquinone, mp 120°-121° C.
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