A solution of sodium nitrate (0.69 g, 10 mmol) in water (5 ml) was added at
0-5°C to a solution of 6-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 room temperature 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 6-chloro-
2,3-dimethoxy-1,4-naphthoquinone, melting point 93-94°C (from etherpetrolium ether).
