Sodium Hydroxide → 3,6-Dichlorophthalic acid

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This compound was prepared by modifications to the literature method of J. Het. Chem., 1995, 32, 907. Zinc dust (165 g, 2.52 mmol) was added portionwise (over 15 min) to a homogenous mixture of 20 (118 g, 0.437 mmol) and NaOH (120 g) in water (1200 mL) stirred at 90° C. (bath) under a nitrogen atmosphere. The resulting heterogeneous mixture was further stirred at 95-100° C. for 5 h, then cooled to room temperature and filtered through a bed of Celite. The filter and residue was washed with water (3×100 mL), and the combined filtrate was acidified with conc. HCl (250 mL) and extracted with EtOAc (2×300 mL). The combined EtOAc solution was dried (Na2SO4), filtered and evaporated under reduced pressure to give crude 3,6-dichlorophthalic acid (100 g). Toluene (1000 mL) was added to this solid, and the mixture was distilled until the distillate was clear (after about 600 mL had been collected). The hot concentrate was gravity-filtered, and the residue was washed with hot toluene (3×50 mL). The combined filtrate was seeded and chilled to give 3,6-dichlorophthalic anhydride (21) as a colourless solid (67.0 g, 72%), m.p. 187-190° C. (lit. m.p. 188-191° C.). 1H NMR identical to literature.

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