1-Chloro-5,8-dimethoxy-4-nitro-9H-thioxanthen-9-one is prepared as follows: A solution of 9.5 g (0.048 mol) of 2-amino-3,6-dimethoxybenzoic acid [P. K. Bannerjee and D. N. Chaudhury, J. Indian Chem. Soc., 86 (4), 257 (1959)], 4.6 ml (0.11 mol) of 50% aqueous NaOH, 60 ml of H2O, and 3.3 g (0.048 mol) of NaNO2 was added slowly to a mixture of 15 ml of concentrated HCl and 20 g of ice chips which had been previously cooled in a salt-ice bath to -5° C. Good stirring was maintained throughout the addition and the temperature was kept below 5° C. After the addition was complete, the mixture was stirred to 0° C. for one hour, neutralized (pH 5.5) with potassium acetate, and added while cold in a thin stream to an 80° C. solution of 22.2 g (0.15 mol) of potassium ethyl xanthate in 75 ml of H2O under N2. Copious N2 evolution (foaming) occurred during the addition, and heat was applied as needed to maintain the temperature at 75°-80° C. The reaction mixture, under N2, was cooled to 20° C. and acidified (pH 3) with concentrated HCl. The oily material which separated was extracted into CH2Cl2 (2X), keeping contact with air to a minimum. The extracts were combined, dried under N2 (MgSO4), and concentrated on a steam bath to a brown oil using a stream of N2. The crude 2,5-dimethoxy-6-thiobenzoic acid was immediately dissolved in 40 ml of hot anhydrous EtOH and added to a premixed, 25° C. mixture of 9.2 g (0.048 mol) of 2,4-dichloronitrobenzene in sodium ethoxide [2.2 g (0.096 g atom) of sodium spheres dissolved in 90 ml of anhydrous EtOH]. The resulting suspension was heated under reflux for 16 hours, concentrated to dryness in vacuo and taken up in 250 ml of ether and 250 ml of H2O. The layers were separated and the aqueous layer was extracted twice with ether to remove organic soluble impurities, and made acidic (pH 1) with concentrated HCl. The solid which formed was collected, dried, and recrystallized from EtOH to give a first crop of product. The mother liquor was concentrated to dryness in vacuo and the residue was crystallized from CH3CN to give a second crop of product. The mother liquor was concentrated to dryness in vacuo and the residue was flash chromatographed over 500 g of silica gel, eluting with CH2Cl2 /MeOH (15/1). Combination of the appropriate fractions, concentration to dryness in vacuo, and crystallization of the residue from acetonitrile provided a third crop of product. All crops were combined to give 7.8 g of 2-[(5-chloro-2-nitrophenyl)-thio]-3,6-dimethoxybenzoic acid, mp 218°-220° C.
