[C:1](OC(=O)C)(=[O:3])[CH3:2].[Cl-].[Al+3].[Cl-].[Cl-].[CH:12]1[C:25]2[S:24][C:23]3[C:18](=[CH:19][CH:20]=[CH:21][CH:22]=3)[S:17][C:16]=2[CH:15]=[CH:14][CH:13]=1.Cl>ClC(Cl)C>[C:1]([C:20]1[CH:21]=[CH:22][C:23]2[S:24][C:25]3[C:16](=[CH:15][CH:14]=[CH:13][CH:12]=3)[S:17][C:18]=2[CH:19]=1)(=[O:3])[CH3:2]Acetic anhydride (22.4 g) in dichloroethane (75 ml) was added to a suspension of aluminium chloride (58.4 g) in dichloroethane (100 ml) at 0° C. This was then added to a solution of thianthrene (43.2 g) in dichloroethane (600 ml) at 0° C. The reaction mixture was stirred for 1 hour at 0° C, then allowed to warm to room temperature and poured onto ice and concentrated hydrochloric acid. The organic layer was separated and the aqueous extracted with dichloroethane. The combined extracts were washed, dried and evaporated to give a brown oily product of crude 2-acetylthianthrene. This was heated to 90° C. for 2.5 hours with sodium hypochlorite solution (containing 14% available chlorine) (270 ml), normal sodium hydroxide solution (600 ml) and 1,4 dioxan (600 ml). After standing for 16 hours fine needles were filtered from the reaction mixture. These were washed and dried to give thianthrene, m.pt. 155° C. The reaction liquors were poured onto ice and concentrated hydrochloric acid. The precipitated solid was filtered off, washed with water, and dissolved in 0.5 Normal sodium hydroxide solution. The mixture was filtered from some insoluble material, and the alkaline filtrates re-acidified with concentrated hydrochloric acid. The precipitated solid was washed with water, and dried in vacuo to yield thianthrene-2-carboxylic acid, m.pt. ca. 220° C. Recrystallisation from glacial acetic acid and acetonitrile raised the melting point to 227°-8° C.
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