[Cl:1][C:2]1[C:15]2[C:14](=[O:16])[C:13]3[C:8](=[CH:9][CH:10]=[C:11]([O:17]C)[CH:12]=3)[S:7][C:6]=2[C:5]([N+:19]([O-:21])=[O:20])=[CH:4][CH:3]=1.B(Br)(Br)Br.CO>C(Cl)Cl>[Cl:1][C:2]1[C:15]2[C:14](=[O:16])[C:13]3[C:8](=[CH:9][CH:10]=[C:11]([OH:17])[CH:12]=3)[S:7][C:6]=2[C:5]([N+:19]([O-:21])=[O:20])=[CH:4][CH:3]=1The requisite 1-chloro-7-hydroxy-4-nitrothioxanthone was prepared as follows: To a suspension of 5.0 g of 1-chloro-7-methoxy-4-nitro-thioxanthone in 50 ml of methylene chloride was added 20 ml of boron tribromide in methylene chloride (1M). The mixture was stirred for two days, and an additional 20 ml of boron tribromide was added. The mixture was stirred for three hours at room temperature and heated under reflux for two hours. An additional 20 ml of boron tribromide was added and the mix was heated under reflux for two hours. The mixture was cooled in an ice bath and 50 ml of methanol was added carefully. The solution was heated under reflux for one hour, and the solvent was removed in vacuo. The residue was boiled in about 50 ml of ethanol, collected, and retreated with hot ethanol to provide the product as an orange powder, mp 240°-254° (dec).
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