[C:1]([C:4]1[CH:5]=[CH:6][C:7]2[C:8](=O)[C:9]3[C:14]([S:15](=[O:19])(=[O:18])[C:16]=2[CH:17]=1)=[CH:13][CH:12]=[CH:11][CH:10]=3)([OH:3])=[O:2].Cl>C(O)(=O)C.[Zn]>[C:1]([C:4]1[CH:5]=[CH:6][C:7]2[CH2:8][C:9]3[C:14]([S:15](=[O:19])(=[O:18])[C:16]=2[CH:17]=1)=[CH:13][CH:12]=[CH:11][CH:10]=3)([OH:3])=[O:2]3-Carboxythioxanthone-10,10-dioxide (prepared as in Reference Preparation 4) (5.0 g), zinc wool (10.0 g) and mercuric acetate (0.2 g) in acetic acid (100 ml) were brought to the boil and concentrated hydrochloric acid (10 ml) was added. Vigorous evolution of hydrogen chloride occurred at first. After 2 hr., further hydrochloric acid (7 ml), was added and the mixture boiled under reflux for further 4 hr. It was then filtered while hot and poured on to ice and water. The precipitated product was filtered off and dried at 80° C. in vacuo, m.p. 229°-248° C. Two recrystallisations from methanol gave the product m.p. 254° C.
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