Cyclization of 2‐chloro‐6‐[(3‐chlorophenyl)thio]benzoic acid ( 2 ) gave a mixture of 1,8‐, 3 , and 1,6‐dichloro‐9 H ‐thioxanthen‐9‐ones 4 . The mixture was converted to 1,8‐diamino‐ 7 , and 1‐amino‐6‐chloro‐9 H ‐thioxanthen‐9‐ones 8 , from which 3 and 4 were prepared separately, respectively. From a mixture of 4 and 3,6‐dichloro‐9 H ‐thioxanthen‐9‐one ( 11 ) obtained by cyclizing 4‐chloro‐2‐[(3‐chlorophenyl)thio]benzoic acid ( 10 ) was separated 11 by conversion of 4 to 8 .
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Okabayashi et al. (1991) studied this question.