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Treatment of indole (1) with toluene-p-sulphonic acid in benzene provided the 2,3′-indole trimer (4), a positional isomer of the well known 3,3′-indole trimer (3). The structure of the new trimer (4) was established by comparative studies with the trimers (3) and (4).
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Ishii et al. (1988) studied this question.
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