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The Pictet–Spengler reaction between Nb-hydroxytryptamine and cysteinals in the presence of trifluoroacetic acid gave, in addition to the normal products, β-carbolines (12)–(17), the unexpected tetracyclic compounds (4)–(9), providing new evidence for a spiroindolenine intermediate.
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Nakagawa et al. (1988) studied this question.
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