Electrophilic Cyclization of Pyrrole

In a series of papers spanning 20 years, Natsume, Muratake, and coworkers reported a versatile synthesis of indoles exploiting the reactivity of the pyrrole ring toward intramolecular electrophilic attack at the C-2 or C-3 position [1-16], In their seminal paper, Natsume and 

Indole Ring Synthesis From Natural Products to Drug Discovery, First Edition. Gordon W. Gribble. 2016 John Wiley Sons, Ltd. Published 2016 by John Wiley Sons, Ltd. 

4-(3-methyl-2-butenyl)indole, and, using direct C-2 inter-molecular electrophilic attack on N-tosylpyrrole followed by intramolecular C-3 electrophilic cyclization, Muratake and Natsume synthesized the tumor promoters lyngbya-toxin A (= teleocidin A-1) and teleocidin A-2 the key indole-forming step is shown in equation 2 [3], In addition to the two isomeric products, the CA 5-methyl compound was also obtained (30%). 

In a tour de force over the next ten years, Natsume, Muratake, and colleagues applied their pyrrole-to-indole methodology to the syntheses of the trikentrins [10-13], pendohnycin [14], ( )- and (5)-pindolol [15], theherbindoles [13, 16], hapalindole O [17], mitosene analogues [18], and the duocarmycins [19, 20]. Several related companion 

7-diarylindoles by the acid-catalyzed rearrangement of 1,4-diaryl-l,4-di(pyrrol-2-yl)but-2-ynes (equation 3) [35]. The venerable acid-catalyzed indole dimerization occurred with these 4,7-diarylindoles on further treatment with trifluoroacetic acid. 

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