Miscellaneous Indole Syntheses from Pyrroles

This short chapter presents several new indole syntheses from pyrroles that do not logically fit into the previous pyrrole chapters but that nevertheless deserve to be revealed as being clever and promising for future applications. 

Reaction of this dialdehyde with sulfones afforded thiepino[4,5-fc]pyrrole 6,6-dioxides. In contrast, A -methyl-pyrrole-3,4-dicarboxyaldehyde (not shown) yielded solely the thiepino[4,5-c]pyrroles or the 6,6-dioxides. Mancini and colleagues effected a Diels-Alder reaction between 2- and 3-nitropyrroles and activated dienes to form indoles in modest yields (equations 3,4) [2-4]. 

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

7-acylindole. Fuji and Ohno employed gold in a catalyzed indole synthesis that also involved a formal [4-1-2] reaction between pyrroles and 1,3-diynes (equation 3) [6], The mechanism is thought to involve electrophilic attack at C-2 of pyrrole by a Ag(I)-diyne complex followed by an intramolecular 6-endo-dig hydroarylation. The initial C-2 adduct can be isolated and further converted to the indole (equation 4). Not unexpectedly, carbazoles were formed from indoles under these conditions (equation 5). 

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