Indoles via Dehydrogenation of Pyrroles

Whereas the dehydrogenation of pyrroles to indoles has been described sporadically in preceding chapters, the present chapter covers those examples from tetrahydroindoles, such as the often encountered 4-keto-4,5,6,7-tetrahydroindole. The dehydrogenation of indoUnes (2,3-dihydroindoles) to indoles is covered in a later chapter. 

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

As we have seen, a large number of pyrrole indole dehydrogenations involved the use of abenzoquinone, usually DDQ, and a selection of the substrates that were converted to indoles is sununarized in Schane 4 [20-29]. In each case the compound shown afforded the fully aromatic indole in some cases the yields were not reported. The oxidation of trifluoro alcohol 9 afforded the keto indole. Space does not permit the illustration of the syntheses of the substrates shown in Schane 4. 

The less-reactive p-benzoquinone has also been employed to dehydrogenate 4,7-dihydroindoles, such as those obtained viaFriedel-Crafts reactions onto the pyrrole C-2 position. A selection of these substrates so oxidized is 

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