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DOI: 10.1055/sos-SD-010-00864

Gallagher, P. T.Science of Synthesis, (200110706.

1H-Indole (50) reacts with ketals derived from 4-acetylpyridine to produce (aminoethyl)carbazoles (Scheme 22).[‌71‌] Thus, 4-acetyl-1-methyl-1,2,5,6-tetrahydropyridine ethylene glycol ketal (51) reacts with indole in aqueous acetic acid during extended reflux. Under the conditions of the reaction the ketal deprotects and isomerizes to a 3,4,5,6-tetrahydropyridine, which is electrophilic towards indole in the 3-position. The intermediate tetrahydrocarbazole aromatizes with loss of water to produce 1-methyl-2-(2-methylamino)ethyl-9H-carbazole (52) in 74% yield. This reaction has also been reported with 1,4-diacylpiperidines and forms carbazoles in modest yields.[‌72‌]

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