In Situ Vinylindole Synthesis of Carbazoles

Wayland E Noland, Steven R. Wann

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The condensation of indole with ketones catalyzed by maleic acid has been utilized as a synthesis of 3-vinylindoles which, acting asdienes, are trapped by in situ Diels-Alder addition to the maleic acid. The resulting tetrahydrocarbazoles undergo double-bond isomerization and selective decarboxylation of the carboxyl group in the indole-2-acetic acid configuration to form 3-R1 23- and 4-R4-substituted-l, 2, 3, 4-tetrahydrocarbazole-2-carboxylic acids which were the products isolated [R3 = R4 = CH3; R3 = CH3, R4 = C2H5; R3 = H, R4 = CH2CH(CH3)2; R3 + R4 = (CH2)3; R3 = C6H5, R4 = CH3; R3 = H, R4 = CH2C(CH3)3], The Diels-Alder reaction is sensitive to steric hindrance in the 3-vinylindole, and the limits have been fairly well defined. Methyl or ethyl esterification converted the acid products to the more soluble esters, which were then dehydrogenated with chloranil in refluxing o-xylene or 3-10% palladium-on-carbonin refluxing o-dichlorobenzene to the corresponding carbazole-2-carboxylate esters, thus providing an overall synthesis of carbazoles in three laboratory steps from indole. The mass spectral fragmentations of all the compounds described are interpreted in detail.

Original languageEnglish (US)
Pages (from-to)4402-4410
Number of pages9
JournalJournal of Organic Chemistry
Issue number24
StatePublished - Jan 1 1979


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