In Situ Vinylindole Synthesis of Carbazoles

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.