Condensed Heteroaromatic Ring Systems. VI. : Synthesis of Indoles and Pyroolopyridines from o-Nitroarylacetylenes

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Catalytic hydrogenation of o-nitrophenylacetaldehyde diethyl acetal over palladium-carbon, followed by intramolecular cyclization of the resulting o-aminophenylacetaldehyde diethyl acetal with hydrochloric acid, gave indole. Similarly, 4-methylindole, ethyl 5-indolecarboxylate, and various pyrrolopyridines were synthesized from the corresponding o-nitroacrylacetaldehyde derivatives. The preparation of the desired o-nitroarylacetaldehydes was accomplished by the condensation of o-halonitroaromatics with trimethylsilylacetylene in the presence of dichloro-bis(triphenylphosphine)palladium as a catalyst, followed by ethanolysis of the resulting o-(tri-methylsilylethynyl)nitroaromatics.