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10.13.1.1.1.2.1.1.3 Variation 3: From 2-(o-Aminoaryl)acetaldehydes

DOI: 10.1055/sos-SD-010-00559

Joule, J. A.Science of Synthesis, (200110415.

The synthesis of pyrrole-ring-unsubstituted indoles by this strategy requires the generation of 2-(o-aminoaryl)acetaldehydes. A direct route to these is the oxidation of 2-(o-aminoaryl)ethanols, e.g. using bis(triphenylphosphine)ruthenium(II) chloride.[‌214‌] The 2-(o-aminoaryl)ethanols 308 are easily prepared via the base-catalyzed condensation of formaldehyde with the methyl group of 1-methyl-2-nitroarenes 307[‌214‌‌216‌] followed by reduction of the nitro group. Remarkably, ring closure to indoles 309 can be brought about using either the precursor 2-(o-nitroaryl)ethanol 308 or the 2-(o-aminoaryl)ethanol 310. In the former case hydrogen, and a heterogeneous hydrogenation catalyst, rhodium/carbon, is included together with the homogenous ruthenium catalyst in the reaction mixture, and in the latter, hydrogen is evolved, quantitatively (Scheme 86).

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