Science of Synthesis

Joule, J. A., Science of Synthesis Knowledge Updates, (2010) 2, 86.

Cyclization of α-(o-aminoaryl) ketones [o-(2-oxoalkyl)arylamines] ‌326‌ (R1 = alkyl or aryl) or α-(o-aminoaryl)aldehydes ‌326‌ (R1 = H) by intramolecular amine-to-carbonyl nucleophilic addition, with loss of water from the species ‌327‌ thus produced, occurs very easily, especially with acid catalysis, giving indoles ‌328‌ (‌Scheme 114‌). Many ways of generating the α-(o-aminoaryl) ketone or α-(o-aminoaryl)aldehyde or masked versions thereof have been devised and it is in these that the various approaches differ. Some of these routes produce o-nitro precursors initially and a variety of the usual methods for reduction of nitro groups are applied. An important and rather direct route to α-(o-nitroaryl) ketones, which are reduced by titanium(III) chloride with ammonium acetate in ethanol at room temperature to give the 1H-indole, is the palladium-catalyzed coupling of 2-bromo- or 2-chloronitroarenes with ketones in the presence of phenols.[‌310‌]

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