Palladium-catalyzed synthesis of 3-indolecarboxylic acid derivatives

Indoles having an ester functionality in the 3-position were prepared from 2-(2-nitrophenyl)propenoic acid derivatives via a palladium-catalyzed reductive N-heteroannulation using carbon monoxide as the ultimate reducing agent. The starting materials were prepared either by a Stille coupling of 2-halo-1-nitrobenzenes with ethyl 2-(tributylstannyl)-2-propenoate or by vicarious nucleophilic substitution of nitrobenzenes followed by a Knoevenagel-type condensation with an aldehyde. Synthesis of an example of a 3-nitrile- and a 3-sulfone-substituted indole is also described using the same type of methodologies.