Synthesis of 3-Carboxylated Indoles through a Tandem Process Involving Cyclization of 2-Ethynylanilines Followed by CO₂ Fixation in the Absence of Transition Metal Catalysts

In this study, a facile synthesis of 3-carboxylated indoles involving a tandem-type cyclization of 2-ethynylanilines and subsequent CO2 fixation at the 3-position of the indole ring is realized. The reaction proceeds efficiently at 65 °C under 10 atm of CO2, giving rise to variously substituted 3-carboxylated indoles, generally in high yields. An inorganic base, such as K2CO3, is the only reagent required, and the addition of transition metal catalysts is not necessary. The method provides a novel, simple, and promising strategy for CO2 fixation in the research field of heterocyclic chemistry.