One‐Flow Synthesis of Substituted Indoles via Sequential 1,2‐Addition/Nucleophilic Substitution of Indolyl‐3‐Carbaldehydes

Abstract

AbstractSubstituted indoles are important as drugs. A number of valuable indoles have been synthesized via nucleophilic substitution at the 3’‐position of indoles. However, the preparation of an indolylmethyl electrophile containing a tertiary carbon at the 3’‐position and its subsequent nucleophilic substitution are challenging owing to the instability of the electrophile. Herein, we demonstrated the rapid one‐flow synthesis of indoles via sequential 1,2‐addition/nucleophilic substitution of indolyl‐3‐carbaldehydes. The use of a microflow technology helped in suppressing the undesired reactions caused by the unstable intermediates, resulting in significantly higher yields and reproducibility compared to those under batch conditions. A crown ether was effective when 1‐alkylindole‐3‐carboxaldehyde was used as a substrate. However, the crown ether exerted a detrimental effect when 1H‐indole‐3‐carboxaldehyde was used. A total of 15 structurally diverse indole derivatives were obtained in generally acceptable to good yields.