Synthesis of pyrroloquinolines from substituted 6-aminoindoles and oxaloacetic ester

Abstract

It was established that the initial condensation of substituted 6-aminoindoles and oxaloacetic ester in boiling benzene with the addition of catalytic amounts of acetic acid takes place exclusively through the carboxyl group of the keto ester with the formation of the corresponding enamines, which successfully undergo thermal cyclization (biphenyl, 280°C) to pyrroloquinolines. Here, irrespective of the nature of substituents at the N-1 and C-5 atoms enamines with a free position 7 are transformed into pyrrolo-[2,3-f]quinolines (structural analogs of vitamin PQQ) while 7-OMe-substituted enamines give pyrrolo[3,2-g]quinolines with linear fusion of the rings.