On the Knorr Synthesis of 6-Bromo-4-methylquinolin-2(1H)-one

Abstract

In the course of our work on infectious diseases, we were led to prepare 6-bromo-2-chloro-4-methylquinoline as a starting material. Since surprisingly little has been reported in the literature, the two synthetic steps to this compound were investigated. The synthesis involves a condensation between β-keto esters and 4-bromoaniline and the cyclization of the resulting anilides into 6-bromoquinolin-2(1H)-one, otherwise known as the Knorr reaction. The ¹H NMR monitoring of the first step allowed us to optimize the conditions leading specifically to the anilide without the occurrence of the alternative crotonate. To illustrate the scope of our finding, few additional anilides featuring electron-attracting groups were prepared. The study of their cyclization revealed some unsuspected steric effect governing this second step. Aside from rectifying a few claims in this chemistry, this study led to a three-step preparation of 6-bromo-2-chloro-4-methylquinoline in 48% overall yield from 4-bromoaniline.