A General Synthesis of Quinoline-2,5,8(1H)-triones via Acylation of 2,5-Dimethoxyaniline with S-tert-Butyl Thioacetates by Application of the Knorr Cyclization

An efficient synthesis of quinoline-2,5,8(1H)-triones bearing alkyl groups at C3 and/or C4 is described. The reaction sequence employed involves Knorr cyclization of 2,5-dimethoxyanilides into 5,8-dimethoxyquinoline systems, followed by oxidative demethylation with cerium ammonium nitrate. The starting 2,5-dimethoxyanilides were prepared by chemoselective acylation by regioselective alkylation of S-tert-butyl acetothioacetate (1) at C2 and/or C4. The introduction of electrophiles other than alkyl groups at C2 on 1 was also studied.