Further Studies on the Cyclization of Aromatic Azomethines Ortho-Substituted with a Trifluoromethyl Group: Synthesis of 2,4-Di-or 2,3,4-Trisubstituted Quinolines

The scope and limitations of the novel synthetic route to quinolines (Strekowski et al. J. Org. Chem. 1990, 55, 4777) have been studied. A direct condensation of 2-(trifluoromethyl)aniline (1) with methyl aryl ketones, methyl heteroaryl ketones, ethyl aryl ketones, methyl vinyl ketones, 1-indanone, 1-tetralone, camphor, and cyclohexanone provides an easy access to the corresponding ketimines. An indirect one-pot preparation of dialkyl ketimines and C-alkyl-substituted amidines derived from 1, but inaccessible by the direct condensation method, is also presented. All these ketimines and amidines are cyclized in the presence of alkylamide, dialkylamide, or alkoxide bases to give a quinoline containing the base function at C-4. Analysis of byproducts of the base-mediated reactions provides strong support for the originally proposed mechanism of the quinoline formation.