The peculiar reaction of 2-arylazo-1-vinylpyrroles with trifluoroacetic acid: a novel synthesis of 2-methylquinolines

2-Arylazo-1-vinylpyrroles 1-6 react with CF₃CO₂H (benzene, reflux, 5 h) in a peculiar way: instead of expected electrophilic addition of CF₃CO₂H to the N-vinyl group, the latter is transferred to the azo group followed by NN bond cleavage to afford substituted 2-methylquinolines 10-12 in up to 56% yields. The reaction was shown (¹H, ¹³C, and ¹⁵N NMR) to start with the protonation of the azo group with further inter- and intramolecular involving of two protonated N-vinyl groups to finally build up the quinoline cycle over the aryl moiety.