Synthesis of pyridinium salts from N-substituted dihydropyridines with BF3OEt2 in the absence of added oxidants

Abstract N -Substituted dihydropyridines were converted into pyridinium salts with BF 3 OEt 2 as aromatization promoter in reactions that could be conducted without the need of the usually added oxidizing agents, such as quinones, nitrates, peroxides, and chromates. An optimization study employing an N -phenyl Hantzsch ester dihydropyridine ( 1a ) to obtain the corresponding N -phenylpyridinium-containing ammonium quaternary salt ( 2a ) was conducted to define the best conditions for the aromatization reaction: 3 equiv of BF 3 OEt 2 at 0 °C, in the dark, under atmospheric air in CH 2 Cl 2 to give yields up to 80%. These conditions could also be applied to additional examples of both N -substituted dihydropyridines and 1,4-dihydropyridines.