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

N-Substituted dihydropyridines were converted into pyridinium salts with BF3OEt2 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 BF3OEt2 at 0 degrees C, in the dark, under atmospheric air in CH2Cl2 to give yields up to 80%. These conditions could also be applied to additional examples of both N-substituted dihydropyridines and 1,4-dihydropyridines. (C) 2015 Elsevier Ltd. All rights reserved.