Ma. Zolfigol et al., An efficient and chemoselective method for the oxidation of 1,4-dihydropyridines under mild and heterogeneous conditions, J CHEM R-S, (4), 2000, pp. 167-169
A combination of inorganic acidic salts or oxalic acid dihydrate and sodium
nitrate in the presence of wet SiO2 was used as an effective oxidising age
nt for the oxidation of dihydropyridines to their corresponding pyridine de
rivatives at 40-50 degrees C with excellent yields.