Stereoselective synthesis of 1,2- and 1,4-dihydropyridines by using cation–π interaction as a conformation-controlling tool

Selective shielding of one side of the pyridinium face by way of intramolecular cation–π complex formation enabled nucleophiles to attack the pyridinium ring from the non-shielded side. As a result, 1,2- and 1,4-dihydropyridines were formed in good stereoselectivities. 1H NMR analysis and ab initio calculations at the RHF/3-21G* level supported the existence of cation–π interaction between the pyridinium and the aromatic ring of the chiral auxiliary.