Regio- and stereoselective synthesis of 1,4-dihydropyridines by way of an intramolecular interaction of a thiocarbonyl or carbonyl with a pyridinium nucleus

Chiral 1,4-dihydropyridines were prepared by the regio- and stereoselective addition of ketene silyl acetals and organometallic reagents to pyridinium salts. In the addition reaction, an intramolecular interaction between the thiocarbonyl or carbonyl with the pyridinium nucleus plays an important role in bringing about the selectivities. The absolute configuration of the newly produced stereogenic center of the 1,4-dihydropyridines was determined by X-ray analysis and CD Cotton effects after conversion into the appropriate derivatives. The working model for the stereoselectivity was proposed based on the ab initio calculations at the RHF/3-21G∗ level.