Conformation-control and stereoselective reactions using cation-π interactions

The existence of two types of intramolecular cation-pi interactions between a phenyl and a pyridinium, and between a thiocarbonyl and a pyridinium in nicotinic amide derivatives have been clarified by H-1 NMR and CD spectroscopies, X-ray structural analysis and calculations. These interactions resulted in the shielding of one side of the pyridinium face by the phenyl or the thiocarbonyl group, which enables nucleophiles to attack the pyridinium ring from the opposite side. Addition of organometallic reagents and ketene silyl acetals gave 1, 4-dihydropyridines in high regio- and stereoselectivities. The stereochemistry of the chiral center of the products was opposite depending on whether the chiral auxiliary is oxazolidine or thiazolidine-2-thione. Moreover, the DMAP derivatives bearing a thiazolidine-2-thione moiety served as asymmetric acylating catalysts. Kinetic resolution of a variety of see-alcohols was performed in the presence of 0.5 mol% of the catalyst with the s values of 7-30.