Generation of chiral N-acylpyridinium ions by means of silyl triflates and their diastereoselective trapping reactions: formation of N-acyldihydropyridines and N-acyldihydropyridones

An efficient method for the generation of chiral N-acylpyridinium salts from various pyridines and a chiral acid chloride promoted by means of silyl triflates as additives is presented. The stereoselective addition of organometallic reagents to the intermediate chiral N-acylpyridinium ions led to α-substituted N-acyldihydropyridines and N-acyldihydropyridones in good yields, whereas no or minor amounts of products were obtained, when standard procedures without silyltriflates as additives were employed. Good diastereoselectivities were observed dependent on which pyridine or organometallic reagents were used. The removal of the chiral auxiliary proceeded smoothly either under basic or reductive reaction conditions.