Generation of chiral N-acylpyridinium ions by means of silyl triflates and their diastereoselective trapping reactions: formation of N-acyldihydropyridines and N-acyldihydropyridones
作者:Cornelia E Hoesl、Markus Maurus、Jörg Pabel、Kurt Polborn、Klaus Th Wanner
DOI:10.1016/s0040-4020(02)00675-0
日期:2002.8
An efficient method for the generation of chiral N-acylpyridinium salts from various pyridines and a chiral acid chloride promoted by means of silyltriflates as additives is presented. The stereoselectiveaddition 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
Asymmetric methylation and phenylation of the chiral pyridinium salt 7, as well as methylation of chiral pyridinium salt 18, with Grignard reagents occurred in good yield and with good-to-excellent diastereoselectivities (Schemes 2 and 3, resp.). These results are best explained by assuming chelate control to govern the asymmetric alkylation/arylation process. The minimum-energy conformations of the
A pronounced asymmetric induction (d.e. = 95 %) was observed during methylation of pyridinium salt 7 with MeMgI which led ultimately to (2R) 2-methyl-2,3-dihydro-4-pyridone 9. This result is best explained by assuming chelate-control during the asymmetric alkylation step.