3-Acylpyridinium ions, synthesis

The electron-poor aromatic ring of 1-acylpyridinium ions is known to easily undergo nucleophilic addition by carbon nucleophiles an example was proposed by Comins and coworkers, who exploited the addition of zinc enolate 68 to enantiopure 67 (R = trans-2-(a-cumyl)cyclohexyl) in a total synthesis of (-F)-cannabisativine (equation 42)125. 

The formation of N-acylpyridinium ions from acyl chlorides and pyridines is effectively promoted by (la) and i-Pr3SiOTf. The N-acylpyridinium ions are usable for the synthesis of dihydropyridines and their derivatives by alkylation with Grignard reagents [128]. This reaction sequence has been applied to asymmetric alkylation of pyridines using a chiral acyl chloride [129]. 

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