Abstract
The addition of RMgX or R 2Mg to nicotinic acid esters, activated with triisopropylsilyl triflate, was investigated. The regioselectivity of this reaction, where 4-unsubstituted and 4-substituted pyridine derivatives were employed as starting materials, was examined. Depending on the structure of the organomagnesium reagent varying ratios of 1,2-, 1,4-, and 1,6-regioisomers were obtained but in any case the 1,4-addition products were clearly predominating. Desilylation of the isomeric pure addition products with LiOH provided stable 4,4-disubstituted dihydropyridines, which could be easily N-alkylated with alkylhalides in high yield. Furthermore, it was shown that the silyl protecting group of the obtained dihydropyridines can be easily replaced by an acyl group.
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