Structural effect in the reductive vinylation/phosphorylation of pyridines with alkyl propiolates and secondary phosphine chalcogenides: protonation vs. zwitterion generation

The non-catalyzed reaction of 2- and 3-substituted pyridines with alkyl propiolates and secondary phosphine chalcogenides (50–52°C, MeCN) produces stereo-, regio- and chemoselectively 1-[(E)-2-(alkoxycarbonyl)ethenyl]-4-chalcogenophosphoryl- 1,4-dihydropyridines in 57–90% yields, the adducts of pyridines with alkyl propiolates being the zwitterionic intermediates. 4-Methylpyridine mainly catalyzes nucleophilic addition of secondary phosphine chalcogenides to alkyl propiolates.