Aslam C. Shaikh, O. Kwon
Apr 29, 2020
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Influential Citations
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Journal
Organic syntheses; an annual publication of satisfactory methods for the preparation of organic chemicals
Abstract
Graphical abstract There has been remarkable progress in organophosphine-catalyzed reactions, 2 especially the processes involving [4C+X] annulations, because of their potential application in building 5–8-membered cyclic products. The phosphine-catalyzed [4 + 2] annulation between 2-alkyl-but-2,3-dienoates and aldimines, first reported by our group in 2003, has become a powerful tool in the construction of substituted tetrahydropyridine derivatives (Scheme 1). 3 According to the generally accepted mechanism, nucleophilic addition of tri-n-bu tylphosphine to the β -position of α -alkyl allenoates results in the formation of a resonance stabilized zwitterionic species A . The nucleophilic addition of the enolate A into N-tosylimine 2 produces sulfonamide B . Through proton transfer, the species B equilibrates with the vinylogous phosphonium ylide C/D . One more proton transfer facilitates the formation of the sulfonamide anion in E , which undergoes conjugate addition to the α , β -enoate, followed by β -elimination of tributylphosphine, resulting in the formation of tetrahydropyridine 3 . This Discussion Addendum focuses on new developments of this [4 + 2] annulation–including its asymmetric versions and synthetic utility–since 2003. When reacting 2-substituted 2,3-butadienoates and N-tosylimines in the presence of tributylphosphine, Kwon’s [4 + 2] annulation proceeds to afford tetrahydropyridines with high efficiency and diastereoselectivity. Specifically, ethyl α -methylallenoate undergoes the [4