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Synthesis of 3,4-dihydropyran-2-ones

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After NHC-catalyzed activation of acetic esters, the catalytically generated triazolium enolate intermediates serve as two-carbon nucleophiles that undergo highly enantioselective reactions with enones and α,β-unsaturated imines to give α-unsubstituted δ-lactones and lactams, respectively.
S. Chen, L. Hao, Y. Zhang, B. Tiwari, Y. R. Chi, Org. Lett., 2013, 15, 5822-5825.


An N-Heterocyclic carbene (NHC)-catalyzed highly enantioselective lactonization of modified enals with enolizable aldehydes proceeds via α,β-unsaturated acylazolium intermediates and yields synthetically important 4,5-disubstituted dihydropyranones.
S. R. Yetra, T. Kaicharla, S. S. Kunte, R. G. Gonnade, A. T. Biju, Org. Lett., 2013, 15, 5202-5205.


Asymmetric Michael addition-lactonization reactions of β,γ-unsaturated α-keto esters with thioesters catalyzed by a proline-derived urea provide 3,4-dihydropyranones and spiro-3,4-dihydrocoumarin-fused 3′,4′-dihydropyranones in high yield with excellent stereoselectivities via bioinspired S-to-O acyl-transfer reactions.
H. Jin, J. Lee, H. Shi, J. Y. Lee, E. J. Yoo, C. E. Song, D. H. Ryu, Org. Lett., 2018, 20, 1584-15888.