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Synthesis of 2-pyrones

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An electrophilic pyrone derivative, which can be readily prepared on a multigram scale, undergoes metal-catalyzed cross-coupling reactions with a variety of nucleophiles to yield 3-substituted-2-pyrones.
F. Frébault, M. T. Oliveira, E. Wöstefeld, Nuno Maulide, J. Org. Chem., 2010, 75, 762-7695.

A palladium-catalyzed cross-coupling reaction of cyclic vinyldiazo ester with aryl iodides provides various 3-aryl-2-pyrones in good yields with high functional group tolerance. The resulting 3-aryl-2-pyrones can be used as diene components in Diels-Alder reactions.
J. Huang, L. Li, H. Chen, T. Xiao, Y. He, L. Zhou, J. Org. Chem., 2017, 82, 9204-9209.

Sequential activation of terminal alkynes and propiolic acids by gold(I) catalysts yields α-pyrones in good yields. Novel cascade reactions involving propiolic acids are reported that give rise to α-pyrones with different substitution patterns.
T. Luo, M. Dai, S.-L. Zheng, S. L. Schreiber, Org. Lett., 2011, 13, 2834-2836.

The oxyindation of carbonyl-ene-yne compounds with indium trihalides provides di-, tri-, and tetrasubstituted 2-pyrones bearing a carbon-indium bond. The metalated 2-pyrones can be oxidized or cross-coupled to multifunctionalized 2-pyrones.
T. Yata, Y. Kita, Y. Nishimoto, M. Yasuda, J. Org. Chem., 2019, 84, 14330-14341.

A gold(I)-catalyzed rearrangement of β-alkynylpropiolactones enables a straightforward synthesis of substituted α-pyrones in good yields.
T. Dombray, A. Blanc, J.-M. Weibel, P. Pale, Org. Lett., 2010, 12, 5362-5365.

Synergistic use of electrochemistry and organometallic catalysis enables a site-selective vinylic C-H functionalization of acrylic acids with alkynes affording α-pyrones with very good yields in an undivided cell. Importantly, common chemical oxidants such as Ag(I) or Cu(II) did not give significant amounts of the desired product in the absence of electrical current under otherwise identical conditions.
Q.-L. Yang, Y.-K. Xing, X.-Y. Wang, H.-X. Ma, X.-J. Weng, X. Yang, H.-M. Guo, T.-S. Mei, J. Am. Chem. Soc., 2019, 141, 18970-18976.

A N-heterocyclic carbene catalyzes a formal [3 + 3] annulation of alkynyl esters with enolizable ketones to provide functionalized 2H-pyran-2-ones in a highly regioselective manner. This simple protocol offers a broad substrate scope and mild metal-free conditions.
Z. Liang, J. Li, L. Wang, Z. Wei, J. Huang, X. Wang, D. Du, J. Org. Chem., 2023, 88, 1836-1843.

An efficient synthesis of 6-substituted 3-(alkoxycarbonyl)-5-aryl-α-pyrones consists of an addition-elimination reaction between benzyl ketone derivatives and dimethyl methoxymethylenemalonate, and an acid-catalyzed condensation reaction. The synthesis is applicable to various multiubstituted α-pyrones in very good yields.
T. Miura, S. Fujioka, N. Takemura, H. Iwasaki, M. Ozeki, N. Kojima, M. Yamashita, Synthesis, 2014, 46, 496-502.

A ruthenium-catalyzed regioselective intermolecular multistep homo- and heterodimerization of substituted propiolates provides α-pyrone-5-carboxylates and α-pyrone-6-carboxylates. The proposed mechanism is strongly supported by experimental evidence.
R. Manikandan, M. Jeganmohan, Org. Lett., 2014, 16, 652-655.

Ruthenium catalyzes an efficient three-component cascade reaction of acrylic acids, ethyl glyoxylate, and p-toluenesulfonamide to provide pyranones. A nucleophilic addition of the vinylic C-H bond to the aldehyde is followed by an intramolecular cyclization and a second insertion to provide substituted butenolides. The elimination of sulfonamides occurs at higher temperature to give the pyranones.
S. Yu, C. Hong, Z. Liu, Y. Zhang, Org. Lett., 2022, 24, 4871-4875.

Addition of a catalytic amount of Cy2NH·HX improves the rate and yields for cyclization reactions of o-(alkynyl)benzoates in the presence of a stoichiometric amount of copper(II)halide to give 4-haloisocoumarins. Under the standard reaction conditions, various 4-haloisocoumarins are provided in good yield.
Y. Liang, Y.-X. Xie, J.-H. Li, Synthesis, 2007, 400-406.

An efficient intramolecular annulation of Morita-Baylis-Hillman (MBH) alcohols of propiolaldehydes in the presence of ICl or PhSeSePh/PhSSPh-CuCl2 provides a wide variety of iodinated or chalcogenated 3-(chloromethyl)-2-pyrones in good yields. The chloromethyl group can easily be converted to other functionalities.
C. R. Reddy, A. D. Patil, Org. Lett., 2021, 23, 4749-4753.