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Synthesis of 3,4-dihydro-2H-pyrans

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An olefin metathesis/double bond migration sequence of allyl ethers to cyclic enol ethers is catalyzed by first and second generation Grubbs' catalysts. These ruthenium carbene complexes were activated to catalyze the double bond migration by additioin of hydride sources, such as NaH or NaBH4.
B. Schmidt, Eur. J. Org. Chem., 2003, 816-819.

Molecular iodine catalyzes practical and mild syntheses of substituted pyrans and furans under solvent-free conditions at ambient temperature and atmosphere, thus offering a facile and practical alternative to currently available reaction protocols. A combination of experimental studies and density functional theory calculations revealed interesting mechanistic insights.
D. P. Pace, R. Robidas, U. P. N. Tran, C. Y. Legault, T. V. Nguyen, J. Org. Chem., 2021, 86, 8154-8171.

A titanocene-catalyzed reductive domino reaction provides gem-difluorobishomoallylic alcohols starting from trifluoromethyl-substituted alkenes and epoxides. Furthermore, diverse 6-fluoro-3,4-dihydro-2H-pyrans have been prepared through derivatization of the cross-coupling products in a single step.
Z. Lin, Y. Lan, C. Wang, Org. Lett., 2020, 22, 3509-3514.

The carbonyl ene reaction of 2-methylenetetrahydropyrans provides β-hydroxydihydropyrans under mild conditions in very good yields. This synthesis of 2-substituted tetrahydropyrans enables a direct introduction of oxygen heterocycles into molecular frameworks.
G. Liang, D. T. Sharu, T. Lam, N. I. Totah, Org. Lett., 2013, 15, 5974-5977.

C2-symmetric bis(oxazoline)-Cu(II) complexes catalyze the inverse electron demand hetero Diels-Alder reaction of α,β-unsaturated carbonyl compounds with electron-rich olefins in high diastereo- and enantioselectivity. α,β-Unsaturated acyl phosphonates and β,γ-unsaturated α-keto esters and amides are effective heterodienes, while enol ethers and sulfides function as heterodienophiles.
D. A. Evans, J. S. Johnson, E. J. Olhava, J. Am. Chem. Soc., 2000, 122, 1635-1649.

Hydrophobic ionic liquids such as [Bmim]PF6 are powerful media for bis(oxazoline)-copper-catalyzed asymmetric hetero-Diels-Alder reactions, that allow a convenient catalyst recycling. The reactivities and stereoselectivities were comparable to those of the corresponding homogeneous reactions. Furthermore, the reaction was remarkably accelerated in [Bmim]PF6 compared to dichloromethane.
Y. J. Shin, C.-E. Yeom, M. Jeong, B. M. Kim, Synlett, 2008, 89-93.

Tuning the reactivity of arylpalladium intermediates enables 5-exo and 6-endo cyclizations of alkynols. These palladium-catalyzed reactions offer a divergent synthesis of oxygen-containing heterocycles, which are of synthetic use for further derivatization. Formal synthesis of an hNK-1 receptor antagonist also showcases the utility this arylative cyclization.
D. Fujino, H. Yorimitsu, A. Osuka, J. Am. Chem. Soc., 2014, 136, 6255-6258.

The combination of IPrAuCl and NaBArF catalyzes an efficient synthesis of functionalized volatile oxa-bicyclo[4.1.0]-hept-4-ene compounds in good yields. The utility of these low-molecular-weight enols was demonstrated by derivatization of some adducts and olfactory evaluation.
R. Laher, C. Marin, V. Michelet, Org. Lett., 2020, 22, 4058-4062.