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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.


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.


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.