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Synthesis of 2H-pyrans

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Phosphine-catalyzed (3 + 3) annulations between allenoates and 1C,3O-bisnucleophiles provide a facile entry to stable 2H-pyrans which are greatly challenging targets in organic synthesis. (3 + 2) Annulations can be achieved with the assistance of base to yield furans.
J. Hu, W. Dong, X.-Y. Wu, X. Tong, Org. Lett., 2012, 14, 5530-5533.


A one-pot sequence via a Ag(I)-catalyzed propargyl-Claisen rearrangement of easily accessed propargyl vinyl ethers, followed by a base-catalyzed isomerization, and 6π-oxaelectrocyclization, leads to the formation of stable 2H-pyrans.
H. Menz, S. F. Kirsch, Org. Lett., 2006, 8, 4795-4797.


DABCO-catalyzed reactions of α-halo carbonyl compounds with dimethyl acetylenedicarboxylate (DMAD) at room temperature gave polysubstituted furans and highly functionalized 2H-pyrans in good yields.
M. Fan, Z. Yan, W. Liu, Y. Liang, J. Org. Chem., 2005, 70, 8204-8207.