A concise
synthetic method for dihydropyrans has been developed
by inverse-electron-demand oxa-Diels–Alder reaction of α-keto-β,γ-unsaturated
esters with α,β-unsaturated hydrazones as electron-rich
olefins. This reaction is catalyzed by Eu(hfc)3 and proceeds
in an endo-selective manner. This umpolung cycloaddition
affords a variety of substituted dihydropyrans stereoselectively in
high yields. In addition, indirect synthesis of formyl-substituted
dihydropyran was achieved by dehydrazonation of the cycloadduct. This
method is expected to be useful for the synthesis of dihydropyrans
and tetrahydropyrans with unusual substitution patterns.
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