A synthesis of densely functionalized 2,3-dihydropyrans using ring-closing metathesis and base-induced rearrangements of dihydropyran oxides

Article abstract of DOI:10.1002/1099-0690(200009)2000:18<3145::AID-EJOC3145>3.0.CO;2-B

The preparation of dihydropyran and dihydrofuran oxides and their rearrangement in the presence of lithium dialkylamides to functionalized 2,3-dihydropyrans or 2,3-dihydrofurans, respectively, is described. The regiochemical outcome of the reaction can be influenced by the relative configuration of the starting epoxides and the steric demand of the base. The 2,3-dihydropyrans obtained were converted stereoselectively to difunctionalized 3,4-dihydropyrans by the carbon-Ferrier reaction, or to fused acetals by addition of dimedone, mediated by ceric ammonium nitrate. The stereochemical results are rationalized by mechanistic proposals.