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

The preparation of dihydropyran and dihydrofuran oxides and their rearrange ment in the presence of lithium dialkylamides to functionalized 2,3-dihydro pyrans 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-dihydropyr ans obtained were converted stereoselectively to difunctionalized 3,4-dihyd ropyrans by the carbon-Ferrier reaction, or to fused acetals by addition of dimedone, mediated by eerie ammonium nitrate. The stereochemical results a re rationalized by mechanistic proposals.