Stereoselective synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via intramolecular hetero-Diels-Alder reaction

The synthesis of 3a,7a-dihydro-3H,4H-furo3,4-c]pyran-1-ones via an intramolecular hetero-Diels-Alder reaction of easily accessible α,β-unsaturated γ-ketoesters was investigated. The reaction was found to proceed in a highly stereoselective way leading to single, cis-configured product isomers. The same diastereomer is formed, independently of the configuration of the enone double bond of the precursor. The respective E- and Z-isomers react either through an endo-E-syn or an exo-Z-syn transition state.