Synthesis of Bicyclic Pyrane Derivatives via Tungsten-Mediated [3 + 3] Cycloaddition of Epoxides with Tethered Alkynes

Propargyltungsten compounds bearing a tethered epoxide were prepared in short steps from readily available materials. In the presence of various Lewis acids, BF₃·Et₂O catalysts (25 mol %) most effectively promote the [3 + 3] cycloaddition of the epoxide with its tethered propargyltungsten group, delivering bicyclic pyranyltungsten compounds in reasonable yields. This cyclization proceeds highly diastereoselectively with tolerance of various functional groups. The stereochemical outcome indicates that the cycloaddition is initiated by the ring opening of the epoxides via an exo-attack of the propargyltungsten group. The resulting pyranyltungsten organometallics were demetalated to yield various bicyclic pyranyl derivatives using different oxidants. This new method provides a short enantiospecific synthesis of bicyclic oxygen compounds if chiral epoxide is used in the cyclization. A mechanistic model is presented to rationalize the reaction pathway of this [3 + 3] cycloaddition.