Construction of alkyl-substituted 7-norbornenones through Diels−Alder cycloaddition of electron-deficient olefins and a cyclopentadienone derivative generated in situ

Dimeric sesquiterpenoids possessing densely substituted 7-norbornenone/7-norbornenol motifs pose a considerable challenge for chemical synthesis. From a strategic perspective, one could envision intermolecular Diels−Alder cycloaddition as a straightforward method for assembling alkyl-substituted 7-norbornenones. However, this approach is hindered by lability of the required dienes, namely alkyl-substituted cyclopentadienones. Here we report a one-pot protocol for construction of alkyl-substituted 7-norbornenones from electron-deficient olefins and a cyclopentenone derivative. DDQ was found to be an effective oxidant for generating a cyclopentadienone intermediate in situ from the enone. A series of sterically congested 7-norbornenone-containing polycyclic compounds were prepared by using this protocol.