Tandem Addition/Electrocyclization/Benzylation of Alkyl Aryl-1,3-dienes and Aromatic Aldehydes: Access to Highly Substituted Indenes.

BF3·Et2O-mediated synthesis of multisubstituted indenes from alkyl aryl-1,3-dienes and aromatic aldehydes through tandem addition/4π-electrocyclization/benzylation via tetrahydroindeno-oxepine/quinone methide followed by an intramolecular 1,6-hydride transfer is described. This novel reaction pathway is established by the isolation of potential intermediates and with the support of deuterium-labeling studies. In addition, the generality of this method is demonstrated by reacting various aromatic aldehydes, which ascertains the role of the electronic effect of aldehydes in the formation of indene derivatives and tetrahydroindeno-oxepines.