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/ and benzylation via tetrahydroindeno-oxepine/quinone methide followed by the intramolecular 1,6-hydride transfer is described. This novel reaction pathway was established by the isolation of potential intermediates and with the support of deuterium labelling studies. In addition, the generality of this method has been demonstrated by reacting various aromatic aldehydes, which ascertained the role of aldehyde's electronic effect on the formation of indene derivatives and tetrahydroindeno-oxepines.