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.