Treatment of α-alkylcinnamaldehydes with orthoesters, alcohols, or thiols in the presence of BF3· OEt2 induces an intramolecular electrophilic aromatic substitution reaction to afford 1- alkoxy-2-alkylindenes. The reaction mechanisms of the indene formation have been elucidated on the basis of the reaction behaviors of β-deuterated α-methylcinnamaldehyde and the NMR studies of the reaction mixture. The transformation process involves ...