Intramolecular electrophilic aromatic substitution of α-alkylcinnamaldehydes affording 1-alkoxy-2-alkylindenes

Treatment of alpha-alkylcinnamaldehydes with orthoesters, alcohols, or thiols in the presence of (BF3OEt2)-O-. 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 beta-deuterated alpha-methylcinnamaldehyde and the NMR studies of the reaction mixture. The transformation process involves successive reactions, i.e., alkoxylation of the carbonyl carbon of alpha-alkylcinnamaldehydes to form acetals, elimination of alkoxide from the acetals to give alkoxycarbenium ion and gamma-alkoxyallyl cation, and intramolecular electrophilic arylation to afford the indene ring structure. (c) 2006 Elsevier Ltd. All rights reserved.