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

Abstract

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 successive reactions, i.e., alkoxylation of the carbonyl carbon of α-alkylcinnamaldehydes to form acetals, elimination of alkoxide from the acetals to give alkoxycarbenium ion and γ-alkoxyallyl cation, and intramolecular electrophilic arylation to afford the indene ring structure. © 2006 Elsevier Ltd. All rights reserved.