Reversal of Chemoselectivity in Diels−Alder Reaction with α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted Acid or Lewis Acid

Abstract
High chemoselectivity was observed in the Diels−Alder reaction of α,β-unsaturated aldehyde and α,β-unsaturated ketone with cyclopentadiene. Using Tf2NH as catalyst, the reaction gave Diels−Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, bulky Lewis acid, B(C6F5)3, gave mainly the cycloadduct of α,β-unsaturated aldehyde and cyclopentadiene.