Reversal of chemoselectivity in Diels-Alder reaction with alpha,beta-unsaturated aldehydes and ketones catalyzed by Brønsted acid or Lewis acid.

[reaction: see text] High chemoselectivity was observed in the Diels-Alder reaction of alpha,beta-unsaturated aldehyde and alpha,beta-unsaturated ketone with cyclopentadiene. Using Tf2NH as catalyst, the reaction gave Diels-Alder adduct derived from alpha,beta-unsaturated ketone as a major product. On the other hand, bulky Lewis acid, B(C6F5)3, gave mainly the cycloadduct of alpha,beta-unsaturated aldehyde and cyclopentadiene.