Metal-free Brønsted acid catalyzed formal [3 + 3] annulation. straightforward synthesis of dihydro-2H-chromenones, pyranones, and tetrahydroquinolinones.

Brønsted acids catalyze the addition of enolizable beta-diketones, beta-ketoesters, and vinylogous amides to alpha,beta-unsaturated aldehydes to lead to substituted chromenones, pyranones, and tetrahydroquinolinones in good yields under mild reaction conditions via a formal [3 + 3] cycloaddition.