The combination of suitably substituted pyrones with electron-rich alkenes allows the Diels-Alder reactions to be performed at atmospheric pressure. Interesting oxygenated cyclohexadienes can be readily accessed using this simple methodology.
Marko, I., & Evans, GR. (1993). Tandem Pericyclic-reactions (tpr) - Away From High-pressure. Tetrahedron Letters, 34(45), 7309-7312. https://doi.org/10.1016/S0040-4039(00)79316-6 (Original work published 1993)