Asymmetric oxa-Diels-Alder reactions using carbonyl compounds as dienes or dienophiles are effective tool for synthesis of various chiral pyran derivatives. [1, 2]  The first asymmetric oxa-Diels-Alder reaction of a,β-unsaturated ketones, which were activated through dienamines, with aldehydes described Xiao [3]. The best yields were achieved with pyrrolidine, but with chiral catalysts the yields dramatically dropped and the best ee was only 40 %.

With regards to the fact, that oxa-Diels-Alder reactions of a,β-unsaturated ketones as precursors of dienes with aldehydes have been described just once untill now [3] and suffer from long reaction time and/or mediocre yields and selectivities, we decided to study reaction of (E)-4-phenylbut-3-en-2-one with 4-nitrobenzaldehyde under various, classical as well as non-classical (ultrasonic or microwave irradiation, ball milling, continuous flow microreactor) conditions. We have tested a range of chiral organocatalysts. As a selection principle we have chosen catalyst ability to participate in an enamine activation of a,β-unsaturated ketone as well ability to hydrogen bond activation.

Our thanks for financial support are due to the Slovak Grant Agency VEGA, grant no. 1/0543/11, and Slovak research and development agency, grant no. APVV-0067-11.

[1] Carmona, D.; Lamata, M. P.; Oro, L. A. Coord. Chem. Rev. 2000, 200-202, 717-772.
[2] Núnez, M. G.; García, P. ; Moro, R. F.; Díez, D. Tetrahedron 2010, 66, 2089-2109.
[3] Lu, L.-Q.; Xing, X.-N.; Wang, X.-F.; Ming, Z.-H.; Wang, H.-M.; Xiao, W.-J. Tetrahedron Lett. 2008, 49, 1631-1635.