An Unexpected Chiral Bisguanidine Catalyzed Hetero-Diels-Alder Reaction

Significance

The authors found that in the presence of chiral C ₂-symmetric bisguanidines, azlactones and chalcones undergo an inverse electron-demand hetero-Diels-Alder reaction rather than the expected Michael addition. The novel catalyst 1 turned out to be superior for this reaction giving the δ-lactone products in good yields and high enantioselectivities for a variety of different chalcones and azlactones. The proposed reaction pathway was supported by submitting a preformed Michael adduct to identical reaction conditions, whereby the corresponding δ-lactone was not formed. Furthermore, the authors show the feasibility to recycle the catalyst and propose a plausible transition state based on X-ray data.