(1R)-(+)-Camphor and Acetone Derived α′-Hydroxy Enones in Asymmetric Diels−Alder Reaction: Catalytic Activation by Lewis and Brønsted Acids, Substrate Scope, Applications in Syntheses, and Mechanistic Studies
2010-03-05T00:00:00Z (GMT) by
The Diels−Alder reaction constitutes one of the most powerful and convergent C−C bond-forming transformations and continues to be the privileged route to access cyclohexene substructures, which are widespread within natural products and bioactive constituents. Over the recent years, asymmetric catalytic Diels−Alder methodologies have experienced a tremendous advance, but still inherently difficult diene-dienophile combinations prevail, such as those involving dienes less reactive than cyclopentadiene or dienophiles like β-substituted acrylates and equivalents. Here the main features of α′-hydroxy enones as reaction partners of the Diels−Alder reaction are shown, with especial focus on their potentials and limitations in solving the above difficult cases. α′-Hydroxy enones are able to bind reversibly to both Lewis acids and Brønsted acids, forming 1,4-coordinated species that are shown to efficiently engage in these inherently difficult Diels−Alder reactions. On these bases, a convenient control of the reaction stereocontrol can be achieved using a camphor-derived chiral α′-hydroxy enone model (substrate-controlled asymmetric induction) and either Lewis acid or Brønsted acid catalysis. Complementing this approach, highly enantio- and diastereoselective Diels−Alder reactions can also be carried out by using simple achiral α′-hydroxy enones in combination with Evans’ chiral Cu(II)-BOX complexes (catalyst-controlled asymmetric induction). Of importance, α′-hydroxy enones showed improved reactivity profiles and levels of stereoselectivity (endo/exo and facial selectivity) as compared with other prototypical dienophiles in the reactions involving dienes less reactive than cyclopentadiene. A rationale of some of these results is provided based on both kinetic experiments and quantum calculations. Thus, kinetic measurements of Brønsted acid promoted Diels−Alder reactions of α′-hydroxy enones show a first-order rate with respect to both enone and Brønsted acid promoter. Quantum calculations also support this trend and provide a rational explanation of the observed stereochemical outcome of the reactions. Finally, these fundamental studies are complemented with applications in natural products synthesis. More specifically, a nonracemic synthesis of (−)-nicolaioidesin C is described wherein a Brønsted acid catalyzed Diels−Alder reaction involving a α′-hydroxy enone substrate is the key step toward the hitherto challenging trisubstituted cyclohexene subunit.