Highly Enantioselective Synthesis of 2,6-Disubstituted and 2,2,6-Trisubstituted Dihydropyrones: A One-Step Synthesis of (<i>R</i>)-(+)-Hepialone and Its Analogues

An efficient enantioselective approach to chiral dihydropyrones has been developed by the hetero-Diels−Alder (HDA) reactions of (E)-4-methoxy-2-trimethylsiloxy-penta-1,3-diene (diene 1) with aldehydes and pyruvates. It has been found that the readily accessible (R)-BINOL-Ti(OiPr)4 (1.1:1) complex was a very effective catalyst for this reaction. Aromatic, heteroaromatic, conjugated, and aliphatic aldehydes afforded the corresponding products in moderate to high isolated yields (up to 99%) with excellent enantioselectivities (up to 99% ee). The first example of highly enantioselective synthesis of 2,2,6-trisubstituted dihydropyrones by the catalytic reaction of diene 1 with pyruvates was reported. The isolated intermediates indicated that this asymmetric HDA reaction proceeded in a Mukaiyama aldol pathway. On the basis of the absolute configurations of the products, a possible mechanism was proposed. Moreover, the catalytic system could be used to synthesize a series of enantioenriched β-hydroxyketones 4. Finally, this methodology was successfully applied for the one-step synthesis of the important natural product (R)-(+)-Hepialone with 88% isolated yield and 94% enantioselectivity.