Diastereo- and enantioselective synthesis of trans-2,3-disubstituted 2,3-dihydropyran-4-one derivatives

Trans-Diastereoselective hetero-Diels-Alder reactions took place in the presence of SiCl4/activator systems. The reactions of aldehydes with a derivative of Danishefsky's diene afforded the corresponding pyrones with high yields and diastereoselectivity upon activating SiCl4 with suitable neutral Lewis bases. Aldol intermediates deriving from a Mukaiyama-type pathway were isolated in many cases. The employment of a chiral activator allowed us to convert Danishefsky's diene (or its disubstituted derivative) into both aldols and pyrones in good to high enantiomeric excesses.