Insights into the stereoselective BF3-catalyzed hetero Diels-Alder reaction of Garner's aldehyde with Danishefsky's diene

A trans-2,6-disubstituted tetrahydropyranone was prepared in high yield via a BF3-mediated hetero Diels-Alder reaction between Danishefsky's diene and Garner's aldehyde. Only two of the four possible reaction products were obtained, and excellent diastereoselectivity toward the exo-syn adduct was observed (de = 80%). The origin of this somewhat unusual selectivity can be attributed to the steric interactions between the bulky protecting groups present in the diene and dienophile.