ZnI2-Catalyzed Diastereoselective [4 + 2] Cycloadditions of β,γ-Unsaturated α-Ketothioesters with Olefins.

The potential of β,γ-unsaturated α-ketothioesters participating in hetero-Diels-Alder reaction has remained unexplored. We report herein the first study of a ZnI2-catalyzed highly diastereoselective inverse electron demand hetero-Diels-Alder reaction of β,γ-unsaturated α-ketothioesters with olefins to access highly substituted 3,4-dihydro-2H-pyrans. All the reactions proceed with cis-selectivity in moderate to excellent yields. Under similar reaction conditions, terminal alkynes undergo direct conjugate 1,4-addition to yield δ,ε-acetylenic α-ketothioesters. Furthermore, the utility of these cycloadducts has been demonstrated by an NBS-MeOH mediated stereospecific efficient access to fully substituted pyran rings. The product bromoethers undergo E2 elimination with DBU, resulting in substituted 3,6-dihydro-2H-pyrans. In addition, the thioester moiety of the products has been used for further transformations, such as amidations and Fukuyama coupling reactions.