Thionoesters as 1,2-Dipolarophiles for [4+2] Cycloaddition with Cyclobutanones

The utility of thionoester as a 1,2-dipolarophile for [4+2] cycloaddition with cyclobutanones is described. The [4+2] cycloaddition reaction provided tetrahydrothiopyran, which is found in biologically active natural products, in high yield with high diastereoselectivity by using readily available TiCl4. This synthetic method was applicable to a wide range of thionoesters and cyclobutanones. The ketone and S,O-ketal functionalities of the product could be reduced with excellent diastereoselectivity. Furthermore, the C-O bond was transformed into a C-C bond, affording contiguous tetrasubstituted carbon centers.