A formal [4+2] cycloaddition of sulfur-containing alkylidene heterocycles with allenic compounds

In this study, we report an enantioselective synthesis of sulfur heterocycles containing the dihydro-2H-pyran moiety. The synthesis strategy is based on quinidine catalyzed formal [4 + 2] cycloaddition between 3-alkylidene benzo[b]thiophenes and allenoates, affording the corresponding cycloadducts in high yields (up to 95%) with high enantioselectivities (up to 99% ee). Moreover, cycloadducts can be isolated by simple filtration from the reaction media.