Substrate‐Controlled Diastereoselectivity Switch in the Formation of Dihydrothieno[3,4‐ c ]coumarins via [4+1] Annulations

An expedient and practical route for realizing a novel, substrate‐controlled diastereoselective switch strategy has been successfully developed. The [4+1] annulation of thioamides with sulfur ylides proceeded smoothly via a Michael addition triggered sequential process, and trans ‐dihydrothieno[3,4‐ c ]coumarins were obtained because of steric hindrance effects between the benzoyl group and the unit of 2 H ‐chromen‐2‐one as shown in the Newman projection. Nucleophilic substitution was observed as the key step when the sulfur ylides were replaced with acetophenones in the [4+1] annulation, and a double π,π‐stacking donor‐acceptor interaction stabilized the transition state that led to the cis ‐dihydrothieno[3,4‐ c ]coumarins. In addition, a facile chemoselective reaction was also investigated, providing a series of thieno[3,4‐ c ]coumarins.