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 2H‐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.