Light‐Induced Cycloaddition of 2,3‐Dihydro‐2,2‐dimethyl‐4H‐thiopyran‐4‐one (a 4‐Thiacyclohex‐2‐enone) to Alkenes and Dienes

The reactivity of (thiacyclic)‐2,3‐dihydro‐2,2‐dimethyl‐4H‐thiopyran‐4‐one ( 1a ) in light‐induced cycloadditions to furan ( F ), acrylonitrile ( AN ), or 2,3‐dimethylbut‐2‐ene ( TME ) is compared to that of (carbocyclic) 5,5‐dimethylcyclohex‐2‐enone ( 1b ). Whereas for the more‐flexible thiacycle, the efficiency of 2+2]‐photocycloadduct formation with AN or TME is generally much lower, the diastereoselectivity regarding the ring fusion in the bicyclo4.2.0]octanes is quite similar for both enones. In contrast, 1a affords exclusively trans‐fused 4+2] cycloadducts with F , while 1b gives predominantly the corresponding cis‐fused products.