Wavelength Control in the Photochemical Addition of Tricyclic [ g ]‐Fused 1‐Thiocoumarins or Coumarins to Alkenes Yielding Mono‐ and Bis([2 + 2]‐Cycloadducts)

On long‐wavelength (>395 nm) irradiation in the presence of 2,3‐dimethylbut‐2‐ene the benzobis(thiinones) 6 and 7 afford 4:1 mixtures of cis ‐ and trans ‐fused monocycloadducts. Similarly, irradiation of thiinobenzopyrandione 8 affords 50% of a 4:1 mixture of cis ‐ and trans ‐fused monocycloadducts at the thiinone moiety and in addition further 50% of the cis ‐fused monocycloadduct at the pyranone ring. Irradiation of 6–8 at shorter wavelength (>340 nm) in the presence of the same alkene furnishes mixtures of all bis(photocycloadducts), of which some were isolated by chromatography and analyzed by 1 H‐NMR spectroscopy. – Mixed bis([2 + 2]‐photocycloadducts) of such molecules become easily accessible, as illustrated by the irradiation of benzobispyranone 9 , first (>350 nm) in the presence of 2,3‐dimethylbut‐2‐ene and then (>300 nm) in the presence of isobutene.