Photochemistry of Isothiocoumarin (= 1 H ‐[2]benzothiopyran‐1‐one)

Irradiation (λ = 350 nm) of 1 H ‐[2]benzothiopyran‐1‐one ( 2 ) in the solid state affords selectively and in good yield 6aα, 6bα, 12bα, 12cα ‐tetrahydrocyclobuta[1, 2‐ c :4, 3‐ c ′]bis([2]benzothiopyran)‐5, 8‐dione ( 3 ), the head‐to‐head (HH) cis ‐ cisoid ‐ cis ‐cyclodimer of 2 , X‐Ray analysis of 2 confirms that this reaction proceeds according to the well‐established topochemical principles. The same dimer 3 is obtained in low yields on irradiation of 10 −1 M solutions of 2 in either MeOH or MeCN, while no conversion at all is observed in benzene. On irradiation of 2 in MeCN in the presence of tetrachloroethene, the [2 + 2] photocycloadduct 4 is formed in good yield, the conversion 2 → 4 being efficiently quenched by naphthalene. In contrast, no reaction is observed ‐on irradiation of 2 in the presence of 2, 3‐dimethylbut‐2‐ene, neither in polar nor in apolar solvents.