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

Irradiation (λ = 350 nm) of 1H‐[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.