Benzo thiophenes photocycloadditions

The benzo[ ]-fused systems participate in a number of [2 + 2] cycloaddition reactions. The photocycloaddition products of benzo ] thiophenes and DMAD are dependent on the irradiation wavelength at 330 nm 240 is formed, while at 360 nm the rearranged product 241 is produced. 

Thiophene on irradiation in presence of dihalogenomaleimides leads to low yields of the 2-substituted products (Scheme 89) (78TL125). When benzo[6]thiophene is irradiated in the presence of dibromomaleic anhydride, both photosubstitution and photocycloaddition are observed (Scheme 90) (77BCJ1797). The ratio of photosubstitution relative to cycloaddition increases with the temperature and polarity of the solvent. A common triplet exciplex has been suggested for both processes. 

Benzo[b]thiophene 1,1-dioxide 38 undergoes [2+2] photocycloaddition with vinyl acetate [120] and l-(diethylamino)propyne [121] to give the corresponding adducts in excellent yields (Scheme 22). The reaction appears synthetically important, and its scope and limitation deserve examination in detail. 

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