1,3-Di-t-butyl- and 1,3-di-t-butyl-4,6-dimethylthieno[3,4-c]thiophenes (1g and 1h) are generated by Pummerer dehydration of the corresponding sulfoxides (4 and 5, respectively) in boiling acetic anhydride and can be trapped with N-phenylmaleimide (NPM). Both endo- and exo-adducts are formed from 1g and NPM, while only exo-adduct is formed from 1h and NPM. Reduction and isomerization of the starting