Construction of Thienothiophene and Thienofuran Ring Systems via Ring Expansion of Difluorothiiranes Generated from Dithioesters.

The reaction of aryl thiophene-2-carbodithioates or thiophene-3-carbodithioates with difluorocarbene generated from BrCF2CO2Li/molecular sieves 4A produced arylsulfanylated 2,2-difluoro-3-thienylthiiranes. In the presence of lithium ion, the thiirane intermediates underwent ring expansion followed by HF elimination, leading to fluorinated thieno[3,2-b]thiophenes or thieno[2,3-b]thiophenes. The reactions of the oxygen analogues, aryl furancarbodithioates, also proceeded to afford the corresponding thieno[3,2-b]furans. Intramolecular fluorine substitution in the produced arylsulfanyl(fluoro)thienofurans allowed for another thiophene ring construction, leading to the synthesis of fused pentacyclic thienothienofurans.