Construction of Thienothiophene and Thienofuran Ring Systems via Ring Expansion of Difluorothiiranes Generated from Dithioesters
Kohei Fuchibe, Ibuki Mukohara, Atsushi Yamada, Daisuke Miyazaki, Ryo Takayama, Junji Ichikawa
Abstract
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.