Synthesis of dithienofurans via cascade copper catalysed dual C-S coupling and ring closure reactions under mild conditions.

We have developed a mild catalytic approach for the synthesis of new dithienofuran derivatives via cascade copper catalysed dual C-S coupling and subsequent ring closure reactions. Sonogashira coupling between perbromofuran and terminal alkynes produced 3,4-dibromo-2,5-dialkynylfuran (1) in good yields. Next, copper catalysed C-S coupling between 1 and Na2S·9H2O and a subsequent ring-closure reaction afforded dithienofuran compounds (2) under mild conditions. We found that this strategy shows broad substrate scope and can be used to prepare not only aryl and heteroaryl but also alkyl substituted dithienofuran derivatives in up to 70% yields. Furthermore, we proposed a mechanism including two catalytic cycles: a typical Cu(i)/Cu(iii) catalytic cycle and a subsequent Cu(ii) induced cyclization mechanism.