An Approach to the Construction of Benzofuran-thieno[3,2-]indole-Cored N,O,S-Heteroacenes Using Fischer Indolization.

A series of 6-benzofuro[2',3':4,5]thieno[3,2-]indoles were readily synthesized from methyl 3-aminothieno[3,2-]benzofuran-2-carboxylates using a one-pot procedure with Fischer indolization as the key step. At the same time, 3-aminothieno[3,2-]benzofuran-2-carboxylates were prepared from 3-chlorobenzofuran-2-carbaldehydes in three steps, including replacement of the Cl atom at the C-3 position of these starting substrates onto the -SCHCOMe moiety, conversion of the CHO group at the C-2 position into the CN group, followed by base-promoted cyclization of the formed carbonitrile. The present route was elaborated by us because we failed to obtain directly the desired 3-aminothiophene-2-carboxylate by reaction of 3-chlorobenzofuran-2-carbonitrile with methyl thioglycolate in the presence of various bases. In turn, 3-chlorobenzofuran-2-carbaldehydes were prepared from benzofuran-3(2)-ones following the Vilsmeier-Haack-Arnold reaction.