A new convenient synthetic route towards 2-(hetero)aryl-substituted thieno[3,2-b]indoles using Fischer indolization

A number of 2-(hetero)aryl-substituted thieno[3,2-b]indoles have been successfully prepared using an efficient transition-metal-free strategy, involving the Fiesselmann synthesis of methyl 5-(hetero)aryl-3-hydroxythiophene-2-carboxylates from 2-bromo-3-(hetero)arylacrylates and methyl thioglycolate, and the transformation of the synthesized 3-hydroxyesters into the corresponding thiophen-3(2H)-ones, followed by their treatment with arylhydrazines to directly form the targeted structures via Fischer indolization. At the same time, structural variety of the obtained thieno[3,2-b]indoles has been achieved due to a wide range of available starting materials, including both 2-bromo-3-(hetero)arylacrylates and arylhydrazines. In addition, two π-extended molecules, namely 1,4-bis(4H-thieno[3,2-b]indol-2-yl)benzene and 2,5-bis(4H-thieno[3,2-b]indol-2-yl)thiophene, have been synthesized in line with the current approach towards 2-(hetero)arylated thieno[3,2-b]indoles.