Abstract
Fiesselmann thiophene synthesis was applied for the convenient construction of thieno[3,2-b]thiophene derivatives. Thus, new 5- or 6-aryl-3-hydroxythieno[3,2-b]thiophene-2-carboxylates were obtained by condensation of 5- or 4-aryl-3-chlorothiophene-2-carboxylates, respectively, with methyl thioglycolate in the presence of potassium tert-butoxide. The saponification of the resulting esters, with decarboxylation of the intermediating acids, gave the corresponding thieno[3,2-b]thiophen-3(2H)-ones. The latter ketones were used to synthesize new N,S-heterotetracenes, namely 9H-thieno[2',3':4,5]thieno[3,2-b]indoles by their treatment with arylhydrazines in accordance with the Fischer indolization reaction.
Highlights
The thieno[3,2-b]thiophene (TT) unit is highly demanded in modern organic synthesis since TT-based compounds have a variety of applications, e.g., in the field of organic electronics
Continuing our previous work in which we used the Fiesselmann thiophene synthesis [16], viz., the interaction of ethyl thioglycolate with ethyl 3-chlorobenzo[b]thiophene-2-carboxylates, to form a benzo[b]thieno[2,3-d]thiophene scaffold [17,18], we have decided to apply a similar approach to the synthesis of aryl-substituted TT derivatives
Methyl 3-chlorothiophene-2-carboxylates 2a–k were further involved in the Fiesselmann reaction with methyl thioglycolate in the presence of potassium tert-butoxide in THF, which afforded aryl-substituted methyl 3-hydroxythieno[3,2-b]thiophene-2-carboxylates 3a–k in 41–78% yields (Scheme 1)
Summary
The thieno[3,2-b]thiophene (TT) unit is highly demanded in modern organic synthesis since TT-based compounds have a variety of applications, e.g., in the field of organic electronics. We elaborated our own procedure for the preparation of aryl-substituted methyl 3-chlorothiophene-2-carboxylates 2, which consists of diazotization of the corresponding 3-aminothiophene-2-carboxylates 1 in acetonitrile solution with an aqueous sodium nitrite (1.1 equiv) solution in the presence of p-toluenesulfonic acid (4.0 equiv) followed by the addition of the formed solution of diazonium salts to a warm suspension of CuCl (5.0 equiv) in acetonitrile. Compounds 2a–k were obtained in 43–83% yield (Scheme 1, see Supporting Information File 1 for more experimental details).
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