Thieno-thiophenes base-catalyzed

The known dibromide 464 was converted in good yield to the dinitrile 465 by reaction with buffered potassium or sodium cyanide. Reaction of 2,5-dimethoxycarbonyl-3,4-dicyanomethylthiophene 465 with thionyl chloride and selenium oxychloride gave thieno[3,4-f]thiophene 466 and selenolo[3,4-f]thiophene 467, respectively (Scheme 57) <2002JOC2453>. In the case of thionyl chloride as the sulfur transfer reagent, an intermediate sulfoxide 468 must be involved, which then suffers a spontaneous base-catalyzed Pummerer reaction to give 466 in high yield. 

The base-catalyzed intramolecular condensation of the aldehyde-ester (171) led to the isomeric thieno[3,2-6]thiophene acid (172), which upon decarboxylation led to thieno[3,2-6]thiophene (3 Scheme 58) (76AHC(19)123). 

The 3-substituted thieno[3,2-6]thiophenes were prepared via the same sequence of reactions using the appropriately substituted acetic acid. For example, methyl 2-acetyl-3-thienylthioacetate (173) underwent base-catalyzed condensation to yield, after decarboxylation, 3-methylthieno[3,2-6]thiophene (174 Scheme 59) (76AHC(19)123). 

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