Thienylthioacetic acids

The cyclization of thienylthioacetic acids " and thienylthioacet-aldehyde acetals " has been studied in connection with work on the chemistry of the thiophthenes. The cyclizations are achieved with concentrated or anhydrous usually in... 

A number of thieno[3,2-6]thiophene derivatives have been synthesized by the acid-catalyzed cyclization of appropriately substituted acid derivatives. 3-Thienylthioacetic acid was cyclized with concentrated sulfuric acid to yield 14% of the ketone (158), which upon... 

Several 2-alkyl-substituted thieno[3,2-6 jthiophenes were also prepared in an analogous fashion (76AHC(19)123). The sulfuric acid cyclization of 2-thienylthioacetic acid (160) has been, reported to give in 9-11% yield a mixture of the unexpected ketone (159) and a thiolactone presumed to be (161 Scheme 52) (76AHC(19)123). 

A similar abnormal cyclization of 2-pyrrolylthioacetic acid (162) by PPA to the ketone (163) has been reported (Scheme 53) (61JOC2615, 61AK(18)15l). Thus, this Friedel-Crafts approach is not of synthetic value in the [2,3-6]-fused series. The acid-catalyzed cyclization of a 3-thienylthioacetic acid to give a thieno[3,4-6]thiophene was successful, but only if the highly reactive 2-position was blocked. For example, 2,5-dimethyl-3-thienylthioacetic acid (164) was cyclized with anhydrous HF to ketone (165), subsequent reduction of which gave 4,6-dimethylthieno[3,4-6]thiophene (166 Scheme 54) <76AHC(19)123>. 

Methyl-substituted thienothiophens have been obtained by condensation of methyl esters of ortho-foxmy - or -acetyl-thienylthioacetic acids. ... 

The cyclization of thienylthioacetic acids and thienylthioacet-aldehyde acetals " has been studied in connection with work on the chemistry of the thiophthenes. The cyclizations are achieved with concentrated H2SO4or anhydrous HF, usually in low yields (10-15%). An interesting rearrangement has been discovered in the cyclization of 2-thienylthioacetic acid (137) with concentrated H2SO4 which leads to 2i -thieno[3 -6]thiophene-3-one (138), identical with the product obtained upon cyclization of 3-thienylthioacetic acid (139). The structure of (138) has been... 

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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