1.3- Dihydrothieno thiophenes, formation

Challenger and Harrison found both thienothiophene 1 and its isomer 2 in the products of the reaction between acetylene and sulfur. To identify these compounds, Challenger et developed syntheses of unsubstituted and 2-alkyl-substituted thieno[3,2-f>]thiophene (2) from thiophene derivatives. Cyclization of (3-thienylthio)acetic acid in the presence of sulfuric acid gave 2,3-dihydrothieno[3,2-6]thiophen-3-one (22) (R = H) in 14% yield reducing the latter with lithium aluminum hydride resulted in thienothiophene (2) formation in 80% yield [Eq. (9)]. Similarly 2-methyl- and 2-ethyl-2,3-dihydrothieno[3,2-/>]thiophen-3-one were obtained from a-(3-thienylthio)propionic and a-(3-tWenylthio)-butyric acids in 30% and 27% yields, respectively their reduction yielded 2-methyl (32%) and 2-ethylthieno[3,2-6]thiophenes (52%). The parent acids were prepared from 3-mercaptothiophene. ... 

Thieno[3,4-3]thiophenes 500 were prepared from thiophen-3-yl-substituted [l,3]dithiol-2-ones 496 in good yields <2003JOG7115>. The photochemical irradiation of 496 leads to thieno[3,4-f]dithiines 497 via a unique ring cleavage reaction and subsequent reductive treatment of dithiines 497 proceeds to give 2,3-dihydrothieno[3,4-/ ]thiophenes 498 and 499. Heating of 499 in the presence of further iodomethane leads to the formation of the thieno[3,4-/ ]-thiophenes 500 (Scheme 63). 

---