PARTIALLY FLUORINATED HETEROCYCLIC-COMPOUNDS .28. THE HYDROLYSIS OF 1,3,4,5,6,7,8-HEPTAFLUORO-2-NAPHTHYLIDENERHODANINE TO FORM RING-FUSED THIOPHENES - A SIGNIFICANT PROPORTION OF A LINEAR NAPHTHO[2,3-B]THIOPHENE DERIVATIVE ACCOMPANYING AN ANGULAR NAPHTHO[1,2-B]THIOPHENE DERIVATIVE

1,3,4,5,6,7,8-Heptafluoronaphthalene-2-carbaldehyde (7), prepared via a Grignard reaction from 2-bromo-1,3,4,5,6,7,8-heptafluoronaphthalene (9) with N-methylformanilide, reacted with rhodanine to give 1,3,4,5,6,7,8-heptafluoro- 2-naphthylidenerhodanine (5). Hydrolysis of (5) with base gave a mixture of 4,5,6,7,8,9-hexafluoronaphtho[2, 3-b]thiophene-2-carboxylic acid (11) and 4,5,6,7,8,9-hexafluoronaphtho[1, 2-b]thiophene-2-carboxylic acid (12) resulting from the intermediate thiolate (6) displacing fluorine at sites 3 and 1 in the naphthalene ring in the ratio of 22:78 respectively, which represents a significantly high proportion of the linearly cyclised product accompanying the angularly cyclised product. Decarboxylation of the mixture of (11) and (12) gave 4,5,6,7,8,9-hexafluoronaphtho[2, 3-b]thiophene (13) and 4,5,6,7,8,9-hexafluoronaphtho[1, 2-b]thiophene (14) respectively, while treatment of the mixture of (11) and (12) with diazomethane gave an inseparable mixture of the methyl 2-carboxylates (15) and (16) respectively. © 1990.