Acetylenedicarboxylic acid thiophene ring

Very little work has been done on fluoro derivatives of thiophenes. 2-Fluorothiophene was obtained in low yield from treatment of 2-iodothiophene with arsenic trifluoride. The action of fluoroboric acid on thiophenediazonium salts was unsuccessful. It may be useful for the preparation of 4-, 5-, 6- or 7-fluorobenzo[6]thiophenes from the appropriate amines. However, these are more conveniently prepared from fluorine-substituted benzenethiols by ring-closure reactions. For example 4,5,6,7-tetrafluorobenzo[6 Jthiophene was obtained by decarboxylation of the corresponding 2,3-dicarboxylic acid (equation 99) prepared by condensation of pentafluorobenzenethiol with diethyl acetylenedicarboxylate (Section 3.15.3.4.1). 2-Fluorothiophene has been prepared from 2-thienyllithium using perchloryl fluoride, and 2-fluorobenzo[ Jthiophene from the 2-lithio derivative in a similar manner (Section 3.14.3.9.1). 

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