Acetylation of 2,3,5,6-tetrafluoroaniline (I), followed by treatment of the resulting acetanilide (II) with P2S5, gave thioamide (III). Cyclization of (III) in the presence of NaH produced the benzothiazole (IV), which was subsequently hydrolyzed to amino thiophenol (V) by means of NaOH in ethyleneglycol.
Alkylation of 3-indolylacetonitrile (VI) with ethyl bromoacetate provided indoleacetate ester (VII). Condensation of the cyano group of (VII) with amino thiophenol (V) in hot EtOH furnished benzothiophene (VI). The ethyl ester group of (VI) was finally hydrolyzed with NaOH to the title carboxylic acid.