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Abstract

The cyclization of 3-[2-(phenylthio)phenyl]propionic acid (I) with polyphosphoric acid gave equal amounts of 4-(phenylthio)indanone (III) and 2H-cyclopenta[k,l]thioxanthene (VI), accompanied with a small amount of the macrocyclic diketone V. Compound VI was evidently formed via the cation XII and 7-(phenylthio)indanone (X) which was quantitatively dehydrocyclized. Similar cyclization of the isomeric acid XIIIa yielded 5-(phenylthio)indanone (XVIII) as the main product; formation of VI could be detected only chromatographicaly. Cyclization of 3-[2-(chloro-5-(phenylthio)phenyl]propionic acid (XIIIb) gave 3-chloro-2H-cyclopenta[k,l]thioxanthene (VII) as the predominant product. From the by-products formed, only 4-chloro-7-(phenylthio)indanone (XI) could be identified, being the precursor of compound VII. Treatment of thioxanthene-9-acetic acid (XXII) with polyphosphoric acid resulted in a cleavage with thioxanthone (XXIII) and thioxanthene (XXIV) as the main products.