The cyclization of 5-(1-cyanoethyl)-2-(phenylthio)phenylacetic acid (I) with polyphosphoric acid at 120 C gives 2-(10-oxo-10,11-dihydrodibenzo[b,flthiepin-2-yl)propionitrile (II), which is then hydrolyzed with KOH in refluxing ethanol-water.