The isomerisation of 1,4-diarvl-2-azetidinones in cold concentrated sulohuric acid to qive the correspondinq 3,4-dihydro-4-arylquinolin-2(1H)-ones (e.a. la + 2a) was first reported hy Knunyants and Gambaryan' and the results were recently confirmed and extended2. We have found that the chloro-comnounds (lb) behave analoqously in cold concentrated sulphuric acid or in polyphosphoric acid (PPA) at 78' to nive the expected 4-aryldihydroquinolinones (2b), but at hiqher temperatures in PPA isomerisation is accompanied by loss of hydroqen chloride, and in some cases followed by ar,vl miqration to qive 3-aryl-quinolin-2(1H)ones (4). For instance, treatment of trans-3-chloro-1,4-diphenyl-2-azetidinone3 (lb, Ar=Ar'=Ph) with PPP at 140' aave with evolution of hydroqen chloride, a white solid (70%) of m.p. 230'. Its analytical and spectral data corresponded to a ohenvlquinolinone (e.a. 3, Ar=Ph), i.e. isomerisation appeared to have been followed by loss of hydrogen chloride. However, comparison (m.p. and spectra) with authentic 3-phenyl-4 and 4-ohenyl-quinolin-2(lH)-one5 showed the product to be the 3-phenyl isomer (4, Ar=Ph). Since in cold concentrated sulphuric acid or PPA at 78'. trans-3-chloro-3,4-dihydro-4-phenylquinolin-2(lH)-one* (2b, Ar=Ph) was isolated, phenyl mioration could not have occurred during fission of the azetidinone nor durinq the subsequent cyclitiion to the 4-arvldihydroquinolone. Treatment of the 3-chloro-3,4_dihydroquinolinone (Zb) with ethanolic potassium hydroxide resulted in dehydrochlorination without phenyl miaration to qive 4-phenylquinolin-2(1H)-one (3, Ar=Ph) identical with the authentic sample. Moreover, the 4-phenylquinolin-2(1H)-one (3, Ar=Ph) does net isomerise to the 3-phenylquinolin-2(1H)-one (4, Ar=Ph) under the reaction conditions (i.e. PPA at 78'). However, treatment of trans-3-chloro-3,4-dihydro-4-phenylquinolin-2(lH)one (2b, Ar=Ph) with PPP at 140' qave the 3-phenylquinolin-2(1H)-one (4) in high yield. Phenyl miqration thus takes place durinq the dehydrochlorination steo by hot PPA but high