5-[(E)-2-arylvinyl]-1,2-dihydro-3H-1,4-benzodiazepin-2-ones are used as starting materials for the preparation of new analgesics with anti-inflammatory and non-psychotropic properties. The alkylation of 5-[(E)-2-arylvinyl]-1,2-dihydro-3H-1,4-benzodiazepin-2-one with an excess of methyl iodide in chloroform in the presence of saturated aqueous solution of potassium carbonate and TBAB as a catalyst for interfacial transfer unexpectedly resulted in the title compound as a product of narrowing of the heterocycle. The structure of (E)-4-(4-chlorostyryl)-3-(dimethylamino)-1- methylquinolin-2(1H)-one was confirmed by mass spectrometry, 1H-NMR spectroscopy and single crystal X-ray diffraction.  The title compound crystallizes in the monoclinic P21/n space group, a=9.1605(5), b=20.5027(8), c=9.4894(5) Å; β=106.429(5) ^o, V=1709.48 Å3, Z=4 with one molecule per asymmetric unit. The C-C bond distances adjust the common values, the C-N bond distances in the heterocyclic ring are equal to 1.378(3) and 1.393(3) Å, C=O distance is equal to 1.225(3) Å. The molecule is characterized by the non-planar skeleton with the twisted angle between the planar chlorophenyl and quinoline fragments of 32.11^o. No specific interactions between the molecules in crystal were found.