Polymorphic forms of cilostazol

Two unique conformational polymorphic forms of the compound 6-[4-(1-cyclo­hexyl-1H-tetrazol-5-yl)­butoxy]-3,4-di­hydro­quinolin-2(1H)-one (cilostazol), C20H27N5O2, have been discovered and characterized using single-crystal X-ray structural analysis. A third polymorph also exists, but acceptable crystals could not be obtained. Features of both reported polymorphic structures include a chair conformation of the cyclo­hexyl ring and puckering in the quinolinone ring. The major feature distinguishing the two polymorphic forms is a rotational twisting of the butoxy chain between the tetrazole and quinolinone rings. This difference in conformation influences the intermolecular forces, and hence the packing of the two mol­ecules during crystallization.