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), C(20)H(27)N(5)O(2), 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.