Abstract: | 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. |