Thermal and structural characterization of two polymorphs of Atovaquone and of its chloro derivative

Atovaquone, 2-4-(4-chlorophenyl)cyclohexyl]-3-hydroxy-1,4-naphthoquinone, is an antimicrobial medicament used to treat or prevent pneumocystis carinii pneumonia, toxoplasmosis and malaria. Two polymorphs of Atovaquone (crystal phases I and III) were isolated and their crystal and molecular structures were determined by single crystal X-ray analysis. In both crystal phases, strong hydrogen bond interactions link adjacent molecules in centrosymmetric dimers. The existence of the different polymorphs is determined by the different orientation of the dimers in the crystal packing. In addition, a crystalline phase of the 2-chloro substituted derivative, which is not stabilized by intermolecular H-bond interactions, was also studied, and compared with those of the pristine (hydroxylic) species. DSC measurements and thermodiffractometry analyses on polycrystalline batches witnessed the 100% purity of the isolated materials and disclosed the crystal-to-crystal interconversion of phase I to phase III upon heating at 210 °C.