Morphotropism and 'Quasi-Isostructurality' in the Three High Z′ Concomitant Polymorphs of Efinaconazole

Abstract

The crystal structures of the three efinaconazole anhydrous forms have been solved by single-crystal X-ray diffraction showing a very rare case of nearly isostructural polymorphism. The analysis of their crystal structures revealed that forms I and III are 'quasi-isostructural' polymorphs and are morphotropically related to form II. The identical conformational and supramolecular environment can explain the concomitant polymorphism phenomenon observed for this important active pharmaceutical ingredient.