Synthesis, characterization and thermal analysis of ursolic acid solid forms

This study performed a solid-state characterization of ursolic acid (UA) crystalline forms, a poorly water-soluble triterpene with anticancer activity. Two new polymorphs (form I, II), two new solvates (propanol and isopropanol solvates), and a known ethanol solvate were determined and elucidated using a combination of multi-techniques, including X-ray single crystal and powder diffraction, Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A colorless single crystal of UA was grown from a propanol solution, and its crystalline structure was determined through X-ray single crystal diffraction. It was determined that the propanol solvate was crystallized in the orthorhombic space group P2₁2₁2₁ with unit-cell parameters a = 7.17200 (8) Å, b = 12.24100 (16) Å, c = 33.8950 (4) Å and Z = 4. The ethanol solvate and propanol solvate were isomorphous crystals. The results of the thermal analysis demonstrate that form I is a meta-stable form, while form II is a stable form that is monotropically related. This paper is an investigation on the landscape of ursolic acid solid forms. Two polymorphs and three solvates were obtained and elucidated using the combination of multi-techniques. The phase transformation and the inter-conversion relationships were studied and outlined by thermal methods and solvent equilibration.