Epalrestat tetra­hydro­furan monosolvate: crystal structure and phase transition

The title compound, epalrestat {systematic name: (5Z)-5-[(2E)-2-methyl-3-phenyl­prop-2-en-1-yl­idene]-4-oxo-2-sulfanyl­idene-1,3-thia­zolidine-3-acetic acid}, crystallized as a tetra­hydro­furan monosolvate, C(15)H(13)NO(3)S(2)·C(4)H(8)O. Epalrestat, an important drug for diabetic neuropathy, has been reported to exist in polymphic, solvated and co-crystal forms. In the mol­ecule reported here, the phenyl ring is inclined to the rhodamine ring by 22.31 (9)°, and the acetic acid group is almost normal to the rhodamine ring, making a dihedral angle of 88.66 (11)°. In the crystal, pairs of O—H⋯O hydrogen bonds are observed between the carb­oxy­lic acid groups of epalerstat mol­ecules, forming inversion dimers with an R (2) (2)(8) loop. The dimers are linked by pairs of C—H⋯O hydrogen bonds, forming chains along [101]. The solvate mol­ecules are linked to the chain by a C—H⋯O(tetra­hydro­furan) hydrogen bond. A combination of thermal analysis and powder X-ray diffraction revealed that title compound desolvated into epalerstat Form II. One C atom of the tetra­hydro­furan solvate mol­ecule is positionally disordered and has a refined occupancy ratio of 0.527 (18):0.473 (18).