Epalrestat tetrahydrofuran monosolvate: crystal structure and phase transition

The title compound, epalrestat {systematic name: (5 Z )-5-[(2 E )-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).