The starting compound thiazolidine-4-one may also be a derivative of thiazolidine-2,4-dione In this case, if R1 is a substituent other than a hydrogen atom, this substituent may have been introduced into the thiazolidine-2,4-dione by means of one of its alkali metal salts, generally the potassium salt formed by treatment with potash in solution in ethanol at room temperature (the corresponding sod