Inclusion Compounds of α-Lipoic Acid Methyl Ester with Urea and Thiourea

Abstract

Recently several works have been reported on stabilization of various pharmaceuticals by formation of urea, thiourea, or deoxycholic acid adducts. Methyl <i>α</i>-lipoate, a relatively unstable hepatotonic, was found to form adducts both with urea and thiourea. X-ray diffraction patterns and infrared spectra showed that these adducts were typical inclusion compounds. Methyl <i>α</i>-lipoate in the urea adduct was relatively stable under exposure to sunlight or to ultraviolet light, whereas it was not so stabilized in the thiourea adduct. It was also found that in these adducts methyl <i>α</i>-lipoate became its free radical under such irradiation and that this radical remained stable in these channels for several hours to one day even after the irradiation was stopped.