Structure elucidation of thioketone analogues of sildenafil detected as adulterants in herbal aphrodisiacs

Two analogues of sildenafil were detected in herbal dietary supplements marketed as aphrodisiacs. Both compounds were identified as thioketone analogues of sildenafil in which the carbonyl group in the pyrimidine ring of sildenafil was substituted with a thiocarbonyl group. The first compound was identified as thiosildenafil, a compound that has recently been reported as an adulterant in health supplements. The structure of the second compound was established using LC–MS, UV spectroscopy, ESI-MSn, NMR and a hydrolytic process. A detailed study of the hydrolysis products of sildenafil, thiosildenafil, and the second unknown compound proved that the second compound, named thiomethisosildenafil, had a structure analogous to sildenafil in which the N-methylpiperazine moiety had been replaced with 2,6-dimethylpiperazine and the oxygen atom of the carbonyl group in the heterocyclic ring had been replaced with a sulfur atom. Under the hydrolytic reaction conditions employed in this study, thioketones hydrolyze to ketones (e.g., thiosildenafil → sildenafil), making this a valuable technique for the structure elucidation of thiosildenafil analogues. Ten herbal dietary supplements, each as a capsule dosage form, were found to contain 8–151 mg of thiomethisosildenafil per capsule, and one herbal dietary supplement was found to contain 35 mg of thiosildenafil per capsule.