Carfentanil is an ultra-potent synthetic opioid. The Russian police force used both carfentanil and remifentanil to resolve a hostage incident in Moscow. This reported use sparked an interest in the pharmacology and toxicology of carfentanil in the human body, and data on its metabolites were later published. However, there have been few studies on the synthesis of carfentanil metabolites, and biological extraction has also put forward large uncertainty in subsequent studies. The aim of the present study is to investigate the synthesis of biphasic metabolites that are unique to carfentanil. The purpose was to produce corresponding metabolites conveniently, quickly, and at low cost that can be used for comparison with published structures and to confirm the administration of carfentanil.