In this work, the synthesis of thiohydantoins from l-amino acids and isothiocyanates was studied, using a high purity method based on the Edman cycle. This process, expected to occur through the Michael addition to a series of 1,4-naphthoquinone derivatives did not lead to the expected final product, rather leading to a colored mixture of compounds, where EPR spectra showed that some of them presented unpaired electrons, indicating that an electron transfer pathway was also involved in the reaction mixture. Detected organic radicals proved to be stable albeit with the use of different experimental conditions. Voltammetric results indicated that the reactions led to the formation of electroactive species, probably derived by homogeneous electron transfer between the reactive quinone moieties and the oxidizable urea functions within the hydantoin species.