Novel antioxidants have been synthesized and characterized by their chemical properties as antioxidants with high superoxide scavenging activity. (2R,3R)-diphenylethylenediamine is a spacer in antioxidants, and we synthesized targets 11a and 11b by conjugation with o-hydroquinone and p-hydroquinone at the two N-termini, respectively. Superoxide scavenging activities of the plant flavonoid-like 11a and 11b were compared with those of known antioxidants, and shown to increase in the following order: curcumin << ascorbic acid < Trolox < (+)-epicatechin < 11a < quercetin ≤ 11b. Compound 11a also formed a solvated 11a-metal complex with metal ions. The 11a-Cu²⁺ complex was shown to have higher superoxide scavenging activity than that of 11a, 11b, Cu²⁺, and the 11a-Al³⁺ and 11a-La³⁺ complexes, whereas the 11a-Al3+ complex increased rather than decreased superoxide levels. The 11a-Al³⁺ complex did not abstract one electron from the SOMO of O₂⁻· since the HOMO and LUMO phases of Al³⁺ do not exist in the center of the 11a-Al³⁺ complex. However, the SOMO of the 11a-Cu²⁺ complex distributed in the center of Cu²⁺ may abstract one electron from the SOMO of O₂⁻·. These results suggest that 11a and 11b are powerful antioxidants.