Novel Antioxidant Caffeate Esters of Black Locust (Robinia pseudoacacia L.) Stem Bark

Antioxidant profiling of black locust (Robinia pseudoacacia L.) stem bark was performed by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay off-line coupled to high-performance liquid chromatography–diode array detection–electrospray ionization mass spectrometry (DPPH–HPLC–DAD–ESI–QMS). The ethyl acetate, methanol, and n-hexane extracts exhibited similar chemical profiles for the antioxidant amphipathic phenolic compounds; however, the ethyl acetate extract was the richest in them, and n-hexane yielded several compounds in higher ratios. A total of 21 compounds displayed antioxidant activity and were further characterized by HPLC–high-resolution tandem MS (HRMS/MS). Nine compounds were identified as previously described antioxidants from the black locust stem bark, including 3-O-caffeoyl oleanolic acid (1), and the oleyl (4), octadecyl (6), gadoleyl (7), eicosanyl (9), (Z)-9-docosenyl (10), docosyl (12), tetracosyl (15), and hexacosyl (17) caffeates. Further ten caffeate and two ferulate esters were tentatively identified as 3-O-caffeoyl oleanolic, ursolic, or betulinic aldehyde (2), and hexadecyl (3), heptadecyl (5), hydroxy-hexacosyl (8), henicosyl (11), tetracosenyl (13), tricosyl (14), hexacosenyl (16), octacosenyl (18), and octacosyl (20) caffeates, and hexacosyl (19) and octacosyl (21) ferulates that were reported for the first time in R. pseudoacacia. Furthermore, this is the first report about the antioxidant activity of the previously undescribed compounds 8, 13, 16, and 18, as well as known compounds 11 and 14.