Isoprenylated Phenolic Compounds from Morus macroura as Potent Tyrosinase Inhibitors

Three new Diels-Alder adducts, macrourins E - G (1: -3: ), one new 2-arylbenzofuran, macrourin H (4: ), and eight known Diels-Alder adducts (5: -12: ) were isolated from Morus macroura. Their structures were elucidated through extensive analysis of spectroscopic data. The ¹H NMR and ECD trends in the determination of the configurations of these Diels-Alder adducts were summarized. The tyrosinase inhibitory activities of all compounds isolated were evaluated, and the new compounds (1: -4: ) as well as the eight known compounds (5: -12: ) were found to be potent with IC₅₀ values ranging from 0.39 to 4.54 µM. Among them, 1 showed the best tyrosinase inhibitory activity with an IC₅₀ value of 0.39 µM, approximately 50 times stronger than the positive control, kojic acid.