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 mu M. Among them, 1 showed the best tyrosinase inhibitory activity with an IC50 value of 0.39 mu M, approximately 50 times stronger than the positive control, kojic acid.