Two new series of biphenyls, analogs of aglycone of natural product fortuneanoside E, were prepared using Suzuki-Miyaura cross-coupling and selective magnesium iodide demethylation/debenzylation, and their mushroom tyrosinase inhibitory activity was evaluated. Most of the 4-hydroxy-3,5-dimethoxyphenyl biphenyl compounds (series II, 20-36) were in general more active than 3,4,5-trimethoxyphenyl biphenyl compounds (series I, 1-19). Structure-activity relationships study showed that monosaccharide substituents, such as glucose, were not necessary and the presence of 4-hydroxy-3,5-dimethoxyphenyl moiety was crucial for inhibitory activity. Among the compounds synthesised, compound 21 (IC50=0.02 mM) was found to be the most active one, which exhibited an activity that was 7 times higher than that of fortuneanoside E (IC50=0.14 mM) and 10 times higher than that of arbutin (IC50=0.21 mM), known as potent tyrosinase inhibitors. The inhibition kinetics analyzed by Lineweaver-Burk plots revealed that compound 21 was a competitive inhibitor (Ki=0.015 mM).
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