Tocotrienol monomers and dimers from the roots of Litchi chinensis with tyrosinase inhibition activity.

Four undescribed modified tocotrienols, including two monomers, litchinols A (1) and B (2), and two walsurol dimers, δ,δ-walsurol (3) and γ,δ-bi-O-walsurol (4), as well as seven known compounds (5-11) were isolated from the roots of Litchi chinensis. The structures of the undescribed compounds were elucidated based on analyses of spectroscopic data and ECD spectra. All tocotrienol derivatives (1-6) were evaluated for their tyrosinase inhibition activity. Only monomers 1-2 and 5-6 displayed potent inhibitory activity and greater than kojic acid. Kinetic analysis revealed that the representative compound 2 was uncompetitive inhibitor with the inhibition constant value of 5.70 µM.