Found: C, 78.36; H, 5.95. been approved as a natural yellow color additive and antioxidant in makeup products for many years [21]. Recently, curcumin was demonstrated to be beneficial to treat some skin diseases [22,23], and one of its derivatives, tetrahydrocurcumin, was recommended to be used in cosmetics as a lighting agent [24]. Furthermore, Lee recently reported that some curcumin analogues exhibited inhibitory activity against tyrosinase [25]. These reports attracted our interest to further study the inhibitory effect of curcumin analogues on tyrosinase. In this study, considering polyphenolic compounds exhibited potent inhibitory activity against tyrosinase, and their 4-hydroxyl groups played a crucial role in some tyrosinase inhibitors [26,27,28], a series of unsymmetrical curcumin analogues (UCAs) bearing 4-hydroxyl groups were synthesized (Scheme 1) and their inhibitory activities against tyrosinase were evaluated. In addition, the inhibition mechanism and acute toxicity of several potent UCAs were also investigated in order to achieve our aims of developing novel tyrosinase inhibitors with potent activities and lower side effects. 2. Results and Discussion 2.1. Chemistry The syntheses of polyphenolic UCAs (compounds 3aCk and 4aCi, Scheme 1) were easily completed by a facile two-step sequence without the need for hydroxyl group protection. This began with an aldol condensation of an aromatic aldehyde (4-hydroxybenzaldehyde or 4-hydroxy-3-methoxy-benzaldehyde) with extra acetone or cyclopentanone under basic conditions (aq. NaOH) to afford a conjugated enone 1 or 2 2. Finally, one more aldol condensation of a different aromatic aldehyde (using various substituted benzaldehyde derivatives) Jervine with the corresponding intermediate 1 or 2 2 under acidic conditions (catalytic amount of conc. HCl) gave the desired UCAs 3 or 4 4 in 50~75% yield over two actions. Thus, twenty polyphenolic UCAs made up of 4-hydroxyl groups on ring A were prepared. Among them, hydroxyl groups are only present at the (3a). Yield = 75%; m.p.: 241C243 C; 1H-NMR = 10.02 (br, 1H), 7.63 (d, = 8.7 Hz, 2H), 7.52 (d, = 8.7 Hz, 2H), 7.34 (d, = 6.6 Hz, 2H), 7.03 (d, = 8.7 Hz, 2H), 6.86 (d, = 8.7 Hz, 2H), 3.90 (s, 3H), 3.03 (s, 4H); ESI-MS: = 304.9 (M+?H); Anal. Calc. for C20H18O3: C, 78.41; H, 5.92. Found: C, 78.36; H, 5.95. (3b). Yield = 72%; m.p.: 278C280 C; 1H-NMR = 10.01 (brs, 1H), 9.62 (brs, 1H), 7.52 (d, = 8.7 Hz, 2H), 7.33C7.32 (m, 2H), 7.22 (s, 1H), 7.16C7.13 (m, 1H), 6.86 (dd, = 8.4, 2.7 Hz, 3H), 3.82 (s, 3H), 3.03 (s, 4H); ESI-MS: = 320.8 (M+?H); Anal. Calc. for C20H18O4: C, 74.52; H, 5.63. Found: C, 74.49; H, 5.65. (3c). Yield = 65%; m.p.: 300 C; 1H-NMR = 10.00 (br, 1H), 9.53 (brs, 1H), 9.19 (brs, 1H), 7.51 (d, = 8.7 Hz, 2H), 7.30 (s, 1H), 7.23 (s, 1H), 7.09 (d, = 1.5 Hz, 1H), 7.00C6.97 (m, 1H), 6.87 (s, 1H), 6.84C6.80 (m, 2H), 3.00 (s, 4H); ESI-MS: = 306.8 (M+?H); Anal. Calc. for C19H16O4: C, 74.01; H, 5.23. Found: C, 73.97; H, 5.24. (3d). Yield = 68%; Jervine m.p.: 241C243 C; 1H-NMR = 10.01 (brs, 1H), 9.97 (brs, 1H), 9.84 (brs, 1H), 7.73 (s, 1H), 7.50 (d, = 8.7 Hz, 2H), 7.39 (d, = 8.7 Hz, 1H), 7.28 (d, Jervine = 2.1 Hz, 1H), 6.85 (d, = 8.4 Hz, 2H), 6.42C6.30 (m, 2H), 3.00 (s, 4H); ESI-MS: = 306.9 (M+?H); Anal. Calc. for C19H16O4: C, 74.01; H, 5.23. Found: C, 73.95; H, 5.25. (3e). Yield = 71%; m.p.: 259C261 C; 1H-NMR = 10.02 (brs, 1H), 9.02 (brs, 1H), 7.52 (d, = 8.7 Hz, 2H), 7.33 (m, 2H), 6.96 (s, 2H), 6.86 (d, = 8.7 Hz, 2H), 3.82 (s, 6H), 3.08C3.03 (m, 4H); ESI-MS: = 350.9 (M+?H); Anal. Calc. for C21H20O5: C, 71.58; H, 5.72. Found: C, 71.50; H, 5.74. (3f). Yield = 75%; m.p.: 233C235 C; 1H-NMR = 10.01 (brs, 1H), 7.66 (s, 1H), 7.51 (d, = 8.7 Hz, 1H), 7.42 (s, 2H), 7.33 (d, = 8.7 Hz, 2H), 6.85 (d, = 8.7 Hz, 2H), 3.03 (s, 4H), 1.42 (s, 18H); ESI-MS: = 403.0 (M+?H); Anal. Calc. for C27H32O3: C, 80.16; H, 7.97. Found: C, 80.12; H, 8.00. (3g). Yield = 75%; m.p.: 241C243C; 1H-NMR = 10.10 (brs, 1H), 10.07 (brs, 1H), 7.52 (d, = 8.7 Hz, 2H), 7.41 (s, 1H), 7.34 (s, 1H), 7.30 (s, 1H), 7.27 (s, 1H), 6.86 (d, = 8.7 Hz, 2H), 3.88 (s, 3H), 3.04 (s, 4H); ESI-MS: = 399.0 (M+?H); Anal. Calc. for C20H17BrO4: C, 59.87; H, 4.27. Found: C, 59.80; H, 4.29. (3h). Yield = 69%; m.p.: 286C288 C; 1H-NMR = 9.75 (brs, Jervine 1H), 8.04 (brs, 1H), 7.83 (s, 2H), 7.53 (d, = 8.7 Hz, 2H), 7.35 (s, 1H), 7.26 IL-2 antibody (d, = 1.2 Hz), 6.86 (d, = 8.7 Jervine Hz, 2H), 3.02 (s, 4H); ESI-MS: = 448.7 (M+?H); Anal. Calc. for C19H14Br2O3: C, 50.70; H, 3.13. Found: C,.