Tyrosinase Inhibitors from the Wood of Artocarpus heterophyllus

Oct 31, 2012 - From the methanolic-soluble extract of the wood of Artocarpus heterophyllus, four new flavones, artocarmins A–D (1–4), and three ne...
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Tyrosinase Inhibitors from the Wood of Artocarpus heterophyllus Nhan Trung Nguyen, Mai Ha Khoa Nguyen, Hai Xuan Nguyen, Ngan Kim Nguyen Bui, and Mai Thanh Thi Nguyen* Faculty of Chemistry, University of Science, Vietnam National University−HoChiMinh City, 227 Nguyen Van Cu Street, District 5, HoChiMinh City, Vietnam S Supporting Information *

ABSTRACT: From the methanolic-soluble extract of the wood of Artocarpus heterophyllus, four new flavones, artocarmins A−D (1−4), and three new chalcones, artocarmitins A−C (5−7), have been isolated together with 13 known compounds. Their structures were determined on the basis of the spectroscopic data. Compounds 1−4, 6, 7, 9− 16, and 20 displayed significant tyrosinase inhibitory activity. The most active compound, morachalcone A (12) (IC50, 0.013 μM), was 3000 times more active as a tyrosinase inhibitor than a positive control, kojic acid (IC50, 44.6 μM). Artocarpus heterophyllus Lam. (Moraceae), known as “jackfruit”, is distributed widely in tropical and subtropical regions of Asia. In Vietnam, A. heterophyllus is known as “Mit”, and this plant is commonly cultivated for its edible fruits, while its wood has been used as an anti-inflammatory, antioxidant, and antiaging agent.1 Previously, prenylflavonoids, stilbenes, triterpenes, and sterols have been isolated from the roots and stems of this plant.2 As part of a search for biologically active compounds with utility against overproduction of melanin in human skin epidermal layers, a screen has been initiated to evaluate natural product extracts for the inhibition of tyrosinase. A methanolic extract of the wood of A. heterophyllus showed significant tyrosinase inhibitory activity with an IC50 value of 2.3 μg/mL. Thus, the constituents of this plant were examined, and four new flavones (1−4) and three new chalcones (5−7) were isolated, together with 13 known compounds (8−20). In this paper, we report the isolation and structure elucidation of the new compounds using spectroscopic techniques, together with their tyrosinase inhibitory activity.

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vanillic acid (18),10 goldfussinol (19),11 and p-coumaric acid (20).12

Artocarmin A (1) showed a quasimolecular ion at m/z 377.0998 [M + Na]+, corresponding to the molecular formula C20H18O6 in the HRESIMS. Its IR spectrum displayed the absorbance of hydroxy (3400 cm−1), phenyl (1605, 1450 cm−1), and carbonyl (1660 cm−1) groups. The 1H NMR spectrum of 1 (Table 1) exhibited signals due to two sets of ortho-coupled aromatic protons at δ 7.90 (2H, d, J = 9.0 Hz) and 6.92 (2H, d, J = 9.0 Hz), an isolated aromatic proton at δ 6.54 (1H, s), and two isolated olefinic protons at δ 6.75 (1H, s) and 5.40 (1H, t, J = 7.0 Hz), together with one methyl, one methylene, one oxymethylene, and a characteristic signal of a hydrogen-bonded hydroxy proton (δ 13.21). On the other hand, the 13C NMR spectrum (Table 1) showed 20 carbon signals including a carbonyl carbon, two sp2 carbons, an sp2 quaternary carbon, an sp2 oxygenated quaternary carbon, a methyl carbon, a methylene carbon, an oxymethylene carbon, and 12 aromatic carbons. These data were similar to those of apigenin,13 a common flavone found in plants, except for the

RESULTS AND DISCUSSION

The MeOH extract of the wood of A. heterophyllus was partitioned between EtOAc and water to give an EtOAc-soluble fraction. The EtOAc fraction was subjected to a series of chromatographic separation and preparative TLC steps to afford four new flavones, artocarmins A−D (1−4), and three new chalcones, artocarmitins A−C (5−7), together with 13 known compounds. The known compounds were identified by analysis of their spectroscopic data and comparison with literature data as 3′-[γ-hydroxymethyl-(Z)-γ-methylallyl]4,2′,4′-trihydroxychalcone (8),3 gemichalcone B (9),3 gemichalcone A (10),3 isogemichalcone B (11),3 morachalcone A (12),4 norartocarpin (13),5 cudraflavone C (14),6 albanin A (15),7 p-hydroxybenzoic acid (16),8 β-resorcylic acid (17),9 © 2012 American Chemical Society and American Society of Pharmacognosy

Received: August 21, 2012 Published: October 31, 2012 1951

dx.doi.org/10.1021/np300576w | J. Nat. Prod. 2012, 75, 1951−1955

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Table 1. 1H and 13C NMR Data (δ, 500 and 125 MHz) for Compounds 1−4 in DMSO-d6 1 δH

position 2 3 4 4a 5 6 7 8 8a 1′ 2′ 3′ 4′ 5′ 6′ 1″ 2″ 3″ 4″ 5″ 1‴ 2‴ 3‴ 4‴ 5‴ 6‴ 7‴ 8‴ 9‴ OH-5 OH-4″ OMe-3‴

6.75, s

6.54, s

7.90, d (9.0) 6.92, d (9.0) 6.92, 7.90, 3.25, 5.40,

d (9.0) d (9.0) d (7.0) t (7.0)

3.75, d (5.0) 1.69, s

2 δC 163.5 102.8 181.8 103.5 158.4 110.8 161.8 93.2 155.1 121.3 128.4 116.0 161.1 116.0 128.4 20.5 121.1 135.2 66.3 13.6

δH 6.76, s

6.55, s

7.91, d (8.8) 6.92, d (8.8) 6.92, 7.91, 3.30, 5.56,

d (8.8) d (8.8) d (7.1) t (7.1)

4.52, s 1.80, s 7.54, d (8.6) 6.78, d (8.6) 6.78, 7.54, 7.54, 6.39,

13.21, s 4.64, t (5.0)

d d d d

(8.6) (8.6) (16.0) (16.0)

3 δC 163.9 103.0 182.0 103.8 158.8 110.2 162.1 93.4 155.5 121.6 128.5 116.1 161.3 116.1 128.5 20.9 126.5 130.3 69.2 13.9 125.4 130.3 115.9 160.0 115.9 130.3 144.9 114.3 166.6

13.27, s

δH 6.75, s

6.55, s)

7.90, d (9.0) 6.92, d (9.0) 6.92, 7.90, 3.29, 5.56,

d (9.0) d (9.0) d (7.0) t (7.0)

4.51, s 1.80, s 7.30, d (1.5)

6.77, 7.10, 7.52, 6.47,

d (8.0) dd (8.0, 1.5) d (16.0) d (16.0)

4 δC 163.7 102.8 181.8 103.6 158.5 110.0 161.9 93.3 155.5 121.3 128.5 116.0 161.1 116.0 128.5 20.8 126.4 130.2 69.1 14.0 125.7 111.3 148.0 149.4 115.6 123.2 145.2 114.5 166.6

13.26, s 3.79, s

δH 6.97, s

6.51, s

6.50, d (2.3) 6.44, 7.73, 3.25, 5.40,

dd (8.8, 2.3) d (8.8) d (7.2) t (7.2)

3.75, d (5.5) 1.69, s

δC 161.5a 106.7 181.9 103.3 158.6b 110.4 161.7a 93.0 155.1 108.7 158.3b 103.3 161.6a 108.0 129.6 20.4 121.2 135.1 66.3 13.5

13.29, s 4.62, t (5.5) 55.8

a,b

These signals may be interchanged.

presence of signals of a set of resonances due to a 4hydroxyprenyl unit, which was confirmed by the HMBC correlations of protons of H-2″ and H-5″ with C-4″ (Figure 1) and the downfield shift of C-4″ (δH 3.75, δC 66.3). The location

of the 4-hydroxyprenyl group was determined at C6 on the basis of the HMBC correlations of H-1″ with C-5, C-6, and C-7 and of a NOESY correlation of a proton of OH-5 with H-1″. Finally, the NOESY correlations of H-1″ with H-5″ and of H2″ with H-4″ indicated an E-configuration for the 2″, 3″ double bond (Figure 1). From this spectroscopic evidence, the structure of artocarmin A was concluded to be 1. Artocarmin B (2) and C (3) were obtained as pale yellow, amorphous solids. The 1H and 13C NMR spectra of 2 and 3 were similar to those of 1, but signals were displayed due to one more trans-p-coumaroyl unit in 2 and one more trans-feruloyl moiety in 3, which were confirmed by the COSY, HSQC, and HMBC spectra (Figure 1). The locations of these moieties were determined at C-4″ by HMBC correlation of H-4″ with the C-9‴ ester carbonyl carbon of these moieties. The NOESY correlations of H-1″ with H-5″ and of H-2″ with H-4″ indicated the configurations of 2 and 3 to be the same as that of 1 (Figure 1). Thus, the structures of artocarmins B and C (3) were concluded to be 4″-trans-p-coumaroylartocarmin A (2) and 4″-trans-feruloylartocarmin A (3), respectively. The molecular formula of artocarmin D (4) was determined as C20H18O7 by HRESIMS. The 1H and 13C NMR spectra of 4 were similar to those of 1, except for the presence of signals due to an ABX system accompanied by the disappearance of signals due to two sets of ortho-coupled aromatic protons in 1 (Table

Figure 1. Connectivities (bold lines) deduced by the COSY spectrum, significant HMBC correlations (arrows), and NOESY correlations (dashed arrows) observed for 1−3. 1952

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7.83 (1H, d, J = 15.5 Hz), an isolated olefinic proton at δ 5.52 (1H, t, J = 7.5 Hz), and one methyl, one methylene, one oxymethylene, and a characteristic signal of a hydrogen-bonded hydroxy proton (δ 13.99). The 13C NMR spectrum (Table 2) showed 20 carbon signals including those corresponding to the above groups, as well as a ketone carbonyl carbon (δ 193.2). These data suggested 5 is a chalcone bearing a prenyl group. Analysis of the COSY, HMQC, and HMBC spectra indicated the chalcone to be 4,2′,4′-trihydroxychalcone. Similar to 1, the 1 H and 13C NMR spectra of 5 showed characteristic peaks for a 4-hydroxyprenyl group. The location of the 4-hydroxyprenyl group was determined to be at C-3′ on the basis of the HMBC correlations of H-1″ with C-2′, C-3′, and C-4′ (Figure 2). The

1). Analysis of the COSY, HSQC, and HMBC spectra of 4 indicated a hydroxy group is present at C-2′. The configuration of 4 was confirmed to be the same as 1 based on the results of NOESY spectrum. From the above finding, the structure of artocarmin D (4) was determined to be 2′-hydroxyartocarmin A. The HRESIMS of artocarmitin A (5) showed a quasimolecular ion at m/z 363.1216 [M + Na]+, consistent with the molecular formula C20H20O5. The 1H NMR spectrum of 5 (Table 2) displayed signals due to three sets of ortho-coupled aromatic protons at δ 7.72 (2H, d, J = 8.5 Hz), 6.92 (2H, d, J = 8.5 Hz), 6.54 (1H, d, J = 9.0 Hz), and 7.97 (1H, d, J = 9.0 Hz), two trans olefinic protons at δ 7.75 (1H, d, J = 15.5 Hz) and Table 2. 1H and 13C NMR Data (δ, 500 and 125 MHz) for Compounds 5−7 in Acetone-d6 5 position 1 2 3 4 5 6 CO α β 1′ 2′ 3′ 4′ 5′ 6′ 1″ 2″ 3″ 4″ 5″ 1‴ 2‴

δH 7.72, d (8.5) 6.92, d (8.5) 6.92, d (8.5) 7.72, d (8.5) 7.75, d (15.5) 7.83, d (15.5)

6.54, d (9.0) 7.97, d (9.0) 3.41, d (7.5) 5.52, t (7.5) 3.89, s 1.80, s

δC 127.7 131.7 116.8 161.1 116.8 131.7 193.2 118.6 145.0 114.5 165.3 115.9 162.9 108.1 130.4 21.9 123.1 136.2 68.5 13.9

δH 7.73, d (8.5) 6.93, d (8.5) 6.93, d (8.5) 7.73, d (8.5) 7.76, d (15.5) 7.84, d (15.5)

6.56, d (9.0) 8.00, d (9.0) 3.47, d (7.5) 5.68, t (7.5) 4.55, s 1.88, s 7.34, d (1.5)

3‴ 4‴ 5‴

6.86, d (8.5) 7.34, d (8.5, 1.5) 7.58, d (16.0) 6.40, d (16.0)

6‴ 7‴ 8‴ 9‴ OH-2′ OMe-3‴ OMe-5‴

6

7 δC 127.5 131.7 116.8 161.1 116.8 131.7 193.1 118.4 145.0 114.4 165.2 115.1 162.9 108.1 130.5 22.0 127.7 131.2 70.2 14.2 127.4 111.3

δH 7.72, d (8.0) 6.92, d (8.0) 6.92, d (8.0) 7.72, d (8.0) 7.74, d (15.0) 7.83, d (15.0)

6.56, d (8.5) 7.99, d (8.5) 3.50, d (7.5) 5.58, t (7.5) 1.75, s 4.96, s 7.04, s

148.8 150.1 116.0

14.06, s 3.91, s

116.8 161.1 116.8 131.7

145.0

130.5 22.1 128.5 131.0 21.6 63.6 126.2 106.9

7.04, s

106.9

145.7

7.61, d (16.0) 6.46, d (16.0)

145.9

56.3

NOESY correlations of H-1″ with H-5″ and of H-2″ with H-4″ indicated that the double bond of this prenyl group has the Econfiguration (Figure 2). Therefore, compound 5 was the geometric isomer of 8, isolated from the same extract, which possesses a Z-configuration. From these data, the structure of artocarmitin A was concluded to be 5. Artocarmitin B (6) was isolated as an amorphous powder. The HRESIMS indicated its molecular formula to be C30H28O8. The 13C NMR data of 6 (Table 2) showed 15 carbons for a 4,2′,4′-trihydroxychalcone unit together with a 4-hydroxyprenyl group similar to 5. The remaining 10 carbons were accounted for by a trans-feruloyl substituent. The location of the 4hydroxyprenyl group was deduced at C-3′, the same as 5, from the HMBC correlations of H-1″ with C-2′, C-3′, and C-4′. A HMBC correlation of proton H-4″ with the C-9‴ ester carbonyl carbon of the feruloyl moiety indicated that the feruloyl moiety is connected with the prenyl unit at C-4″. The configuration of the 2″, 3″ double bond of 6 was determined to be the same as that of 5 from the NOESY spectrum, with 6 being a geometric isomer of 10, isolated from the same extract. Thus, the structure of artocarmitin B (6) was concluded to be 4″-trans-feruloylartocarmitin A. Artocarmitin C (7) was found to possess a molecular formula of C31H30O9 on the basis of its HRESIMS and NMR data. The 1 H and 13C NMR spectra of 4 (Table 2) also closely resembled those of artocamitin B (6), but they showed the presence of one more methoxy group (δH 3.89, δC 56.7) and a set of metacoupled aromatic protons instead of signals due to an ABX system in 6. The methoxy group was placed at C-5‴ by analysis of the COSY, HSQC, and HMBC spectra (Figure 2), indicating

114.4 165.1 115.1 162.9 108.1

124.0

115.8

Figure 2. Connectivities (bold lines) deduced by the COSY spectrum, significant HMBC correlations (arrows), and NOESY correlations (dashed arrows) observed for 5 and 7.

193.0 118.4

149.0 139.5 149.0

167.3 13.99 s

δC 127.5 131.7

116.2 167.6

14.07, s 3.89, s 3.89, s

56.7 56.7 1953

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Table 3. Tyrosinase Inhibitory Activity of the Isolated Compounds compound 1 2 3 4 5 6 7 a

IC50, μMa 18.7 8.4 40.0 47.3 >100 66.2 20.6

± ± ± ±

0.1 0.1 0.4 0.4

± 0.6 ± 0.2

IC50, μMa

compound

>100 55.3 73.6 82.2 0.013 17.3 21.4

8 9 10 11 12 13 14

± ± ± ± ± ±

0.5 0.7 0.8 0.002 0.1 0.2

compound 15 16 17 18 19 20 kojic acid

IC50, μMa 1.01 9.3 >100 >100 >100 2.3 44.6

± 0.05 ± 0.1

± 0.1 ± 0.4

Each value represents the mean ± SD of three determinations. 1.8 μg/mL), fr. 3 (0.3 g; IC50 11.5 μg/mL), fr. 4 (1.2 g; IC50 0.5 μg/ mL), fr. 5 (1.3 g; IC50 1.4 μg/mL), and fr. 6 (42.3 g; IC50 < 0.1 μg/ mL). Fraction 2 was subjected to silica gel column chromatography, eluted with EtOAc−hexane (0−30%), to afford two subfractions (2-1, 450 mg; 2-2, 1.5 mg). Subfraction 2-1 was recrystallized with acetone− hexane to give 15 (50.0 mg), while subfraction 2-2 was rechromatographed with 2% MeOH−CHCl3, followed by preparative TLC with 3% MeOH−CHCl3, to give 10 (6.8 mg), 11 (5.7 mg), 17 (5 mg), and 18 (6.7 mg). Fraction 3 was chromatographed further using MeOH−CHCl3 (0− 30%) to afford two subfractions (3-1, 110 mg; 3-2, 130 mg). Subfraction 3-1 was separated by preparative TLC with 2% MeOH− CHCl3 to give 1 (7.0 mg) and 4 (8.3 mg). Subfraction 3-2 was rechromatographed on silica gel with 2% MeOH−CHCl3, followed by final purification using preparative TLC with 40% acetone−hexane, to give 2 (5.0 mg), 3 (6.0 mg), and 8 (8.5 mg). Fraction 4 was subjected to silica gel column chromatography, eluted with MeOH−CHCl3 (0−50%), to afford three subfractions (41, 450 mg; 4-2, 150 mg; 4-3, 400 mg). Subfraction 4-1 was chromatographed further using a MeOH−CHCl3 gradient system, followed by reversed-phase preparative TLC with 70% acetone−H2O, to give 13 (9.0 mg), 14 (7.5 mg), and 16 (6.5 mg). Subfraction 4-2 was recrystallized using acetone−hexane to give 9 (12.0 mg). Fraction 5 was chromatographed further with a MeOH−CHCl3 gradient system, with additional separation by reversed-phase preparative TLC with 50% MeOH−H2O, to give 5 (5.0 mg), 6 (8.5 mg), 7 (5.5 mg), and 12 (7.8 mg). Fraction 6 was chromatographed further using a MeOH−CHCl3 gradient system, with final purification effected by preparative TLC with 10% MeOH−CHCl3, to give 19 (6.5 mg) and 20 (20.8 mg). Artocarmin A (1): pale yellow, amorphous solid; IR νmax (CHCl3) 3400, 1660, 1605, 1450 cm−1; 1H and 13C NMR (DMSO-d6, 500 MHz), see Table 1; HRESIMS m/z 377.0998 (calcd for C20H18O6Na [M + Na]+, 377.1001). Artocarmin B (2): pale yellow, amorphous solid; IR νmax (CHCl3) 3398, 1650, 1610, 1400 cm−1; 1H and 13C NMR (DMSO-d6, 500 MHz), see Table 1; HRESIMS m/z 523.1353 (calcd for C29H24O8Na [M + Na]+, 523.1369). Artocarmin C (3): pale yellow, amorphous solid; IR νmax (CHCl3) 3410, 1680, 1604, 1440 cm−1; 1H and 13C NMR (DMSO-d6, 500 MHz), see Table 1; HRESIMS m/z 553.1446 (calcd for C30H26O9Na [M + Na]+, 553.1475). Artocarmin D (4): pale yellow, amorphous solid; IR νmax (CHCl3) 3450, 1670, 1600, 1450 cm−1; 1H and 13C NMR (DMSO-d6, 500 MHz), see Table 1; HRESIMS m/z 371.1110 (calcd for C20H19O7 [M + H]+, 371.1131). Artocarmitin A (5): yellow, amorphous solid; IR νmax (CHCl3) 3420, 1680, 1605, 1460 cm−1; 1H and 13C NMR (acetone-d6, 500 MHz), see Table 2; HRESIMS m/z 363.1216 (calcd for C20H20O5Na [M + Na]+, 363.1208). Artocarmitin B (6): yellow, amorphous solid; IR νmax (CHCl3) 3400, 1670, 1600, 1450 cm−1; 1H and 13C NMR (acetone-d6, 500 MHz), see Table 2; HRESIMS m/z 539.1674 (calcd for C30H28O8Na [M + Na]+, 539.1682). Artocarmitin C (7): yellow, amorphous solid; IR νmax (CHCl3) 3425, 1650, 1605, 1460 cm−1; 1H and 13C NMR (acetone-d6, 500

the presence of a trans-sinapoyl moiety in 7. The location of this sinapoyl substituent was determined at C-5″ of the prenyl unit by the HMBC correlation of H-5″ with C-9‴ of the sinapoyl moiety. The 13C NMR spectrum of the prenyl chalcone moiety in 7 showed identical chemical shifts to those of the corresponding carbon signals for 5 and 6, except for the chemical shifts of C-4″ and C-5″, which were shifted to high field and low field, respectively, compared with 5 and 6. This indicated that the configuration of the 2″, 3″ double bond of 7 is Z, which was confirmed by the NOESY correlations of H-1″ with H-5″ and of H-2″ with H-4″ (Figure 2). Thus, the structure of artocarmitin C was concluded to be 7. These compounds were examined for their tyrosinase inhibitory activity (Table 3). The assay was carried out at four different concentrations ranging from 0.01−100 μM, and compounds 1−4, 6, 7, 9−16, and 20 displayed significant concentration-dependent inhibition. Compounds 1−3, 7, 12− 16, and 20 showed more potent inhibitory activities (IC50 values 0.013 to 40.0 μM) than a positive control, kojic acid (IC50, 44.6 μM), a well-known tyrosinase inhibitor currently used as a cosmetic skin-whitening agent. The most active compound, morachalcone A (12) (IC50, 0.013 μM), was 3000 times more active in tyrosinase inhibitory activity than kojic acid. Therefore, the traditional use of A. heterophyllus for antioxidant and skin-care agents in Vietnam might be attributable to the tyrosinase inhibitory activity of its flavonoid, chalcone, and phenolic constituents.

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EXPERIMENTAL SECTION

General Experimental Procedures. IR spectra were measured with a Shimadzu IR-408 spectrophotometer in CHCl3 solution. NMR spectra were taken on a Bruker Avance III 500 spectrometer (Bruker Biospin) with tetramethylsilane as an internal standard, and chemical shifts are expressed in δ values. HRESIMS measurements were carried out on a Bruker microTOF-QII spectrometer. Column chromatography was performed with BW-820MH Si gel (Fuji Silisia, Aichi, Japan). Analytical and preparative TLC were carried out on precoated Merck Kieselgel 60F254 or RP-18F254 plates (0.25 or 0.5 mm thickness). Plant Material. The wood of A. heterophyllus was collected at the Seven-Mountain area, An Giang Province, Vietnam, in August 2010, and identified by Ms. Hoang Viet, Faculty of Biology, University of Science, Vietnam National University−HoChiMinh City. A voucher sample of the wood part (AN-2985) has been deposited at the Department of Analytical Chemistry, Faculty of Chemistry, University of Science, Vietnam National University−HoChiMinh City. Extraction and Isolation. The dried powdered wood of A. heterophyllus (5.8 kg) was extracted with MeOH (15 L, reflux, 3 h × 3) to yield a MeOH extract (310 g; IC50 2.3 μg/mL). The extract was partitioned between EtOAc and water to give an EtOAc-soluble fraction (64.2 g; IC50 1.0 μg/mL). The EtOAc-soluble fraction was subjected to silica gel column chromatography with acetone−hexane to give six fractions: fr. 1 (0.3 g; IC50 > 100 μg/mL), fr. 2 (2.3 g; IC50 1954

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MHz), see Table 2; HRESIMS m/z 569.1750 (calcd for C31H30O9Na [M + Na]+, 569.1788). Tyrosinase Inhibitory Assay. All the samples were first dissolved in DMSO and used at concentrations of 100, 50, 25, and 10 μg/mL (or μM for pure compounds). The tyrosinase inhibitory activity assay was performed as previously described by Arung et al.14 The assay mixtures consisting of 1900 μL of test solution in 0.1 M phosphate buffer pH 6.8 and 100 μL of enzyme solution (15 U/mL in 0.1 M phosphate buffer pH 6.8) was prepared immediately before use. After preincubation at room temperature for 30 min, the reaction was initiated by the addition of 1000 μL of substrate solution (1.5 mM Ldihydroxyphenylalanine in 0.1 M phosphate buffer pH 6.8). The assay mixture was incubated at room temperature for 7 min, and the absorbance at 475 nm was measured with a Shimadzu UV-1800 spectrophotometer. Kojic acid, a known tyrosinase inhibitor, was used as positive control.

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ASSOCIATED CONTENT

S Supporting Information *

1

H, 13C, DEPT, COSY, HSQC, HMBC, and NOESY NMR and MS spectra of new compounds (1−7) are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (84)-907426331. Fax: (84)-838-350-096. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grant 104.01-2010.44 from Vietnam’s National Foundation for Science and Technology Development (NAFOSTED).

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REFERENCES

(1) Vo, V. C. An Giang Medicinal Plants; Science & Technology Publisher: An Giang, Vietnam, 1991. (2) Baliga, M. S.; Shivashankara, A. R.; Haniadka, R.; Dsouza, J.; Bhat, H. P. Food Res. Int. 2011, 44, 1800−1811. (3) Chung, M. I.; Lai, M. H.; Yen, M. H.; Wu, R. R.; Lin, C. N. Phytochemistry 1997, 44, 943−947. (4) Delle Monache, G.; De Rosa, M. C.; Scurria, R.; Vitali, A.; Cuteri, A.; Monacelli, B.; Pasqua, G.; Botta, B. Phytochemistry 1995, 39, 575− 580. (5) Arung, E. T.; Shimizu, K.; Kondo, R. Planta Med. 2006, 72, 847− 850. (6) Hano, Y.; Matsumoto, Y.; Shinohara, K.; Sun, J.; Nomura, T. Heterocycles 1990, 31, 1339−1344. (7) Ferrari, F.; Messana, I.; Mesquita de Araujo, M. do C. Planta Med. 1989, 55, 70−72. (8) Wang, M. F.; Kikuzaki, H.; Zhu, N. Q.; Sang, S. M.; Nakatani, N.; Ho, C. T. J. Agric. Food Chem. 2000, 48, 235−238. (9) Bergeron, R. J.; Wiegand, J.; McManis, J. S.; McCosar, B. H.; Weimar, W. R.; Brittenham, G. M.; Smith, R. E. J. Med. Chem. 1999, 42, 2432−2440. (10) Chatsumpun, M.; Sritularak, B.; Likhitwitayawuid, K. Chem. Nat. Compd. 2010, 46, 634−635. (11) Luo, Y.; Zhou, M.; Qi, H.; Li, B.; Zhang, G. Planta Med. 2005, 71, 1081−1084. (12) Goetz, G.; Fkyerat, A.; Métais, N.; Kunz, M.; Tabacchi, R.; Pezet, R.; Pont, V. Phytochemistry 1999, 52, 759−767. (13) Harborne, J. B. The Flavonoids: Advances in Research; Chapman and Hall: London, 1982; p 60. (14) Arung, E. T.; Kusuma, I. W.; Iskandar, Y. M.; Yasutake, S.; Shimizu, K.; Kondo, R. J. Jpn. Wood Res. Soc. 2005, 51, 520−525. 1955

dx.doi.org/10.1021/np300576w | J. Nat. Prod. 2012, 75, 1951−1955