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Rearranged Benzophenones and Prenylated Xanthones from Garcinia propinqua Twigs Cholpisut Tantapakul,† Wong Phakhodee,† Thunwadee Ritthiwigrom,‡ Sarot Cheenpracha,§ Uma Prawat,⊥ Suwanna Deachathai,† and Surat Laphookhieo*,† †
Natural Products Research Laboratory, School of Science, Mae Fah Luang University, Tasud, Muang, Chiang Rai 57100, Thailand Department of Chemistry, Faculty of Science, Chiang Mai University, Sutep, Muang, Chiang Mai 50200, Thailand § School of Science, University of Phayao, Maeka, Muang, Phayao 56000, Thailand ⊥ Laboratory of Natural Products Chemistry, Faculty of Science and Technology, Phuket Rajabhat University, Rassada, Muang, Phuket 83000, Thailand ‡
S Supporting Information *
ABSTRACT: The first phytochemical investigation of Garcinia propinqua has led to the isolation and identification of three new compounds, including two rearranged benzophenones, doitunggarcinones A (1) and B (2), and a xanthone, doitunggarcinone C (3), together with seven known compounds (4−10). The structures of 1−3 were elucidated on the basis of spectroscopic methods, including UV, IR, NMR, and MS. The antibacterial activity of the 10 isolates was evaluated against Escherichia coli TISTR 780, Salmonella typhimurium TISTR 292, Staphylococcus aureus TISTR 1466, and methicillin-resistant Staphylococcus aureus (MRSA) SK1.
showed 33 carbons attributable to six methyls [δ 29.7 (C-20), 26.1 (C-19), 25.8 (C-14), 22.5 (C-25), 18.4 (C-26), and 17.8 (C-15)], seven methylenes, of which one is a terminal olefinic unit [δ 110.0 (C-24), 47.4 (C-10), 36.4 (C-22), 32.7 (C-21), 32.5 (C-4), 29.0 (C-16), and 25.1 (C-11)], seven methines, of which four are aromatic and one olefinic [δ 133.6 (C-31), 126.9 (C-32), 126.4 (C-33), 123.5 (C-30), 118.7 (C-12), 56.8 (C17), and 56.4 (C-5)], and 13 quaternary carbons, of which three are keto groups [δ 213.2 (C-2), 203.1 (C-9), 200.2 (C27), 150.3 (C-29), 145.6 (C-23), 136.4 (C-28), 134.3 (C-13), 91.7 (C-7), 70.2 (C-3), 69.2 (C-8), 63.1 (C-1), 41.5 (C-6), and 37.2 (C-18)], as determined by a DEPT experiment. The 1H NMR spectrum showed the presence of a 1,2-disubstituted benzene ring [δ 7.69 (1H, d, J = 7.6 Hz, H-33), 7.54 (1H, dt, J = 7.6, 0.8 Hz, H-31), 7.34−7.38 (1H, m, H-32), and 7.34−7.38 (1H, m, H-30)], a 3-methylbut-2-enyl (isoprenyl) unit [δ 5.04 (1H, t, J = 7.6, 7.2 Hz, H-12), 2.26 (2H, m, H-11), 1.65 (3H, s, H-14), and 1.55 (3H, s, H-15)], and a 3-methylbut-3-enyl (terminal isoprenyl) unit [δ 4.70 (1H, brs, H-24a), 4.66 (1H, brs, H-24b), 2.20 (1H, m, H-21a), 2.06 (1H, m, H-21b), 2.05 (1H, m, H-22a), 1.81 (1H, m, H-22b), and 1.71 (3H, s, H-25)]. The remaining signals were two methines [δ 2.66 (1H, m, H17) and 1.81 (1H, m, H-5)], three methylenes [δ 2.20 (1H, m, H-16a), 2.06 (1H, m, H-16b), 2.05 (1H, m, H-4a), 1.81 (1H, m, H-10a), and 1.60 (2H, m, H-4b and H-10b)], and three methyls [δ 1.45 (3H, s, H-26), 1.35 (3H, s, H-19), and 1.08
Garcinia belongs to the plant family Clusiaceae, which is distributed throughout tropical and subtropical countries. The genus Garcinia produces many types of secondary metabolites including xanthones,1−4 flavones,3 terpenoids,4 and benzophenones.5−7 Some of these compounds possess interesting biological and pharmacological activities, including antimicrobial,7 antidepressant,7 anti-HIV,7,8 antioxidant,7 and cytotoxic7−10 effects. In a continuing search for bioactive metabolites from Thai medicinal plants, we now report on the first phytochemical investigation of G. propinqua twigs collected from Doi Tung, Chiang Rai Province, resulting in two new rearranged benzophenones, doitunggarcinones A (1) and B (2), and a new xanthone, doitunggarcinone C (3), along with seven known compounds, xerophenone A (4),11 dulxanthone B (5),12 5-O-methylxanthone V1 (6),6 10-O-methylmacluraxanthone (7),13 macluraxanthone (8),14 gartanin (9),15 and morusignin J (10).16 Compounds 1−10 have been evaluated for antibacterial activity. The acetone extract of G. propinqua twigs was subjected to silica gel column chromatography to yield 10 compounds, of which three are new chemical structures (1−3). Doitunggarcinone A (1), [α]25D −133.3 (c 0.015, CHCl3), was obtained as a white solid (mp 80−82 °C). The molecular formula, C33H40O4, was deduced from the ESITOFMS, which showed a pseudomolecular ion peak at m/z 501.2997 [M + H]+ (calcd 501.2999). The IR spectrum indicated the presence of carbonyl (1739, 1709, 1676 cm−1) and hydroxy (3465 cm−1) functionalities, while the UV spectrum showed absorption maxima at λmax 252 and 288 nm. The 13C NMR spectrum © XXXX American Chemical Society and American Society of Pharmacognosy
Received: July 13, 2012
A
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Chart 1
(3H, s, H-20)]. From the above analysis of the 1H and 13C NMR spectroscopic data, it was implied that compound 1 is a tetracyclo[4.3.2.11,6.03,7]decane derivative similar to that of garcibracteatone, as isolated from G. bracteata.6 However, the main difference found was that compound 1 displays 1H NMR and 13C NMR data for a 3-methylbut-3-enyl unit at C-5 instead of the 3-methylbut-2-enyl group as in garcibracteatone. The relative configuration of 1 is similar to that of garcibracteatone due to the same sign of specific rotation ([α]25D −133.3 (c 0.015, CHCl3) for doitunggarcinone A and [α]25D −1 (c 1.0, CHCl3) for garcibracteatone). The cross-peaks between H-11 and H-25, H-17 and H-19, and H-5 and H-10 in the NOESY spectrum also supported the above assignment. The structure of 1 (doitunggarcinone A) was therefore established as shown. Detailed assignments of the protons and carbons as well as HMBC correlations are shown in Table 1. Doitunggarcinone B (2), [α]27D −129 (c 0.054, CHCl3), was obtained as a colorless gum. The pseudomolecular ion peak at m/z 525.2964 (calcd 525.2975) [M + Na]+ in the ESITOFMS corresponded to the molecular formula of C33H42O4. The IR spectrum indicated the presence of carbonyl (1723 cm−1) and hydroxy (3588 cm−1) functionalities, and the UV spectrum showed absorption maxima at λmax 245 and 338 nm. The 13C NMR spectrum showed 33 carbons, of which six signals corresponded to methyls [δ 26.0 (C-14 and C-20), 22.5 (C25), 19.3 (C-26), 17.9 (C-15), and 17.8 (C-19)], eight to methines, of which five are aromatic and two olefinic [δ 130.4 (C-31), 128.0 (C-30 and C-32), 127.9 (C-29 and C-33), 119.8 (C-17), 119.3 (C-12), and 47.8 (C-5)], and seven to methylenes, of which one is olefinic [δ 109.8 (C-24), 46.5 (C-10), 40.2 (C-4), 36.3 (C-22), 32.8 (C-21), 29.5 (C-16), and 24.9 (C-11)]. In addition, 12 quaternary carbons were assigned, of which three are keto/enol groups [δ 210.8 (C-2), 198.9 (C9), 174.8 (C-27), 145.7 (C-23), 134.8 (C-28), 133.9 (C-18), 133.5 (C-13), 109.5 (C-8), 83.4 (C-7), 63.3 (C-1), 62.6 (C-3), and 47.4 (C-6)]. The 1H NMR spectrum of 2 exhibited signals for a monosubstituted aromatic ring [δ 7.47 (3H, m, H-29, H-
31, and H-33) and 7.44 (2H, m, H-30 and H-32)], two sets of 3-methylbut-2-enyl units [δ 5.17 (1H, m, H-12), 2.45 (1H, m, H-11a), 2.16 (1H, m, H-11b), 1.64 (3H, s, H-14) and 1.51 (3H, s, H-15), and 4.92 (1H, m, H-17), 2.45 (1H, m, H-16a), 2.16 (1H, m, H-16b), 1.71 (3H, s, H-20), and 1.64 (3H, s, H-19)], a 3-methylbut-3-enyl unit [δ 4.66 (1H, brs, H-24a), 4.62 (1H, brs, H-24b), 2.16 (1H, m, H-21a), 1.97 (1H, m, H-21b), 1.97 (1H, m, H-22a), 1.79 (1H, m, H-22b), and 1.67 (3H, s, H-25)], a chelated hydroxy proton (δ 15.32, s, OH-27), and two isolated methylene groups [δ 1.97 (1H, m, H-4a), 1.68 (1H, m, H-10a), 1.55 (1H, m, H-10b), and 1.50 (1H, m, H-4b)]. These signals are similar to that of nemorosonol,17 a tricyclo[4.3.1.03,7]decane derivative, first isolated from Clusia nemorosa (Clusiaceae). However, the 3-methylbut-2-enyl group at C-5 of nemorosonol was replaced by a 3-methylbut-3-enyl unit. The relative configurations at C-1, C-3, C-5, and C-6 were determined by a NOESY experiment. The cross-peaks between H-11, H-16, and H-21 implied that three isoprenyl units at C-1, C-3, and C-5 are in the same orientation, but different from that of Me-26. The structure 2 therefore was assigned to doitunggarcinone B. Doitunggarcinone B may be a precursor of doitunggarcinone A via an intramolecular cycloaddition, then tautomerization and oxidative rearomatization (Figure 1).6 Doitunggarcinone C (3) was obtained as a yellow solid (mp 92−93 °C). The pseudomolecular ion peak at [M + H]+ m/z 425.1962 (calcd 425.1959) in the ESITOFMS corresponded to a molecular formula of C25H28O6. Compound 3 showed UV absorption bands at λmax 242, 313, and 364 nm, indicating a typical xanthone chromophore.14,18,19 In the IR spectrum, hydroxy and carbonyl functionalities were observed at 3343 and 1606 cm−1, respectively. The 1H NMR spectrum displayed signals for a chelated hydroxy proton (δ 13.08, 1H, s, OH-1), two ortho-coupled aromatic protons [δ 7.95 (1H, d, J = 8.8 Hz, H-8) and 7.00 (1H, d, J = 8.8 Hz, H-7)], and two methoxy groups [δ 4.11 (3H, s) and 3.81 (3H, s)]. The remaining signals indicated two 3-methylbut-2-enyl (isoprenyl) units. One unit had signals at δ 5.25 (1H, m, H-2′), 3.42 (2H, d, J = 6.8 B
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Table 1. NMR Spectroscopic Data (400 MHz, CDCl3) for Doitunggarcinones A (1) and B (2) doitunggarcinone A (1) position
δC, type
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
63.1, 213.2, 70.2, 32.5, 56.4, 41.5, 91.7, 69.2, 203.1, 47.4, 25.1, 118.7, 134.3, 25.8, 17.8, 29.0, 56.8, 37.2, 26.1, 29.7, 32.7, 36.4, 145.6, 110.0,
C C C CH2 CH C C C C CH2 CH2 CH C CH3 CH3 CH2 CH C CH3 CH3 CH2 CH2 C CH2
25 26 27 28 29 30 31 32 33 OH-7 OH-27
22.5, 18.4, 200.2, 136.4, 150.3, 123.5, 133.6, 126.9, 126.4,
CH3 CH3 C C C CH CH CH CH
δH (J in Hz)
doitunggarcinone B (2) δC, type
HMBC
2.05, 1.60, m 1.81, m
2, 3, 5, 6, 7, 21 21, 26
1.81, 1.60, m 2.26, m 5.04, t (7.6, 7.2)
1, 2, 6, 7, 9, 11, 26 1, 2, 9, 10, 12, 13 11, 14, 15
1.65, 1.55, 2.20, 2.66,
s s 2.06, m m
12, 13 12, 13, 14 2, 3, 7, 8, 18 7, 8, 9, 16, 18
1.35, 1.08, 2.20, 2.05,
s s 2.06, m 1.81, m
17, 18, 20, 29 17, 18, 19, 29 6, 22, 23 5, 21, 23, 24
4.70, 4.66, 1.71, 1.45,
brs brs s s
22, 25
7.34−7.38, m 7.54, dt (7.6, 0.8) 7.34−7.38, m 7.69, d (7.6) 2.82, brs
22, 23, 24 6, 7, 10
18, 19, 31 33 29, 33 27, 28, 32 3, 7
63.3, 210.8, 62.6, 40.2, 47.8, 47.4, 83.4, 109.5, 198.9, 46.5, 24.9, 119.3, 133.5, 26.0, 17.9, 29.5, 119.8, 133.9, 17.8, 26.0, 32.8, 36.3, 145.7, 109.8,
C C C CH2 CH C C C C CH2 CH2 CH C CH3 CH3 CH2 CH C CH3 CH3 CH2 CH2 C CH2
22.5, 19.3, 174.8, 134.8, 127.9, 128.0, 130.4, 128.0, 127.9,
CH3 CH3 C C CH CH CH CH CH
δH (J in Hz)
HMBC
1.97, 1.50, m 1.63, m
2, 3, 6, 7, 21 4, 21
1.68, 1.55, m 2.45, 2.16, m 5.17, m
1, 2, 6, 9 1, 2, 9, 10, 12, 13 1, 11, 14, 15
1.64, 1.51, 2.45, 4.92,
s s 2.16 m m
12, 13, 15 12, 13, 14 2, 3, 4, 7, 17 3, 19, 20
1.64, 1.71, 2.16, 1.97,
s s 1.97, m 1.79, m
17, 18, 20 17, 18, 19 4, 5 5, 21, 23, 24
4.66, 4.62, 1.67, 1.15,
brs brs s s
22, 25
7.47, m 7.44, m 7.47, m 7.44, m 7.47, m 5.08, m 15.32, s
22, 23, 24 6, 7, 10
27, 30, 31 28, 32 30, 32 28, 32 27, 30, 31 3, 7, 10 8, 9, 27, 28
Figure 1. Plausible biosynthetic pathway of doitunggarcinone A (1).
placed at C-3 since these protons, as well as methylene protons H-1′ (δ 3.42) and H-1″ (δ 3.58), showed HMBC correlations with C-3 (δ 163.4), while the signal at δ 3.81 was located on C5 due to HMBC correlations of H-7 (δ 7.00) and OMe (δ 3.81) with C-5 (δ 133.7). The structure of doitunggarcinone C was assigned as 3. Detailed assignments of the protons and carbons as well as HMBC correlations are shown in Table 2. As summarized in Table 3, all compounds were evaluated for their antibacterial activity against Gram-positive (MRSA SK1
Hz, H-1′), 1.81 (3H, s, H-5′), and 1.70 (3H, s, H-4′). This isoprenylated unit was located at C-2 of the xanthone skeleton due to the methylene protons H-1′ (δ 3.42), which showed 2J and 3J HMBC correlations with C-2 (δ 117.6) and C-1 (δ 159.1), respectively (Table 2). The other unit showed 1H NMR signals at δ 5.25 (1H, m, H-2″), 3.58 (2H, d, J = 6.4 Hz, H-1″), 1.84 (3H, s, H-5″), and 1.68 (3H, s, H-4″) and was placed at C4, because of the 2J and 3J HMBC correlations of H-1″ with C-4 (δ 113.0) and C-4a (δ 152.7). The methoxy group at δ 4.11 was C
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Elmer UV−vis spectrometer. The IR spectra were recorded using a Perkin-Elmer FTS FT-IR spectrometer. The NMR spectra were recorded using a 400 MHz Bruker or a 500 MHz Varian Unity INOVA spectrometer. Chemical shifts are recorded in parts per million (δ) in CDCl3 or acetone-d6 with TMS as an internal standard. The ESITOFMS data were measured on a Bruker Daltonics MicroTOF mass spectrometer. Quick column chromatography (QCC) and column chromatography (CC) were carried out on silica gel 60 H (Merck, 5−40 μm) and silica gel 100 (Merck, 63−200 μm), respectively. Precoated plates of silica gel 60 F254 were used for analytical purposes. Plant Material. The twigs of G. propinqua were collected from Doi Tung, Chiang Rai Province, Thailand, in September 2011. The plant was identified by Mr. Martin van de Bult (Doi Tung Development Project, Chiang Rai, Thailand), and the specimen (MFU-NPR0031) was deposited at Natural Products Research Laboratory, School of Science, Mae Fah Luang University. Extraction and Isolation. Chopped and dried twigs of G. propinqua (1.90 kg) were extracted with acetone over a period of 3 days at room temperature. Removal of the solvent provided the crude acetone extract (183.72 g), which was then subjected to QCC over silica gel, eluting with a gradient of ethyl acetate (EtOAc)−hexanes (100% hexanes to 100% EtOAc), providing 12 fractions (A−L). Fraction A (8.42 g) was further separated by QCC (5% EtOAc− hexanes) and yielded six subfractions (AA−AF). Subfraction AA (4.94 g) was separated by CC with 30% CH2Cl2−hexanes to provide five subfractions (AA1−AA5). Compound 2 (35.1 mg) was purified from subfraction AA5 (415.8 mg) by CC with 5% EtOAc−hexanes. Compound 1 (8.3 mg) was obtained from subfraction AD (355.9 mg) by repeated CC (40% CH2Cl2−hexanes). Fraction B (268.0 mg) was subjected to separation over Sephadex LH-20 with MeOH to afford compound 4 (21.8 mg). Fraction D (7.12 g) was subjected to QCC (100% hexanes to 100% CH2Cl2) to yield six subfractions (DA−DF). Subfraction DE (2.31 g) was further purified by repeated CC (40% CH2Cl2−hexanes), affording compound 10 (3.9 mg). Fraction G (9.27 g) was subjected to repeated QCC (100% hexanes to 100% EtOAc) to yield 11 subfractions (GA−GK). Compounds 3 (7.5 mg), 6 (14.0 mg), and 9 (3.7 mg) were derived from subfraction GF (473.1 mg) using CC (60% CH2Cl2−hexanes). Subfraction GG (548.4 mg) was further purified by CC (70% CH2Cl2−hexanes) to give compound 7 (4.0 mg). Fractions K and L (6.90 g) were combined and subsequently subjected to passage over Sephadex LH-20 using MeOH as solvent to afford three subfractions (KA−KC). Subfraction KB (500 mg) was subjected to purification by CC (20% acetone−hexanes) to obtain four fractions (KB1−KB4). Compounds 5 (5.2 mg) and 8 (1.1 mg) were derived from subfraction KB2 (47.1 mg) by CC (100% CH2Cl2). Doitunggarcinone A (1): white solid (CHCl3); mp 80−82 °C; [α]25D −133.3 (c 0.015, CHCl3); UV (MeOH) λmax (log ε) 252 (4.00), 288 (3.42) nm; IR (neat) νmax 3465, 2926, 1739, 1709, 1676, 1454 cm−1; 1H and 13C NMR (CDCl3, 400 MHz), see Table 1; ESITOFMS m/z 501.2997 [M + H]+ (calcd for C33H41O4, 501.2999). Doitunggarcinone B (2): colorless gum; [α]27D −129.0 (c 0.054, CHCl3); UV (MeOH) λmax (log ε) 245 (4.27), 338 (4.10) nm; IR (neat) νmax 3588, 2928, 1723, 1614, 1587, 1446, 1375 cm−1; 1H and 13 C NMR (CDCl3, 400 MHz), see Table 1; ESITOFMS m/z 525.2964 [M + Na]+ (calcd for C33H42NaO4, 525.2975). Doitunggarcinone C (3): yellow solid (CHCl3); mp 92−93 °C; UV (MeOH) λmax (log ε) 242 (4.44), 313 (4.10), 364 (4.00) nm; IR (neat) νmax 3343, 2925, 1640, 1606, 1583, 1434 cm−1; 1H and 13C NMR (CDCl3, 400 MHz), see Table 2; ESITOFMS m/z 425.1962 [M + H]+ (calcd for C25H29O6, 425.1959). Antibacterial Assay. Escherichia coli TISTR 780, Salmonella typhimurium TISTR 292, and Staphylococcus aureus TISTR 1466 were obtained from the Microbiological Resources Center of the Thailand Institute of Scientific and Technological Research, and MRSA SK1 was obtained from the Department of Microbiology, Faculty of Science, Prince of Songkla University, Thailand. The MICs were determined by a 2-fold serial dilution method using Mueller-Hinton broth, according to the Clinical and Laboratory Standards Institute recommendations.25
Table 2. NMR Spectroscopic Data (400 MHz, CDCl3) for Doitunggarcinone C (3) position OH-1 2 3 4 4a 4b 5 6 7 8 8a 9 9a 1′ 2′ 3′ 4′ 5′ 1″ 2″ 3″ 4″ 5″ OMe-3 OMe-5
δC, type 159.1, 117.6, 163.4, 113.0, 152.7, 154.4, 133.7, 149.7, 112.3, 122.0, 114.8, 181.0, 105.4, 22.5, 122.5, 131.9, 25.7, 17.9, 22.7, 123.0, 132.1, 25.6, 18.0, 61.6, 61.8,
δH (J in Hz)
C C C C C C C C CH CH C C C CH2 CH C CH3 CH3 CH2 CH C CH3 CH3 CH3 CH3
HMBC
13.08, s
1, 2, 9a
7.00, d (8.8) 7.95, d (8.8)
5, 6, 8a 4b, 6, 9
3.42, d (6.8) 5.25, m
1, 2, 3, 2′, 3′ 4′, 5′
1.70, 1.81, 3.58, 5.25,
s s d (6.4) m
2′, 3′, 5′ 2′, 3′, 4′ 3, 4, 4a, 2″, 3″ 4″, 5″
1.68, 1.84, 4.11, 3.81,
s s s s
2″, 3″, 4″ 2″, 3″, 5″ 3 5
Table 3. Biological Activity of Isolated Compounds from G. propinqua Twigs antibacterial activity (MIC, μg/mL) compound 1 2 3 4 5 6 7 8 9 10 vancomycin gentamycin a
MRSA SK1 a
inactive 64 128 128 16 inactivea 4 4 inactivea not tested 1
S. aureus
E. coli
S. typhimurium
128 64 128 inactive 32 inactivea 2 4 128 128 0.25
128 128 128 128 128 128 128 128 128 128
128 128 not tested 128 128 128 128 not tested 128 not tested
0.25
0.125
Inactive at >128 μg/mL.
and Staphylococcus aureus TISTR 1466) and Gram-negative (Escherichia coli TISTR 780, Salmonella typhimurium TISTR 292) bacteria. Only two compounds, 7 and 8, showed good activity against MRSA SK1 and S. aureus TISTR 1466, with minimum inhibitory concentration (MIC) values in the range 2−4 μg/mL, while all remaining compounds showed weak antibacterial activity (MIC 64−128 μg/mL) against both Gram-positive and Gram-negative bacteria.
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EXPERIMENTAL SECTION
General Experimental Procedures. Melting points were determined on a Buchi B-540 visual thermometer. The optical rotation [α]D values were determined with a Bellingham and Stanley ADP400 polarimeter. UV−vis spectra were recorded with a PerkinD
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The test substances were dissolved in DMSO. Vancomycin and gentamycin were used as standard drugs.
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ASSOCIATED CONTENT
* Supporting Information S
1
H and 13C NMR, DEPT, and 2D spectra for compounds 1−3 are provided free of charge via the Internet at http://pubs.acs. org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +66-5391-6238. Fax: +66-5391-6776. E-mail: s.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Thailand Research Fund (TRF Advanced Research Scholar). Laboratory facilities and partial financial support from Mae Fah Luang University are also acknowledged. We are indebted to Mr. N. Chimnoi, Chulabhorn Research Institute, Bangkok, for recording mass spectrometric data. We would like to extend our appreciation to Prof. S. Pyne (University of Wollongong) for editing the manuscript.
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