Anthraquinones from hedyotis pinifolia - Le Hoang Duy

581 Journal of Chemistry, Vol. 47 (5), P. 581 - 585, 2009 ANTHRAQUINONES FROM HEDYOTIS PINIFOLIA Received 19 Oct 2007 LE HOANG DUY1, NGUYEN KIM PHI PHUNG2 1Pham Van Dong University, Quang Ngai Province 2University of Natural Sciences, National University Ho Chi Minh City ABSTRACT Hedyotis pinifolia Wall ex G.Don (Vietnamsese name An điền l¸ th«ng), family of Rubiaceae, has not yet been chemically studied. From the aerial parts of H. pinifolia, four anthraquinones had been isolated: 1,6-dihydroxy-7-methoxy-2-methylanthraquinone (1), 1,6-dihydroxy-2- methylanthraquinone (2), 3,6-dihydroxy-2-methylanthraquinon (3) and 1,3,6-trihydroxy-2- methylanthraquinone (4). Their chemical structures were established by spectroscopic analysis. O O CH3H3C-O HO OH 1 2 3 45 6 7 8 9 10 11 12 13 14A B C (1) O O CH3 HO OH 1 2 3 45 6 7 8 9 10 11 12 13 14A B C (2) O O CH3 HO 1 2 3 45 6 7 8 9 10 11 12 13 14A B C (3) O O CH3 HO OH 1 2 3 45 6 7 8 9 10 11 12 13 14A B C (4) OH OH Keywords: Rubiaceae, Hedyotis pinifolia, anthraquinone. I - INTRODUCTION Several species of Hedyotis genus (Rubiaceae) are used in traditional medicine in a number of Asian countries including Vietnam [1, 2]. There are approximately 180 species recorded of which 56 were identified in Vietnam [2, 3]. Hedyotis pinifolia Wall ex G. Don is a small herb (up to 25 cm tall) commonly found in sandy areas from Hue to the South of Vietnam2. Previous studies showed that the genus Hedyotis contained anthraquinones1-6, but there have been no anthraquinone compounds isolated from Hedyotis pinifolia. From the aerial parts of Hedyotis pinifolia, we isolated four anthraquinones (1-4). The structural elucidation of these compounds was reported. II - EXPERIMENTAL 1. General procedures Melting points were determined on a Maquene hot stage apparatus and were uncorrected. MS, 1H-, 13C-, DEPT- and 2D- NMR, H-H COSY were recorded in The Institute of Chemistry, Vietnamese Academy of Science and Technology, Cau Giay Dist., Hanoi. For analytical and preparative TLC, Merck TLC aluminium sheets silica gel 60 GF254 were utilized. For column chromatography (CC), silica gel Merck 60 GF254 and silica gel Merck 60 (0.040 - 0.063 mm) were used. 2. Plant material The aerial parts of H. pinifolia were collected at the seaside of Long Hai commune, 582 Ba Ria – Vung Tau province, Vietnam in December 2005. A voucher specimen was prepared and deposited by Msc. Vo Thi Phi Giao, Faculty of Biology, University of Natural Sciences, Vietnam National University of Ho Chi Minh City. 3. Extraction and isolation 2.6 kg of ground, air-dried aerial parts of H. pinifolia were macerated in ethanol for 24 h and the extraction was repeated many times. After removal of the solvent under reduced pressure, 80.0 g of crude ethanol extract was obtained. This extract was then subjected to fast dry column chromatography to obtain different fractions: petroleum ether extracts A (6.97 g) and B (1.53 g), chloroform extract (18.09 g), ethyl acetate extracts A (5.73 g) and B (12.68 g) and methanol extracts A (29.46 g) and B (1.39 g), respectively. The chloroform extract was rechromatographed on CC, purified by TLC and recrystallized in acetone to yield compounds 1 and 2. The same work for ethyl acetate extract A and a mixture of 3 and 4 with the ratio of (1:1) was isolated. 4. 1,6-Dihydroxy-7-methoxy-2-methylanthr- quinone (1) 2 mg, orange needles (recrystallized in acetone). Melting point: 189-1900C. ESI-MS m/z: 284.9 [M+H]+ (calc. for C16H12O5). 1H- NMR (500 MHz, DMSO), δppm: 12.92 (1H, s, 1-OH), 7.65 (1H, d, J=7.5 Hz, H-3), 7.59 (1H, d, J=7.5 Hz, H-4), 7.58 (1H, s, H-5), 7.55 (1H, s, H-8), 3.98 (3H, s, 7-OCH3), 2.29 (3H, s, 2- CH3). 13C-NMR (125 MHz, DMSO) see table 1. HMBC see figure 1. 5. 1,6-Dihydroxy-2-methylanthraquinone (2) 2.5 mg, orange needles (recrystallized in acetone). Melting point: 182-1840C. ESI-MS m/z: 255.0 [M+H]+ (calc. for C15H10O4). 1H- NMR (500 MHz, acetone-d6), δppm: 13.19 (1H, s, 1-OH), 7.64 (1H, d, J = 7.5 Hz, H-3), 7.68 (1H, d, J = 7.5 Hz, H-4), 7.63 (1H, d, J = 2.5 Hz, H-5), 7.33 (1H, dd, J = 8.5, 2.5 Hz, H-7), 8.22 (1H, d, J = 8.5 Hz, H-8), 2.06 (3H, s, 2- CH3). 13C-NMR (125 MHz, acetone-d6) see table 1. HMBC see figure 1. 6. Anthraquinones 3 and 4 12 mg, orange needles (recrystallized in acetone); ESI-MS m/z 254.9 [M+H]+, 253.0 [M- H]- (calc. for C15H10O4 for 3); m/z 270.9 [M+H]+, 269.0 [M-H]- (calc. for C15H10O5 for 4). 1H-NMR (500 MHz, acetone-d6) and 13C-NMR (125 MHz, acetone-d6) see Table 1. HMBC see figure 2. H-H COSY see figure 3. O O H H H H CH3 OH H HO O O H H H O CH3 OH H HO H O O H H H O CH3 H H3CO HO H O O H H H O CH3 H H HO H 10 11 12 5 6 7 8 (1) (2) 1 2 3 4 14 13 9 10 11 12 5 6 7 8 1 2 3 4 14 13 9 10 11 12 5 6 7 8 (3) (4) 1 2 3 4 14 13 9 10 11 12 5 6 7 8 1 2 3 4 14 13 9 Figure 1: HMBC correlations of compounds 1 - 4 583 Figure 2: Part of two-dimensional HMBC spectrum of the mixture 3 and 4 O O H H H H CH3 OH H HO O O H H H OH CH3 OH H HO 1 2 3 4 14 13 9 10 11 12 5 6 7 8 1' 2' 3' 4'14' 13' 9' 10' 11' 12' 5' 6' 7' 8' (3) (4) Figure 3: H-H long-range COSY correlations of 3 and 4 III - RESULTS AND DISCUSSION From the chloroform extract, compounds 1 and 2 were isolated. Compound 1 exhibited a melting point of 189 - 190oC. The MS spectrum showed a pseudomolecular ion peak at m/z 284.9 [M+H]+, indicating the molecular formula to be C16H12O5. The 1H-NMR spectrum showed a characteristic downfield signal for the chelated hydroxyl group at δ 12.92 (1H, s). The 1H-NMR 584 spectrum also revealed the presence of four aromatic protons, a methoxyl group at δ 3.98 (3H, s) and a methyl group at δ 2.29 (3H, s). The 13C-NMR spectrum showed 16 carbon atoms, which includes two carbonyl downfield signals at δ 188.1 and δ 180.7, indicative of the presence of the chelated and nonchelated carbonyl. The exact location of the substituents was established based on HSQC and HMBC. There are four signals in the aromatic region and the pattern suggested an anthraquinone with two disubstituted rings. A set of ortho coupled doublets at δ 7.65 (1H, J = 7.5 Hz) and 7.59 (1H, J = 7.5 Hz) indicated that the two aromatic protons were next to each other. These two aromatic protons combining with one hydroxyl and one methyl group were located on ring C. In ring A, there were two proton singlets at δ 7.58 and 7.55 and also one methoxyl and one hydroxyl group. The data caused us the hesitation in the assignment of the structure of the product: whether it was 1,6-dihydroxy-7- methoxy-2-methylanthraquinone or 1,7- dihydroxy-6-methoxy-2-methylanthraquinone. Later, when we isolated other anthraquinones in this herb and observed that the quinones of this herb always possessed substituents of 2-methyl and 6-hydroxyl, so we proposed 1 was 1,6- dihydroxy-7-methoxy-2-methylanthraquinone. Table 1: 1H (500 MHz) and 13C (125 MHz) chemical shifts of 1-4 1a 2b 3b 4b Position δC δH (J, Hz) δCc δH (J, Hz) δCc 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1-OH 2-CH3 7-OCH3 159.8 133.6 136.9 118.5 109.0 153.0 153.4 112.1 188.1 180.7 126.8 127.5 114.8 131.2 - 15.6 56.1 161.7 135.4 137.4 119.5 113.6 164.5 122.1 130.7 189.2 182.7 127.8 126.5 115.9 132.6 - 16.0 - 7.99 (s) - - 7.60 (s) 7.57 d (2.5) - 7.27 (dd, 8.5 , 2.5) 8.12 (d, 8.5) - - - - - - - 2.36 (s) - 130.8 (d) 132.9 (s) 161.4 (s) 112.2 (d) 113.1 (d) 163.3 (s) 121.9 (d) 130.4 (d) 181.5 (s) 183.5 (s) 127.2 (s)d 126.7 (s) 134.7 (s)d 127.3 (s)d - 16.5 (q) - - - - 7.30 (s) 7.58 (d, 8.0) - 7.29 (dd, 8.0 , 2.5) 8.16 (d, 8.0) - - - - - - 13.37 (s) 2.16 (s) - 163.9 (s) 118.9 (s) 163.3 (s) 107.9 (d) 113.5 (d) 163.9 (s) 122.0 (d) 130.4 (d) 187.3 (s) 182.8 (s) 127.2 (s)d 136.7 (s)d 110.3 (s) 130.4 (s)d - 8.3 (q) - aSpectrum run in DMSO; bSpectrum run in acetone-d6; cMultiplicities were determined by DEPT experiment; dDetermined based on ChemNMR C-13 estimation Compound 2 exhibited a melting point of 182 - 184oC. The MS spectrum showed a pseudomolecular ion peak at m/z 255.0 [M+H]+, 30 amu less than that of 1, corresponding to molecular formula of C15H10O4. The 1H-NMR spectra showed signals of one peri-hydroxyl at δ 13.19 and one methyl group at δ 2.06. A disubstituted ring C was indicated by the two coupled aromatic protons at δ 7.64 (d, J = 7.5 Hz, H-3) and δ 7.59 (d, J = 7.5 Hz, H-4). And a monosubstituted ring A was indicated by the three coupled protons at δ 7.63 (d, J = 2.5 Hz, H-5), δ 7.33 (dd, J = 8.5 and 2.5 Hz, H-7) and δ 8.22 (d, J = 8.5 Hz, H-8). The 13C-NMR 585 spectrum showed 15 carbon atoms, like 1, which included two carbonyl groups at δ 189.2 and 182.7, presented of the chelated and nonchelated carbonyl, respectively. The HSQC and HMBC correlation NMR spectra (Figure 1) strongly supported the structure of 2 was 1,6- dihydroxy-2-methylanthraquinone. The ethyl acetate extract was subjected to a series of chromatographic procedures, leading to the isolation of 12 mg of fine orange needles. Initial inspection of the 1H and 13C-NMR spectra of this product indicated the doubling signals for most protons and carbons. These facts suggested that perhaps the product was a dimeric anthraquinone of more than 29 unique carbon nuclei. LC-MS spectrum of the product revealed two equally intensitive peaks at the time of 11.6 and 12.9 minutes. The mass spectrum positive mode showed that these peaks corresponded to the molecular ion of m/z 254.9 [M+H]+ and 270.9 [M+H]+, in agreement with the molecular formula of C15H10O4 and C15H10O5, respectively. This result suggested that the product was a mixture of two structurally related compounds with the ratio of (1:1). Extensive efforts to separate these two compounds by silica gel CC, preparative TLC and to check by C-18 TLC with variety of solvent systems proved unsuccessful. Therefore, the structure elucidation and full 1H and 13C-NMR assignments of compounds 3 and 4 (Table 1) were performed in the inseparative mixture. Compounds 3 and 4 were determined exactly based on the 2D-NMR spectra: HSQC, HMBC and H-H long-range COSY correlations. Compound 3 had molecular formula of C15H10O4 with the presence of five aromatic protons, one methyl and two hydroxyl groups. In ring C, two proton singlets presented at δ 7.99 (H-1) and 7.60 (H-4). And a monosubstituted ring A was indicated by the three coupled protons at δ 7.57 (d, J = 2.5 Hz, H-5), 7.27 (dd, J = 8.5 and 2.5 Hz, H-7) and 8.12 (d, J = 8.5 Hz, H-8). There were 15 carbon atoms in 13C-NMR spectrum, including two carbonyl groups at δ 183.5 and 181.5, and one methyl group at δ 16.5. The HSQC, HMBC and H-H long-range COSY confirmed the location of methyl group at C-2 and two hydroxyl groups at C-3 and C-6 based on two- and three-bond correlations (Figure 1 and 2). These evident spectra supported the structure of 3 to be 3,6- dihydroxy-2-methylanthraquinone. Compound 4 had molecular formula of C15H10O5, 16 amu more than the one of 3, with the presence of four aromatic protons, one methyl group and two hydroxyl groups. In ring C, difference from ring C of 3, only one proton singlet presented at δ 7.30 (H-4) and one peri- hydroxyl group at δ 13.37 (C1-OH). Compound 4 also had a monosubstituted ring A similar to 3, with three coupled protons at δ 7.58 (d, J = 2.5 Hz, H-5), 7.29 (dd, J = 8.0; 2.5 Hz, H-7) and 8.16 (d, J = 8.0 Hz, H-8). The 13C-NMR spectrum showed 15 carbon atoms which include two downfield carbonyl signals at δ 187.3 and 182.8, indicative of the presence of the chelated and nonchelated carbonyl groups, respectively. The HSQC, HMBC and H-H long- range COSY (Figure 2 and 3) confirmed that the structure of 4 was 1, 3, 6-trihydroxy-2- methylanthraquinone. REFERENCES 1. Rohaya Ahmad, Khozirah Shaari, Nordin Hj. 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