A new xanthone from the lichen Parmotrema praesorediosum (Parmeliaceae)

Cite this paper: Vietnam J. Chem., 2020, 58(6), 765-769 Article DOI: 10.1002/vjch.202000070 765 Wiley Online Library © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH A new xanthone from the lichen Parmotrema praesorediosum (Parmeliaceae) Huynh Bui Linh Chi 1* , Nguyen Kim Phi Phung 2 1 Dong Nai University, 3 Le Quy Don, Tan Hiep, Bien Hoa City, Dong Nai Province 76000, Viet Nam 2 University of Science, National University HCM City, 227 Ly Thuong Kiet, District 5, Ho Chi Minh City, 70000 Viet Nam Submitted May 1, 2020; Accepted May 28, 2020 Abstract Two xanthone compounds, praesorexanthone A (1) and praesorexanthone B (2), were isolated from the lichen Parmotrema praesorediosum (Nyl.) Hale (Parmeliaceae). Among them, praesorexanthone B is a new natural compound. Their chemical structures were unambiguously determined by the analysis of 1D and 2D NMR and high resolution ESI mass spectroscopic data, as well as by comparison of their NMR data with the ones in the literature. Keywords. Parmotrema praesorediosum, lichen, lichen substances, tetrahydroxanthone-chromanone dimer, hexahydroxanthone-chromanone dimer. 1. INTRODUCTION The xanthone family has been studied for over a century since the early 1900s and has been investigated and reviewed, particularly for the monomeric species and plant derived species. [1] Several reviews on xanthones have been published in recent years, covering xanthones from fungi, lichens and bacteria, structure-activity relationships, antimalarial properties of xanthones, biological activity, chemical synthesis of xanthone cores, and their biosynthesis. [2] The field of xanthone chemistry is growing rapidly in recent years with publications on novel naturally-occurring xanthones. For example, in 2019 D.T. Huy and co-workers reported the isolation of a new trichlorinated xanthone, isodemethylchodatin from the foliose lichen Parmotrema tsavoense (Krog and Swinscow) Krog and Swinscow (Parmeliaceae). [3] This paper reports the isolation and structure elucidation of a xanthone-chromanone dimer from the lichen Parmotrema praesorediosum (Nyl.) Hale, including praesorexanthone A and B via spectroscopic data analysis and comparison with those reported in the literature (figure 1). Among them, praesorexanthone B is a new natural compound. Figure 1: Structures of two isolated xanthones 2. MATERIALS AND METHOD 2.1. General The high resolution electrospray ionization mass spectroscopy (HR-ESI-MS) was performed on an exactive mass spectrometer (Thermo Fisher Scientific). The 1H-NMR (500 MHz), 13C-NMR (125 MHz), HSQC, HMBC spectra were recorded by mean of INOVA-500 spectrometer. The optical Vietnam Journal of Chemistry Huynh Bui Linh Chi et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 766 rotations were measured on a Jasco DIP-370 digital polarimeter. The IR spectra were measured on Shimadzu FTIR-8200 infrared spectrophotometer. Column chromatography was carried out using silica gel normal-phase (230-400 mesh). Analytical TLC was carried out on silica gel plates (Kieselgel 60 F254, Merck). 2.2. Plant material The lichen thalli of Parmotrema praesorediosum were collected on the bark of Dipterocarpus sp. at Tan Phu forest, Dong Nai province, Vietnam in June 2009 and identified by Dr. Vo Thi Phi Giao, Faculty of Biology, University of Science, National University - Ho Chi Minh City. A voucher specimen (No US-B020) was deposited in the Herbarium of The Department of Organic Chemistry, Faculty of Chemistry, University of Science, of National University - Ho Chi Minh City, Vietnam. 2.3. Extraction and isolation The powder of the lichen Parmotrema praesorediosum (3.0 kg) was exhaustively extracted with methanol at room temperature. After filtrated, the methanol solution was evaporated at the reduced pressure to provide the crude methanol extract (450.0 g), which was subjected to silica gel solid phase extraction and eluted consecutively with the solvents with various polar to afford extract PE (40.5 g), extract C (105.6 g), extract EA (50.2 g), extract A (45.8 g) and extract M (37.9 g). The extract C (105.6 g) was applied to silica gel column chromatography and eluted with the solvent systems of petroleum ether - ethyl acetate (9:1-0:10) then methanol to give twenty three fractions, coded C1 to C23. Fraction C20 (23.9 g) was repeatedly subjected to silica gel column chromatography, eluted with chloroform-methanol (stepwise, 10:0-9:1) and subjected to pre TLC with different kinds of solvents (chloroform-methanol, 98:2 and toluene-acetone, 8:2) to afford two compounds 1 (61.9 mg) and 2 (21.4 mg). Praesorexanthone A (1): Light yellow gum. [α]D 20 +1.07 (c = 2.152, CHCl3). IR (KBr) max cm -1 : 3502 (OH), 1792 (C=O lactone), 1741 (C=O ester), 1648 (C=C), 1270 (CO). HR-ESI-MS m/z 639.1710 [M+H] + (Calcd. for C32H30O14 + H, 639.1715). 1 H and 13 C NMR (CDCl3) see table 1. Praesorexanthone B (2): Light yellow gum. [α]D 20 -2.17 (c = 1.861, CHCl3). IR(KBr) max cm -1 : 3410 (OH), 1790 (C=O lactone), 1744 (C=O ester), 1624 (C=C), 1215 (CO). HR-ESI-MS m/z 655.1660 [M+H] + (Calcd. for C32H30O15 + H, 655.1664). 1 H and 13 C NMR (CDCl3) data see table 1. 3. RESULTS AND DISCUSSION Compound 1, obtained as a light yellow gum, was identical with versixanthone D, [4] a tetrahydroxanthone-chromanone dimer isolated from a culture of the mangrove-derived fungus Aspergillus versicolor, as judged by comparison of its mass, NMR data (table 1), especially of the coupling constants and NOESY experiments. However, its optical rotation,   20 D +1.07 (c 2.152, CHCl3), was opposite in sign from that of versixanthone D, therefore compound 1 was the enantiomer of the latter and was named as praesorexanthone A. Compound 2 was isolated as a light yellow gum. Its molecular formula was determined as C32H30O15 from the HR-ESI-MS spectrum. Its IR absorptions implied the presence of hydroxyl (3410 cm -1 ), lactone group (1790 cm -1 ), ester carbonyl group (1744 cm -1 ) and aromatic (1624, 1587 cm -1 ) functionalities. Analysis of 1 H, 13 C, DEPT and HSQC spectra revealed that compound 2 possessed three conjugated carbonyl carbons (C 198.8, 194.2 and 191.9), three carboxyl carbons (C 175.1, 169.0 and 167.9), twelve aromatic carbons (C 160.6, 159.1, 158.5, 157.6, 141.4, 141.2, 118.4, 117.5, 107.5, 107.4, 107.3 and 106.6), three oxygenated quaternary carbons (C 89.5, 84.4 and 71.9), two oxygenated methines (C 82.8 and 74.0), two methoxy groups (C 53.7 and 53.5), three aliphatic methylenes (C 43.5, 39.7 and 36.8), two aliphatic methines (C 33.5 and 32.1) and two methyl groups (C 18.5 and 14.9). The comparison of the 13 C-NMR data of 2 and 1 (table 1) in the same deuterated solvent CHCl3, showed the similarity in their AD rings and there were two changes in the EF rings with the replacement of the oxygenated aromatic carbon C-8 (C 177.7) and the quaternary aromatic carbon C-8a (C 101.6) in 1 by a carbonyl carbon (C 198.8) and an oxygenated aliphatic quaternary carbon (C 89.5), respectively, in 2. The presence of these functional groups strongly affected the chemical shift values of adjacent carbons, e.g. C-7 (C = -7.2), C-9 (C = -4.7) and C-10a (C = +12.9). The differences in the EF rings were also observed in the comparison of the 1 H-NMR data with the disappearance of the Vietnam Journal of Chemistry A new xanthone from the lichen © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 767 Table 1: 1 H- and 13 C-NMR (CDCl3) data for compounds 1 and 2 No. 1 2 δH J (Hz) δC δH J (Hz) δC 1 159.4 160.6 2 117.8 118.4 3 7.52 d (8.5) 141.3 7.48 d (8.5) 141.2 4 6.63 d (8.5) 107.7 6.67 d (8.5) 107.4 4a 158.4 157.6 5 3.92 dd (11.0, 2.0) 77.0 4.51 br d (10.5) 74.0 6 2.42 m 29.3 2.06 qdd (17.0, 10.5, 5.5) 32.1 7 2.32 dd (19.0, 10.5) 36.3 2.49 dd (10.5, 4.5) 43.5 2.74 dd (19.0, 6.0) 2.97 m 8 177.7 198.8 8a 101.6 89.5 9 187.2 191.9 9a 106.9 106.6 10a 84.8 71.9 11 1.17 d (6.5) 18.0 1.22 d (6.5) 18.5 12 170.3 167.9 13 3.73 s 53.3 3.67 s 53.5 1-OH 11.75 s 11.68 s 5-OH 2.92 br s 2.80 br s 8-OH 13.77 s 8a-OH 4.94 br s 1' 159.2 159.1 2' 118.1 117.5 3' 7.45 d (8.5) 140.2 7.54 d (8.5) 141.4 4' 6.62 d (8.5) 107.3 6.58 d (8.5) 107.3 4'a 158.3 158.5 5' 4.82 d (6.5) 82.7 4.82 d (7.0) 82.8 6' 2.98 m 33.5 2.97 m 33.5 7' 2.71 dd (17.0, 8.0) 36.7 2.72 dd (17.0, 8.0) 36.8 2.48 dd (17.0, 7.5) 2.48 dd (17.0, 7.5) 8' 174.9 175.1 8'a 9' 3.20 3.28 d (17.5) d (17.5) 39.8 194.1 3.19 3.28 d (17.5) d (17.5) 39.7 194.2 9'a 107.5 107.5 10'a 11' 12' 1.34 d (7.5) 84.4 14.9 169.1 1.32 d (7.5) 84.4 14.9 169.0 13' 3.77 s 53.7 3.75 s 53.7 1'-OH 11.93 s 11.88 s chelated hydroxyl proton of C8-OH (H 13.77) in 1 and the appearance of an aliphatic hydroxyl proton (H 4.94) in 2. These findings suggested that 2 should be an oxidative derivative of 1. The sole double bond in the F ring in 1 was firstly peroxidized and then a pair of electron of the hydroxyl group on C-8 displaced toward this three-membered ring to open it and the Vietnam Journal of Chemistry Huynh Bui Linh Chi et al. © 2020 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 768 consequence was the formation a new hydroxyl group (8aOH) and a new carbonyl group (C-8) in 2. This was suitable with the molecular formula of 2 (C32H30O15) possessing one oxygen atom more than the one of 1 (C32H30O14). Up to this point, the planar structure of 2 was constructed of a hexahydroxanthone moiety and a lactone- chromanone one as 1. The connection between these two moieties at 22' was proved by the ROESY correlations of the hydroxyl group at C-1 with the aromatic proton H-3' as well as of the hydroxyl group at C-1' with the aromatic proton H-3. In the lactone-chromanone moiety, the connection between the chromanone unit and the lactone moiety was suggested by the HMBC correlation of H-5' (H 4.82) with C-12' (C 169.0). The substitution pattern of 2 was confirmed by COSY, HMBC and ROESY correlations (figure 2). Figure 2: Key COSY, HMBC and ROESY correlations of 2 In the -butyrolactone moiety C ring of 2, the cis configuration of H-5' and H-6' as well as the syn arrangement of the hydrogen H-5' and the methoxycarbonyl group at C-10'a were supported by the compatible chemical shift values and the coupling constants ( 3 JH-5',H-6' = 7.0 Hz) of 2 compared to those of 1 and versixanthone D. [4] In the F ring of 2, the pseudodiaxial conformation between the carbinolic hydrogen H-5 (H 4.51) and the methine hydrogen H-6 (H 2.06) was confirmed by the large coupling constant ( 3 JH-5,H-6 = 10.5 Hz). The ROESY correlations of H- 5/H3-11 and of H-6/H3-13 supported the stereochemistry of substituents in the F-ring as shown in figure 2. Except the carbon 8a, the relative configurations of asymmetric carbons in 2 were proposed to be the same as those in 1 on the basis of NMR data, especially of the NOESY experiment and coupling constants as well as by consideration of the biogenetic origin. The absolute configurations of 1 and 2 remain questionable. Compound 2 was a new natural xanthone derivative and was named as praesorexanthone B. Xanthones are a class of secondary metabolites of lichens [5] and to the best of our knowledge, after lichexanthone, a xanthone isolated from Parmotrema lichexanthonicum, [6] compounds 1 and 2 are two other examples of xanthone derivatives of this genus. 4. CONCLUSION From Parmotrema praesorediosum (Nyl.) Hale, Parmeliaceae growing in Vietnam, two xanthone compounds were isolated and elucidated. This is the fisrt time these compounds are known in Parmotrema genus, among them, compound 2 was a new natural xanthone. Acknowledgements. We are grateful to the Government of Vietnam (Project 322, MOET) for the fellowship to B.L.C.H. We are grateful to Dr. Vo Thi Phi Giao for identification of the Parmotrema specimens. Thanks are also due to Kobe Pharmaceutical University for helping us in experiment. REFERENCES 1. T. Wezeman, S. Brase, K. Masters. Xanthone dimers: a compound family which is both common and privileged, Nat. Prod. Rep., 2015, 32, 6-28. 2. K. Masters, S. Brase. Xanthones from fungi, lichens, and bacteria: The natural products and their synthesis, Chem. Rev., 2012, 112, 3717-3776. 3. T. H. Duong, M. A. Beniddir, J. Boustie, K. P. P. Nguyen, W. Chavasiri, G. Bernadat, P. L. Pogam. DP4-assisted structure elucidation of isodemethylchodatin, a new norlichexanthone derivative meager in H-atom, from the lichen Parmotrema tsavoense, Molecules, 2019, 24, 1527. 4. G. Wu, G. 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Corresponding author: Huynh Bui Linh Chi Dong Nai University 3 Le Quy Don, Tan Hiep district, Bien Hoa City, Dong Nai 76000, Viet Nam E-mail: hainhanchi@yahoo.com.vn.