A novel caged-prenylxanthone from Garcinia bracteata

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Chinese Chemical Letters 21 (2010) 443–445 www.elsevier.com/locate/cclet

A novel caged-prenylxanthone from Garcinia bracteata Zhi Na *, Hua Bin Hu, Qing Fei Fan Xishuangbanna Tropical Botanical Garden, The Chinese Academy of Sciences, Mengla 666303, China Received 17 August 2009

Abstract A novel caged-prenylxanthone xanthone, neobractatin (1), was isolated from the twig of Garcinia bracteata. Its structure was elucidated by spectroscopic methods, especially 2D NMR techniques. # 2009 Zhi Na. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Garcinia bracteata; Caged-prenylxanthone; Neobractatin

Garcinia bracteata C.Y. Wu ex Y.H. Li, a plant belonging to the family of Guttiferae, is distributed in the south of Yunnan and Guangxi Province of China [1]. Previous phytochemical investigations on G. bracteata resulted in the isolation of caged-prenylxanthones and benzophenones [2,3]. Caged-prenylxanthones were only isolated from several Garcinia species, such as G. bracteata, G. cantleyana [4], G. gaudichaudii [5], G. hanburyi [6], G. morella [7] and G. scortechinii [8]. As a part of our search for bioactive natural products from tropical plants, a careful examination of the twig of this species led to the isolation of a novel caged-prenylxanthone, neobractatin (1), and its structure was elucidated on the basis of a comprehensive analysis of the 1H NMR, 13C NMR and 2D NMR spectra. Herein, we report the isolation and structure elucidation of neobractatin. The twigs of G. bracteata were collected from Xishuangbanna Tropical Botanical Garden (XTBG), in Yunnan Province, China, in August 2008, and authenticated by Prof. Guo Da Tao in XTBG. A voucher specimen (No. 20080801) was deposited in ethnobotany research group of XTBG. The air-dried twig of G. bracteata (6.5 kg) was powered and then extracted with 95% EtOH (20 L  3) at room temperature. The combined solution was concentrated to dryness under vacuum. The crude extract was suspended in water and successively partitioned with petroleum ether and ethyl acetate. The combined extract of petroleum ether was evaporated to give a deep-brown gum (154 g), which was subjected to silica gel column eluted with a gradient of petroleum ether–ethyl acetate (95:5–40:60) to afford six fractions (Fr.1–Fr.6). Fr.3 (8.6 g) [petroleum ether–ethyl acetate (80:20)] was further purified by using silica gel column chromatography repeatedly to provide compound 1 (35 mg). Compound 1 was obtained as yellow amorphous powder, ½a27 D 9.4 (c 0.24, MeOH), UV lmax (log e) (MeOH): 208 (4.59), 346 (4.19) nm. The HR-ESI-MS spectrum showed a [M+Na]+ peak at m/z 487.2085 (calcd. 487.2096), corresponding to a molecular formula of C28H32O6. The IR spectrum showed a broad band at 3425 cm1 due to a

* Corresponding author. E-mail address: [email protected] (Z. Na). 1001-8417/$ – see front matter # 2009 Zhi Na. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.12.030

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Z. Na et al. / Chinese Chemical Letters 21 (2010) 443–445

Fig. 1. Structure of 1 and key HMBC, ROESY correlations for 1.

Table 1 NMR data for neobractatin (1) and 1-O-methylneobractatin (2) d in ppm, J in Hz. Position

1 C (1) CH (2) C (3) C (4) C (4a) C (5) C (6) CH (7) CH (8) C (8a) C (9) C (9a) C (10) C (11) CH (12) CH2 (13) Me (14) Me (15) CH2 (16) CH (17) C (18) Me (19) Me (20) CH2 (21) CH (22) C (23) Me (24) Me (25) OH–C (1) a b c

dC

dH a

b,c

2

5.97 (s, 1H)

6.07 (s, 1H)

3.77 (dd, 1H, J = 5.0, 7.0 Hz) 7.15 (d, 1H, J = 7.0 Hz)

3.69 (dd, 1H, J = 5.0, 7.0 Hz) 6.99 (d, 1H, J = 7.0 Hz)

6.30 4.81 4.69 1.53 1.51 2.30 1.84 2.20

(dd, 1H, J = 10.0, 18.0 Hz) (d, 1H, J = 18.0 Hz) (d, 1H, J = 10.0 Hz) (s, 3H) (s, 3H) (d, 1H, J = 14.0 Hz) (dd, 1H, J = 10.0, 14.0 Hz) (dd, 1H, J = 5.0, 10.0 Hz)

6.49 (dd, 1H, J = 10.0, 18.0 Hz) 5.47 (d, 1H, J = 18.0 Hz) 5.34 (d, 1H, J = 10.0 Hz) 1.74 (s, 3H) 1.61 (s, 3H) 2.46 (d, 1H, J = 14.0 Hz) 1.88 (dd, 1H, J = 10.0, 14.0 Hz) 2.18 (dd, 1H, J = 4.0, 10.0 Hz)

1.21 1.25 2.27 1.84 4.90

(s, 3H) (s, 3H) (dd, 1H, J = 8.0, 14.0 Hz) (dd, 1H, J = 8.0, 14.0 Hz) (t, 1H, J = 8.0 Hz)

1.32 (s, 3H) 1.36 (s, 3H) 2.50 (dd, 1H, J = 8.0, 15.0 Hz) 1.93 (dd, 1H, J = 8.0, 15.0 Hz) 5.11 (t, 1H, J = 8.0 Hz)

1.63 (s, 3H) 1.53 (s, 3H) 12.67 (s, 1H)

Recorded in DMSO-d6. Data of Ref. [2]. Recorded in CDCl3.

1.74 (s, 3H) 1.58 (s, 3H)

1a

2b,c

162.1 97.1 167.0 112.6 159.1 199.9 78.9 44.5 135.5 133.2 177.9 100.5 83.6 40.1 151.0 106.2

160.5 95.8 163.1 112.7 161.8 200.6 78.9 44.1 131.2 138.3 175.0 106.0 83.7 41.4 150.3 111.9

28.3 27.4 32.0

28.4 27.3 32.2

41.4 83.6 26.5 29.2 30.1

42.5 83.6 26.8 29.5 29.7

117.6 135.4 17.9 25.7

117.3 136.0 18.1 25.9

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hydroxyl group, an unconjugated carbonyl group at 1751 cm1, and a chelated ortho-hydroxyl carbonyl group at 1637 cm1. The 1H NMR spectrum revealed the presence of a chelated hydroxy group at d 12.67 (s, 1H, 1-OH), an olefinic proton of an a,b-unsaturated carbonyl unit at d 7.15 (d, 1H, J = 7.0 Hz, H-8), two coupled protons at d 6.30 (dd, 1H, J = 10.0, 18.0 Hz, H-12) and d 4.81 (d, 1H, J = 18.0 Hz, Ha-13), 4.69 (d, 1H, J = 10.0 Hz, Hb-13) which were assigned to two olefinic protons of a 3-methylbut-1-enyl, and a 3-methylbut-2-enyl olefinic proton at d 4.90 (t, 1H, J = 8.0 Hz, H-22). The 13C NMR spectrum revealed 28 carbon signals, which were sorted by DEPT experiment as six methyls, three methylenes, six methines, and thirteen quaternary carbons including two carbonyl carbons at d 199.9, and 177.9. In the 1H NMR spectrum, characteristic signals at d 3.77 (dd, 1H, J = 5.0 and 7.0 Hz, H-7), 2.20 (dd, 1H, J = 5.0 and 10.0 Hz, H-17), and 1.84 (dd, 1H, J = 10.0 and 14.0 Hz, Hb-16) were shown. Together with the presence of three oxygenated quaternary carbon signals at d 83.6 (C-18), 83.6 (C-10), and 78.9 (C-6) and the evidence of correlations between H-16 with C-5, C-7, C-8a, C-10, and C-17 and correlations between H17 with C-6, C-7, C-16 and C-18 in the HMBC spectrum (Fig. 1), compound 1 was presumed to be a caged prenylated xanthone [2]. The NMR data of compound 1 resembled with those of 1-O-methylneobractatin [2], isolated from the same species, except for a hydroxyl group instead of a methoxyl group substituted at C-1 (Table 1). The attachment of the hydroxyl group at C-1 was assigned by the correlations between dH 12.67 with C-1 (dC 162.1) and C-2 (dC 97.1) and C-9a (dC 100.5) in the HMBC spectrum. Moreover, the C-1 signal of compound 1 appeared at more down field (d 162.1) than that of 1-O-methylneobractatin (d 160.5), due to the hydroxyl at C-1 chelated with carbonyl at C-9. ROESY correlations (Fig. 1) existed between protons H-7, H-17, 3H-19, H-8, 2H-21, and H-22, meaning that these protons have the same orientation. Therefore, compound 1 was elucidated as the structure in Fig. 1, and named as neobractatin. Acknowledgment We gratefully acknowledge financial support of this work by the National Natural Science Foundation of China (No. 20702061). References [1] [2] [3] [4] [5] [6] [7] [8]

Zhongguozhiwuzhi, vol. 50 (2), Science Press, Beijing, 1990, p. 99. O. Thoison, J. Fahy, V. Dumontet, et al. J. Nat. Prod. 63 (2000) 441. O. Thoison, D.D. Cuong, A. Gramain, et al. Tetrahedron 61 (2005) 8529. K.A. Shadid, K. Shaari, F. Abas, et al. Phytochemistry 68 (2007) 2537. J. Wu, Y.J. Xu, X.F. Cheng, et al. Tetrahedron Lett. 42 (2001) 727. Y. Sukpondma, V. Rukachaisirikul, S. Phongpaichit, Chem. Pharm. Bull. 53 (2005) 850. G.S.R.S. Rao, S. Rathnamala, V. Surendranath, Tetrahedron Lett. 14 (1974) 1259. V. Rukachaisirikul, P. Painuphong, Y. Sukpondma, et al. J. Nat. Prod. 66 (2003) 933.