Xanthones from Vismia latifolia

Short Report

Xanthones from Vismia latifolia

Marcelo H. dos Santosa^*, Tanus J. Nagem^b, Marilda C. da Silva^a and Luiz G. F. e Silva^a

^aDepartamento de Química, ICEx, Universidade Federal de Minas Gerais,

Av. Antônio Carlos, 6627, Pampulha, 31270-901, Belo Horizonte, MG, Brazil.

^bDepartamento de Química, Universidade Federal de Ouro Preto, 35400-000,

Campus Morro do Cruzeiro, Ouro Preto, MG, Brazil.

Este trabalho relata o isolamento e a identificação de uma nova xantona, 1,4,8-triidroxixantona (1,4,8-triidroxi-9H-xanten-9-ona), isolada das raízes de Vismia latifolia (Guttiferae). Quatro outras xantonas conhecidas também foram isoladas: 1,5-diidroxi-8-metoxixantona, 1,7-diidroxixantona, 1,6-diidroxi-7-metoxixantona e 1,3,5,6-tetraidroxixantona, sendo as duas últimas inéditas no gênero Vismia. As estruturas foram estabelecidas através das técnicas de espectroscópicas de UV, IV, EM e RMN (1D e 2D).

A new xanthone,1,4,8-trihydroxyxanthone (1,4,8-trihydroxy-9H-xanthen-9-one), was isolated from the roots of Vismia latifolia (Guttiferae). Four other known xanthones were isolated: 1,5-dihydroxy-8-methoxyxanthone, 1,7-dihydroxyxanthone, 1,6-dihydroxy-7-methoxyxanthone and 1,3,5,6-tetrahydroxyxanthone. The last two compounds were isolated for the first time from a Vismia species. The structures were established by UV, IR, MS, 1D and 2D NMR spectroscopic techniques.

Keywords: Vismia latifolia, Guttiferae, 1,4,8-trihydroxyxanthone, xanthones

Introduction

Vismia latifolia Choisy (Syn. Hypericum latifolium Aubl.), a tree known popularly in Bahia (Brazil) as "pau-de-sangue", is used as a tonic and febrifugal agent¹. This species belongs to the Guttiferae family, subfamily Hypericoideae and tribe Vismieae. Previous papers have reported the presence of anthranoids, terpenoids, flavonoids and xanthones from Vismia species^2-6. As part of a chemotaxonomic study of the Guttiferae (Vismia genus), in the present paper we have identified the new compound 1,4,8-triidroxixantona (1) and four other known xanthones 2, 3, 4 and 5 from V. latifolia. The last two compounds (4 and 5) were isolated for the first time in this genus. The occurrence of xanthones with a simple oxygenation pattern 1,4,8- (or 1,5,8-) in Vismia genus was described only in V. guaramirense² and V. parviflora³.

Results and Discussion

Purification of an ethanol extract of the roots of Vismia latifolia by silica gel CC resulted in the isolation of a new trioxygenated xanthone 1,4,8-triidroxixantona (1). The molecular formula was deduced to be C₁₃H₈O₅ from its [M]⁺ at m/z 244 in the mass spectrum and from the NMR spectra. A conjugated carbonyl group was identified by an absorption band at 1640 cm^-1 in the IR spectrum⁷. The UV spectral data of 1 (experimental section) was characteristic of the xanthone chromophore⁸ and the sodium acetate addition caused a batochromic shift; the consecutive addition of H₃BO₃ did not modify the UV spectrum, indicating the absence of the ortho hydroxyls. The ¹HNMR spectrum of 1 indicated the presence of signals corresponding to three aromatic hydrogens [d_H, 6.82 (dd, J 8.4 and 0.8 Hz, H-7), 7.05 (dd, J 8.4 and 0.8 Hz, H-5) and 7.76 (t, J 8.4 Hz, H-6)] in an ABC-type system due to 1,2,3-trisubstituted benzene ring, three hydroxyl groups [d_C 8.61 (s, HO-4), 11.05 (s, HO-1) and 11.83 (s, HO-8)] and two ortho-coupled hydrogens [d_H 6.70 (H-2) and 7.37 (H-3) (d, J 8.8 Hz)]. The ¹³C NMR spectrum of 1 showed 13 carbon signals: eight non hydrogenated carbons, including one carbonyl group (d_C 187.53) and five methine aromatic carbons. These data (experimental section) suggested a trisubstituted xanthone (1). The unambiguous attribution was established by means of two-dimensional NMR spectroscopy techniques. The chemical shift assignments of methine carbons C-2, C-3, C-5, C-6 and C-7 in the ¹³C NMR were achieved by a HMQC experiment [¹J(CH)]. The other chemical shifts were assigned by long-range [²J(CH) and ³J(CH)] correlation observed in the HMBC spectrum (Figure 1). Two possible structures were compatible with the ¹H NMR data (experimental section): 1,2,8-trihydroxyxanthone or 1,4,8-trihydroxyxanthone. The exclusion of the 1,2,8-pattern was based on the chemical shift of H-2 at d_H 6.70 (1), when compared with the corresponding H-3 (d_H 7.34) in the partial structure of 1,2,5-trihydroxyxanthone⁹. In addition, the C-H three bond correlation between the chelated hydrogen at d_H 11.05 (HO-1) and a carbon at d_C 110.51 (C-2), to which H-2 is attached (d_H 6.70, d, J 8.8 Hz, ortho coupling), excludes definitively the 1,2,8-pattern. The chemical shift of the hydrogen H-3 at d_H 7.37 (d, J 8.8 Hz, ortho-coupling constant) showed C-H long-range correlation (HMBC) with the carbons at d_C 154.21 [C-1, ³J(CH)], 138.25 [C-4, ²J(CH)] and 144.81 [C-4a, ³J(CH)], revealing that the two hydroxyl groups were in positions 1 and 4. Furthermore, another hydroxyl hydrogen at d_H 11.83 (HO-8) was correlated with carbon signals at d_C 108.60 [C-8a, ³J(CH)] and 111.47 [C-7, ³J(CH)], the latter, bonded to H-7 (d_H 6.82, dd, J 8.4 and 0.8 Hz) that showed ortho-coupling with H-6 (d_H 7.76) and meta-coupling with H-5 (d_H 7.05). These data are in good agreement with those observed in the literature for an A-ring in the partial structure of euxanthone (2)^3,10-11.These results established as 1 the structure of the new trioxygenated xanthone¹ (Figure 1).

Other xanthones were identified as 1,7-dihydroxy-xanthone (euxanthone) (2)^3,10, 1,5-dihydroxy-8-methoxy-xanthone (3)^2,3, 1,3,5,6-tetrahydroxyxanthone (4)¹² and 1,6-dihydroxy-7-methoxyxanthone (5)¹³ by comparison with authentic samples (2 and 3), melting points and spectroscopic data in the literature.

Experimental

General procedures

Melting points were obtained on a Mettler FP 80 HT.IR spectra were determined using a Shimadzu/IR - 408 spectrometer. ¹H and ¹³C, NOESY, HMQC and HMBC spectra were recorded using a Bruker DRX-400 spectrometer.

Collection

Roots and stems of Vismia latifolia were collected in Bahia, Brazil in January 1996. A voucher specimen (register number 4580) is deposited at the Herbarium of the CEPLAC-CEPEQ (Centro de Pesquisas do cacau - Ilhéus - Bahia)

Extraction and fractionation of roots

The dried and ground roots (824 g) were extracted (at room temp.) with n-hexane (7.64 g) and EtOH (68.0 g ) in succession. The ethanol extract was washed with ethyl ether and filtered. The soluble portion was evaporated under vacuum yielding 11.8 g of a residue that was chromatographed on silica gel (Merck) (220 g) CC and eluted with n-hexane-EtOAc, EtOAc-EtOH and EtOH. The twenty seven fractions obtained yielded six groups (A₁-A₆). A₅ (frs 12-17, 1.6 g) was rechromatographed on silica gel (30 g) CC using CH₂Cl₂-EtOAc, EtOAc-EtOH and EtOH as eluents. The twenty-four fractions obtained yielded five groups (A_5a-A_5e). From A_5b (frs 4-6) compound 1 (3.2 mg) was obtained as a yellow solid by recrystallization from CHCl₃ solution. A_5c (fr 7, 738 mg) was rechromatographed in silica gel (22 g) CC using n-hexane-CH₂Cl₂, CH₂Cl₂-EtOAc, EtOAc-EtOH and EtOH as eluents yielding forty-four fractions; from fr (21) compound 2 (3.0 mg) was isolated as a yellow solid by successive silica gel CC. A_5d (frs 12-16) was washed with MeOH and the insoluble green solid obtained was rechromatographed successively on silica (Merck) flash-CC using n-hexane-acetone (3:2) as eluent yielding compound 3 (8.6 mg) as a yellow solid. A₆ (frs 18-25, 562 mg) was rechromatographed on silica gel (12 g) CC using n-hexane-EtOAc, EtOAc-EtOH and EtOH as eluent. The twenty-five fractions obtained yielded three groups (A_6a-A_6c). From A_6b (frs 13-17) compound 4 (2.0 mg) was obtained as a white amorphous solid by cellulose CC using EtOAc, EtOAc-EtOH and EtOH as eluent.

Extraction and fractionation of stems

Dried and ground stems (4.4 kg) were extracted (at room temp.) successively with n-hexane (17.35 g) and EtOH (50.0 g). The ethanol extract was chromatographed on silica gel (500 g) (Merck) CC and eluted with n-hexane-CHCl₃, CHCl₃-EtOAc, EtOAc-EtOH and EtOH. The one hundred fifty fractions obtained yielded twenty groups (B₁-B₂₀). Group B₇ (frs 26-31, 1.2 g) was rechromato-graphed on silica gel (24 g) CC using n-hexane-CH₂Cl₂, CH₂Cl₂-EtOAc, EtOAc-EtOH and EtOH as eluents; compound 5 (6.0 mg) was obtained as a pale yellow amorphous solid from frs (19-24) by washing with acetone.

1,4,8-Trihydroxyxanthone 1

Yellow needles, mp 247-249 °C (CHCl₃). UV l_max/nm (MeOH) (log e): 205 (3.7), 235 (3.8), 255 (4.0), 270 (3.8), 340 (3.5); + NaOAc: 210 (4.5), 250 (3.9), 275 (3.6), 340 (3.5). IR n_Max. (KBr)/cm^-1 3425, 3040, 1640, 1600, 1510, 1475. Positive EIMS (70 eV) m/z (rel. int.): 245 [M+1]⁺ (18.2), 244 [M]⁺ (100), 243 (9.4), 216 (2.5), 215 (2.4), 108 (5). ¹H NMR (400 MHz, acetone-d₆) d 6.70 (1H, d, J 8.8 Hz, H-2), 6.82 (dd, J 8.4, 0.8 Hz, H-7), 7.05 (dd, J 8.4, 0.8 Hz, H-5), 7.37 (d, J 8.8 Hz, H-3), 7.76 (t, J 8.4 Hz, H-6), 8.61 (s, HO-4), 11.05 (s, HO-1),11.83 (s, HO-8). ¹³C NMR (100 MHz acetone-d₆) d 108.12 (CH-5), 111.47 (CH-7), 110.51 (CH-2), 138.25 (C-4), 108.60 (C-8a), 108.60 (C-9a), 154.21 (C-1), 125.51 (CH-3), 157.21 (C-10a), 162.35 (C-8), 144.81 (C-4a), 138.91 (CH-6), 187.53 (C9).

Acknowledgements

The authors thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento Pessoal de Ensino Superior (CAPES) for financial support and Dr. Rodinei Augusti (UFMG) for MS measurements.

Received: July 14, 1999

Published on the web: September 15, 2000

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Publication Dates

• Publication in this collection
  15 Jan 2001
• Date of issue
  Oct 2000

History

• Received
  14 July 1999
• Accepted
  15 Sept 2000