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Journal of the Brazilian Chemical Society

Print version ISSN 0103-5053On-line version ISSN 1678-4790

J. Braz. Chem. Soc. vol.11 n.2 São Paulo Mar./Apr. 2000

http://dx.doi.org/10.1590/S0103-50532000000200007 

Article

 

Chemical constituents from Ouratea floribunda: complete 1H and 13C NMR assignments of atranorin and its new acetyl derivative

 

Mário G. de Carvalhoa*, Geizi J. A. de Carvalhoa and Raimundo Braz-Filhob

aDepartamento de Química, Instituto de Ciências Exatas, Universidade Federal Rural do Rio de Janeiro, 23851-970, Seropédica - RJ, Brazil
bSetor de Química de Produtos Naturais-LCQUI- CCT, Universidade Estadual do Norte Fluminense, 28015-620, Campos - RJ, Brazil.

 

 

O fracionamento cromatográfico do extrato hexânico da madeira de Ouratea floribunda (Ochnaceae) forneceu, além dos triterpenos friedelina (1), friedelanol (2) e lupeol (3), o depsídeo atranorina (4). As estruturas das substâncias isoladas foram determinadas através da análise dos dados espectroscópicos de RMN 1H e 13C, massas e comparação com dados da literatura. A atribuição inequívoca dos deslocamentos químicos dos átomos de carbono e hidrogênio do depsídeo 4 e do seu novo derivado monoacetilado 4a foi realizada através de técnicas uni- e bidimensionais de RMN (1H-1H COSY e NOESY, 1H-13C HMBC e HMQC) e está sendo descrita pela primeira vez na literatura.

 

Chromatographic fractionation of the hexane extract from the wood of Ouratea floribunda (Ochnaceae) afforded friedelin (1), friedelanol (2), lupeol (3) and the depside atranorin (4). The structure elucidation of the isolated compounds was performed by spectrometric analysis involving comparison with literature data. The unambiguous assignments of 1H and 13C NMR data of atranorin 4 and its acetyl derivative 4a are reported for the first time and involved 1H-1H homonuclear (COSY and NOESY) and 1H-13C heteronuclear (HMQC and HMBC) NMR experiments.

Keywords: Ouratea floribunda, Ochnaceae, atranorin, monoacetylatranorin

 

 

Introduction

The genus Ouratea (Ochnaceae) comprises ca 300 tropical species occurring mainly in South America1. Some species of this genus have been shown to possess antiviral (Ouratea lucens, from Panama)2, antimicrobial (O. parviflora, from Brazil)3 as well as pain relief (O. reticulata, from Guinea) activities4. The species O. spectabilis, from Brazil, has been reported to be used in folk medicine for the treatment of rheumatic and gastric distress. Recently, the inhibition of bovine lens aldose reductase by a biflavone isolated from this species has been described5.

The chemical study of this genus has led to the isolation of a biscatequin (Ouratea sp.)6, a cyanidin and a flavonoid (O. affinis)7, biisoflavanones, an isoflavone and triterpenes (O. hexasperma)8, biflavones (O. spectabilis)5 and recently we reported the isolation of biflavones, flavonoid glycosides, chloroisoflavonoids9, norisoprenoids, lignans, a diterpene10, triterpenes and steroids from O. semiserrata11.

As part of our chemical and pharmacological studies of Brazilian plants we reported the investigation of some Ouratea species8-11. In those studies, we also detected the inhibition of murine tumour growth and an antiproliferative effects and activation of apoptosis on Ehrlich tumour cells by biflavones isolated from two species belonging to this genus12.

In this first study of O. floribunda we report the presence of known triterpenes (1 - 3) and describe the spectroscopic data of the depside 4 and of its new monoacetyl derivative 4a. The structure elucidation of the isolated compounds was performed by spectrometric analysis involving comparison with literature data. The unambiguous assignments of 1H and 13C NMR data of atranorin 4 and of its monoacetyl derivative 4a are reported for the first time and involved 1H-1H homonuclear (COSY and NOESY) and 1H-13C heteronuclear (HMQC and HMBC) NMR experiments.

 

Results and Discussion

The hexane extract of the dry wood of Ouratea floribunda (St. Hill) Eng. was submitted to chromatography on a silica gel column to afford three triterpenes 1 - 3 (3.4 x 10-2 % of dry wood weight) and the depside 4 (9.0 x 10-4 % of dry wood weight). The known triterpenes friedelin (1), friedelanol (2) and lupeol (3) were identified by their 1H and 13C NMR spectral data analysis and comparison with literature values13-15.

2a06fg.gif (5819 bytes)

The molecular formula of 4 was deduced as C19H18O8 by a combination of LRMS (m/z 375 [MH]+, 15%, C19H19O8), 1H and 13C (HBBD and DEPT) NMR spectra analysis. Its 1H NMR spectrum showed the presence of singlet signals at dH 2.07, 2.53, 2.67 and 3.97 corresponding to three methyl groups bonded to sp2 carbons and one methoxyl group, respectively. This spectrum also revealed six deshielded singlet signals corresponding to two aromatic hydrogens (dH 6.39 and 6.50), a formyl hydrogen (dH 10.30) and three quelated hydroxyl groups (dH 11.94, 12.49 and 12.54). The comparative analysis of HBBD, DEPT and HMQC NMR spectra (Table 1) showed twelve quaternary sp2 carbon signals, three CH (dC 112.8, 115.9 and 193.8) and four CH3 (dC 9.4, 24.0, 25.6 and 52.3) consistent with the data obtained by 1H NMR analysis. The chemical shifts of quaternary sp2 carbons [dC 102.8, 108.8, 110.3, 116.7, 139.9, 152.0, 152.4, 162.8, 167.5, 169.1, 169.7 (C=O), and 172.2 (C=O)] were used to propose two aromatic rings sustaining five substituents (vide infra) and one hydrogen atom [dH 6.50 (s) and 6.39 (s)] each.

The HMBC experiments showed 2,3JCH long-range correlations (Table 1) between the 3H-9' (dH 2.53) and carbons C-5' (dC 139.9, 2JCH), CH-6' (dC 115.9, 3JCH) and C-4' (dC 110.3, 3JCH); between 3H-8' (dH 2.07) and C-2' (dC 116.7, 2JCH), C-3' (dC 162.8, 3JCH) and C-1' (dC 152.0, 3JCH); and between H-6' (dH 6.50) and C-1' (dC 152.0, 2JCH), (C-2' (dC 116.7, 3JCH) and C-4' (dC 110.3, 3JCH). Therefore, the two methyl groups CH3-8' and CH3-9' are at the same ring. The other cross peaks (Table 1) are also in agreement with the location of one methoxycarbonyl and one hydroxyl group in this same aromatic ring. These deductions and the peaks at m/z 196 (100 %), 194 (90 %) and 179 (95 %) observed in the mass spectrum (Scheme 1) are in agreement with the presence of a methyl 2-hydroxy-3,6-dimethylbenzoate moiety. The nuclear Overhauser enhancements observed in 1H-1H-NOESY spectrum was used to confirm the location of 3H-9' (dH 2.53) ortho to H-6' (dH 6.50). The remaining 2,3JCH cross peaks observed in the HMBC spectrum were used to establish the correlation between the signals of two hydroxyl, HO-2 (dH 12.49) with C-2 (dC 169.1, 2JCH) and C-3 (dC 108.5, 3JCH), HO-4 (dH 12.54) with C-3 (dC 108.5, 3JCH), C-4 (dC 167.5, 2JCH) and CH-5 (dC 112.8, 3JCH), and to assign the chemical shifts for C-2, C-3, C-4 and C-5 (Table 1). The observed long-range correlation involving the remaining methyl group CH3-9 (dC 25.6) and the aromatic hydrogen H-5 (dH 6.39) led to the assignments of C-1 and C-6 (Table 1). The NOE observed between H-5 (dH 6.39) and 3H-9 (dH 2.67) in 1H-1H NOESY experiment was also used to confirm the location of 3H-9 ortho to H-5. These data and the peaks at m/z 196 (100 %),195 (20%), 179 (95 %) and 151 (40 %) observed in the mass spectrum (Scheme 1) are in accordance with the presence of a 2,4-dihydroxy-3-aldehyde-6-methyl-benzoate moiety. Thus, these data allowed to deduce the structure of 4, a natural product classified as depside and known as atranorin, isolated from lichens. The structure was previously determined only by 1H NMR data and confirmed through synthesis16-18.

The acetylation of 4 at 60oC yielded the hydrolysis product, methyl2-hydroxy-4-hydroxy-3,6-dimethylbenzoate (4b). The same acetylation carried out at room temperature gave the new monoacetate 4a, whose structure was confirmed by complete 1H and 13C NMR spectra assignment (Table 1). As was expected, the chemical shift of H-6' was shifted from dH 6.50 in 4 to dH 6.91 in 4a, with its location at ortho or at para positions and, in accordance with the presence of H-6' para in relation to the 3'-hydroxyl group in 419.

This is the first report for 13C NMR data assignments for the depside 4 [3-formyl-2,4-dihydroxy-6-methylbenzoic acid 3-hydroxy-4-(methoxycarbonyl)-2,5-dimethylphenyl ester] and the derivative 4a [3-formyl-2,4-dihydroxy-6-methylbenzoic acid 3-acetoxy-4-(methoxycarbonyl)-2,5-dimethylphenyl ester].

2a06sc1.gif (15915 bytes)

Depsides are aromatic products of polyketide origin which are especially well represented in lichens, the most characteristic being formed by the condensation of two or three simple orcinol or b-orcinol-type phenolic moieties linked by an ester bond. A large number of structurally related metabolites have been detected in several lichens. Although recently a tridepside fenviorin was isolated from the plant Frullania nisqualensis20. Atranorin is a para-b-orcinol depside bearing CO2Me, CHO and OH groups, frequently found in lichens, it seems that the wood of O. floribunda used in this study was contaminated. Among the aromatic lichen compounds, atranorin is the most frequent allergenic agent responsible for airborne contact dermatitis21,22.

 

Experimental

General experimental procedures

Melting points were determined on a Kofler hot-stage apparatus and are uncorrected. The NMR spectra of compounds 1 - 3 were taken on a Bruker AC 200 (1H: 200 MHz; 13C: 50.3 MHz) spectrometer. The 1H (400 MHz) and 13C (100 MHz) NMR (1D and 2D) spectra of 4 and its acetyl derivative 4a were recorded on a Varian UN 400 spectrometer. HMQC and HMBC experiments were optimized for 1JCH = 140 Hz and nJCH (n=2 and 3) = 9 Hz, respectively. Mass spectra were obtained with a VG Quattro instrument. FTIR spectra were recorded as a film on a Perkin-Elmer 1500 spectrometer. Column chromatography and on TLC plates were performed using silica gel Merck.

Plant material

Ouratea floribunda (St. Hill) Engl. was collected at an environmentally preserved area near Centro de Pesquisas Morro do Chapéu, Belo Horizonte, Minas Gerais, Brazil, in 1996. Identification of the plant material was performed by Jorge L. Silva by comparison with a herbarium specimen (no 6944) deposited at the Herbarium José Badini in the Instituto de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto, Minas Gerais State, Brazil.

Extraction and isolation of the constituents

Dried wood material (1334 g) was powdered and sequentially extracted with hexane, EtOAc and MeOH by maceration at room temperature. The hexane extract was evaporated under vacuum to yield 4.3 g of residue. This residue was subjected to chromatography on silica gel column using hexane, dichloromethane and methanol with a gradient of polarity from dichloromethane to methanol. Thirty three fractions of 125 mL each were collected. Fractions 1-5 yielded a mixture of 1 and 2 (120 mg, m. p. 248-250 oC). Fractions 9-10 yielded 3 (330 mg, m. p. 160-161oC). Fractions 11-14 gave 4 (12 mg, m. p. 191-192 oC).

Atranorin (4)

Colorless crystals from hexane, m. p. 191-192 oC. IR (KBr) nmax (cm-1): 3450, 1770, 1730, 1650, 1580, 1260, 1145. 1H (400 MHz, CDCI3) and 13C (100 MHz, CDCl3) NMR: Table 1. CIMS: Scheme 1.

Acetylation of atranorin (4)

Compound 4 (9 mg) was dissolved in pyridine (1.0 mL) and acetic anhydride (1.0 mL), and the solution was allowed to stand for 24 h at room temperature. Usual work-up yielded 4a (gum, 7 mg), amorphous powder from hexane, m. p. 105-106 oC, IR (film) nmax (cm-1): 1772, 1731, 1649, 1572, 1260, 1150. 1H NMR (400 MHz, CDCl3) and 13C NMR (100 MHz, CDCI3): Table 1. CIMS m/z (rel. int.): 417 ([M + H]+, 2), 239 (20), 221 (25), 207 (100), 197 (73), 179 (62).

The acetylation of 4 at 60 oC yielded methyl 2-acetoxy-4-hydroxy-3,6-dimethylbenzoate (4b): gum, 1H NMR (400 MHz, CDCl3) dH: 6.52 (s, H-5), 3.83 (s, MeO-7), 2.30 (s, 3H), 1.99 (s, 3H).

 

Acknowledgements

The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for research fellowships and for grants to G. J. A. de C.. We thank Mr. J. L. Silva and Dr. A. A. Werle (Universidade Federal de Ouro Preto, Minas Gerais, Brazil) for the collection and identification of plant material. We thank Dr D. G. I. Kingston, Virginia Polytechnic Institute and State University-USA, for mass, 1H and 13C NMR spectrometers facilities and Dr. Victor Runjanek for reading the manuscript.

 

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Received: June 1, 1998

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