FURANOEREMOPHILANES FROM SENECIO CLIVICOLUS WEDDELL

MarceCo Davila^a,b; Olov Sterner^a; Nelson Hinojosa^b*

Keywords: Furanoeremophilanes, Senecio clivicoulus

ABSTRACT

A phytochemical investigation of the dried aerial parts of Senecio clivicolus Weddell led to the isolation of four furanoeremophilane sesquiterpenes. Their structures and relative configuration were established by NMR and HRMS-ESI analyses, and by comparison with data reported in the literature. Their presence in S. clivicolus is reported for the first time.

INTRODUCCIÓN

Senecio represent the largest genus of the family Asteraceae and has more than 1500 species [1]. Senecio species are used in traditional medicine for many purposes, such as a remedy for gastric-ulcer and stomach pain [2], chest pain, cough, fever and running nose [3, 4]. In the north región of Argentina S. graveolens is used to counteract mountain sickness, digestive and cough suppressant[5]. Of the 114 species of Senecio reported to grow in Bolivia [6], Senecio clivicolus is a perennial shrub growing in the mountainous regions. The leaves of S. clivicolus have been used to relieve the stomach pain [7] and as a anti-diarrhea remedy [8]. Moreover, the extract has been reported to be used to treat skin fungal infections[9].Only one phytochemical investigation of S. clivicolus has been reported so far [10], in which alpha-farnesene, germacrene D and 1-pentadecene were isolated and characterized. This study reports the isolation and chemical characterization of four furanoeremophilanes (Figure 1) from an ethanol extract of the dried aerial parts of S. clivicolus: decompostin (1), 6-acetoxy-9-oxo-10H-furanoeremophilane (2), 1-hydroxy-6-acetoxy-9-oxo-10H-furanoeremophilane (3) and la-acetoxy-6-acetoxy-9-oxo-10H-furanoeremophilane (4).

Figure 1. Structures of compounds 1-4.

RESULTADOS Y DISCUSIÓN

The elemental composition of compound 1 was determined to be C₁₇H₂₀O₄, based on the ID NMR spectra (¹H and ¹³C NMR data for all four compounds are given in Table 1) as well as HRMS-ESI data. 1 consequently has eight degrees of unsaturation, and as the NMR data show the presence of three carbon-carbon double bonds and two carbonyl groups 1 is tricyclic. In the ^XH NMR spectrum a signal corresponding to a furan ring protón was observed at 7.41 (1H, q, 7=1.0; 12-H), which in the COSY spectrum correlates with the methyl signal at 1.93 (3H, d, 7=1.0; 13-H₃). In addition to this, and besides a methyl group obviously belonging to an acetyl group (5 2.20, 3H, s), the protón spectrum indicated the presence of another two methyls by the signáis at 5 1.09 (3H, s; 14-H₃) and 50.99 (3H, d, 7=6.8; I5-H3). COSY and HMBC correlations from these as well as 1-H (5 6.94, 1H, ddd, 7=4.9, 3.2, 0.8) close the left ring, and show that the acetoxy substituted C-6 is next to C-5 and that the carbonyl group C-9 is adjacent to C-10. HMBC correlations from 6-H, 12-H and 13-H₃ reveal all components of the furan ring, and the final bond between C-8 and C-9 is inevitable. The relative configuration of 1 was elucidated based on the NOESY correlations observed between H-6 and H-4 as well as between 14-H₃ and 15-H₃. The structure of compound 1 isolated here is identical to decompostin, previously reported from Cacalia decomposita[íí] and Psacalium beamanii[l2]. The HRMS-ESI of compound 2 indicated that its elemental composition is C₁₇H₂₂O₄, 2 consequently has one unsaturation less than 1. Comparison of the spectroscopic data of 1 and 2 revealed that the C-l/C-10 double bond in 1 is a single bond in 2, and COSY as well as HMBC correlations established the structure. The relative configuration of 2 was determined based on NOESY correlations between the three protons 4-H, 6-H and 10-H, and 2 was found to be identical to 6-acetoxy-9-oxo-10ai/-furanoeremophilane, previously reported from S. chilensis and S. patagonicus[13]. However, the ¹³C NMR data reported [13] are significantly different from those recorded here, indicating that it is necessary to correct the chemical shifts for C-l, C-2, C-3, C-14 and C-15 in the literature. The NMR data of compound3 (C17H22O5 according to HRMS-ESI) and 4 (C₁₉H₂₄O₆ according to HRMS-ESI) are similar to those of compoundsl and 2, with the exception for the signáis assigned to C-l and C-10. In both 3 and 4 C-l is oxygenated while C-10 is protonated, and extensive 2D NMR experiments show that, compared to 1, the C-l/C-10 double bond had added water in 3 and acetic acid in 4. NOESY correlations were observed between 1-H and 14-H₃, as well as between 4-H, 6-H and 10-H in both compounds, establishing their relative configuration. Based on these data the structures were established as la-hydroxy-6-acetoxy-9-oxo-10-furanoeremophilane 3 and loc-acetoxy-6p-acetoxy-9-oxo-10a//-furanoeremophilane 4. Compounds 3 and 4 have previously been reported from S. santelisis[l4].

Spectra recorded in: ^aDichloromethane-d₂ ^bChloroform-d. * Overlapping. Assignments were based on COSY,HMQC, HMBC, DEPT and NOESY experiments

EXPERIMENTAL

General experimental procedures

The optical rotations were measured with a Perkin-Elmer 341 polarimeter at 20°C. HRMS-ESI spectra were recorded with a Waters Q-TOF Micro system spectrometer, using H₃PO₄ for calibration and as internal standard. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) were measured with a Bruker DRX400 spectrometer; the spectra were recorded in chloroform-d (solvent residual signáis at _H 7.26 and _C 77.16) and dichloromethane-d₂ (solvent residual signáis at _H 5.32 and _C 53.84). The chemical shifts () are given in ppm, and coupling constants (J) in Hz. Vacuum liquid chromatography (VLC) and centrifugal preparative TLC (CTLC) separations were carried out using TLC grade silica gel (Merck), while column chromatography were run on silica gel 60 (230-400 mesh, Merck). TLC analyses were carried out on silica gel GF₂5₄ pre-coated plates (Merck); chromatograms were visualized under a UV lamp (254 nm) and by spraying with vanillin (6%)-sulfuric acid (1.5%)-ethanol solution, followed by heating.

Plant material

The whole aerial parts of Senecio clivicolus Weddell were collected from south of Cochabamba-Bolivia at 2900 meters above sea level in April 2008. A voucher specimen (MZ-3741) was deposited at National Herbarium "Herbario Nacional Martin Cárdenas" at Cochabamba-Bolivia.

Extraction and isolation

The air-dried powdered plant material (500 g) was extracted with 95% ethanol (3x1 L) for 3 days at room temperature. Removal of the solvent from the fíltrate under reduces pressure provided an extract (70 g). Part of the extract was suspended in ethanol-water (80:20) and successively partitioned with hexane and chloroform. The chloroform fraction (15 g) was precipitated with ethyl acétate to yield a dark brown precipítate (550 mg) and a dark liquid, which was subjected to vacuum liquid chromatography on silica gel using heptane-ethyl acétate (80:20) as solvent. Ten main fractions were collected (1-10). Fraction 4 (570 mg) was precipitated with methanol to give a yellow precipítate that was purified by centrifugal preparative TLC with heptane-ethyl acétate (80:20) to yield compound 1 (15 mg). Fraction 3 (3.37 g) was precipitated with methanol to give compound 2 (20 mg). Furthermore, fraction 7 (350 mg) was washed with heptane and then subjected to column chromatography on silica gel eluted with toluene-diethyl ether (25:3.5) to give six fractions (A-F). Compound 3 (5 mg) was purified from fraction F by column chromatography using a mixture of heptane-ethyl acétate (70:30) as the eluent. Finally, fraction C contains compound 4 (60 mg).

1  was obtained as a white amorphous solid. mp 195-198 °C. []_D²⁰ -60° (c 0.60, CHC1₃). ¹H NMR (CD₂C1₂ 400 MHz) and ¹³C NMR (CD₂C1₂100 MHz), see Table 1. HRMS-ESI calculated for C₁₇H₂₀O₄ (M+H)⁺ 289.1440. Found: 289.1445.

6-acetoxy-9-oxo-10H-furanoeremophilane (2).

2 was obtained as a white amorphous solid. mp 147-150 °C. []_D²⁰ -72° (c 0.70, CHC1₃) ^XH NMR (CDC1₃ 400 MHz) and ¹³C NMR (CDC1₃ 100 MHz), see Table 1. HRMS-ESI calculated for C₁₇H₂₂O₄ (M+H)⁺ 291.1596. Found: 291.1586.

1 a-hydroxy-6jB-acetoxy-9-oxo-10aH-furanoeremophilane (3).

3 was obtained as a yellowish oil. [oc]_D²⁰ -12° (c 0.20, CHC1₃) *H NMR (CDC1₃ 400 MHz) and ¹³C NMR (CDC1₃100 MHz), see Table 1. HRMS-ESI calculated for Ci₇H₂₂O₅ (M+H)⁺ 307.1545. Found: 307.1555.

1 -acetoxy-6-acetoxy-9-oxo-10H-furanoeremophilane (4).

4 was obtained as a white amorphous solid. mp 149-151 °C. [oc]_D^2O-84° (c 0.37, CHC1₃). *H NMR (CDC1₃ 400 MHz) and ¹³C NMR (CDC1₃ 100 MHz), see Table 1. HRMS-ESI calculated for C₁₉H₂₄O₆ (M+H)⁺ 349.1651. Found: 349.1654.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from Swedish International Development Cooperation Agency (SIDA). We thank Lie. Modesto Zarate from the National Herbarium "Herbario Nacional Martin Cárdenas", for the taxonomic determination of the plant material.

NOTAS

^aCentre for Analysis and Synthesis, Lund University, P.O. Box 124, SE-221 00, Lund, Sweden.;

^bCentro de Tecnología Agroindustrial, Facultad de Ciencias y Tecnología, Universidad Mayor de San Simón, P.O. Box, Cochabamba, Bolivia.

CorrespondÍng author: hinoiosanel@vahoo.com.ar

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