Eremophilane Sesquiterpenes from Ligularia myriocephala

• Abstract
• Introduction
• Materials and Methods
• General
• Plant material
• Extraction and isolation
• X-ray crystal data and structure refinement of 1
• Results and Discussion
• Acknowledgements
• References

Abstract

Five new eremophilenolides, 1β-angeloyloxy-6β,10β-dihydroxy-8β-methoxyeremophil-7(11)-en-8α,12-olide (1), 1β-angeloyloxy-6β,10α-dihydroxy-8α-methoxyeremophil-7(11)-en-8β,12-olide (2), 1β,6β-diangeloyloxy-8β,10β-dihydroxyeremophil-(11)-en-8α,12-olide (3), 1β,6β-diangeloyloxy-8α,10 α-dihydroxyeremophil-7(11)-en-8β,12-olide (4), 1β-angeloyloxy-8-oxoeremophil-6,9-dien-12-oic acid methyl ester (5) and one known compound, 8β,10β-dihydroxyeremophilenolide (6) were isolated from the extract of the whole plant of Ligularia myriocephala Ling. Their structures and stereochemistry were elucidated by various spectroscopic methods including intensive 2D-NMR techniques (COSY, gHMQC, gHMBC and ¹H-¹H NOESY) and HR-ESI-MS. A single-crystal X-ray experiment was performed for compound 1.

Introduction

Members of the genus Ligularia (Compositae) have been used for a long time to cure pulmonary tuberculosis, hemoptysis, urinary tract blockages, rheumatism, common cold, pharyngitis, laryngitis, hepatitis, bronchitis and asthma. For possible pharmaceutical uses, Ligularia species have been studied by our group for several years [1], [2], and have been found to be an important source of sesquiterpenes of the eremophilane type [3], [4], [5], [6], [7]. Ligularia myriocephala is limited to Teibei, northwestern China and has been used as a folk medicine to reduce phlegm, relieve cough, cure pellagra and rheumatic edema [8]. In the course of our investigations on natural sesquiterpenes, we selected L. myriocephala, whose chemical constituents have not been reported previously. One known eremophilenolide and five novel eremophilane sesquiterpenes have been isolated from this species.

Materials and Methods

General

Melting points were obtained on an X-4 digital display micro-melting point apparatus, and are uncorrected. Optical rotations were taken on a polarimeter 341 (Perkin Elmer) in CHCl₃ solution. UV spectra were observed on a Shimadzu UV-240 spectrophotometer in MeOH solution. IR spectra were measured on a Nicolet NEXUS 670 FT-IR spectrometer. ¹H-NMR (400.16 Hz), ¹³C-NMR (100.32 Hz) and 2D-NMR were recorded on a Varian INOVA 400 MHz FT-NMR spectrometer with TMS as internal standard. HR-ESI-MS was recorded on a Bruker APEX II. Silica gel (200 - 300 mesh) was used for CC and silica GF₂₅₄ (10 - 40 μ) for TLC. Spots were detected on TLC under UV light or by heating after spraying with 98 % H₂SO₄:EtOH = 5 : 95 (v:v).

Plant material

The whole plant of L. myriocephala was collected in August 2001 in Ling Zhi County, Teibei, P. R. China, and was identified by Prof. Guo-Liang Zhang, Department of Biology, Lanzhou University, Lanzhou, P. R. China. A voucher specimen (No. 2001 - 20) has been deposited in our Key Laboratory for Natural Medicine of Gansu Province.

Extraction and isolation

The dried whole plant of L. myriocephala (4.4 kg) was powdered and extracted at room temperature with petroleum ether (60 - 90 °C)-Et₂O-MeOH (v:v:v, 1 : 1:1) (10 L each time, 4 × 7 days). The solvent was removed under reduced pressure to give a residue (110.0 g), which was subjected to column chromatography (CC) over silica gel (200 - 300 mesh, 2000 g), eluting with a gradient of petroleum ether (60 - 90 °C)-acetone (v:v, 50 : 1, 40 : 1, 30 : 1, 10 : 1, 5 : 1, 3 : 1 and 1 : 1; 500 mL for each eluent) and finally with MeOH (2000 mL). Based on differences in composition indicated by TLC, 8 crude fractions [F1 (v:v 50 : 1, 10.0 g), F2 (v:v 40 : 1, 5.0 g), F3 (v:v 30 : 1, 4.0 g), F4 (v:v 10 : 1, 15.0 g), F5 (v:v 5 : 1, 10.0 g), F6 (v:v 3 : 1, 8.0 g), F7 (v:v 1 : 1, 5.0 g), and F8 (MeOH, 15.0 g)] were obtained. F2 was separated by CC over silica gel (200 - 300 mesh, 100 g) with petroleum ether (60 - 90 °C)-EtOAc (v:v, 20 : 1, 1000 mL) to give 1 (35 mg). The mother liquid was separated by repeated preparative TLC (silica GF₂₅₄, 10 - 40 μ) with petroleum ether (60 - 90 °C)-CHCl₃-EtOAc (v:v:v, 10 : 1:1, 3 × 50 mL, three times, R_f = 0.15) to afford an inseparable mixture of 3 and 4 (11 mg) which could not be separated further by CC and RP-HPLC. F3 was subjected to CC on silica gel (80 g) with a gradient of petroleum ether (60 - 90 °C)-EtOAc (v:v, 10 : 1, 1000 mL, 8 : 1, 900 mL, 5 : 1, 700 mL) as eluent. The eluate (v:v, 8 : 1) was purified by petroleum ether (60 - 90 °C)-acetone (v:v, 5 : 1, 800 mL) to give 2 (10 mg). F4 was purified by CC on silica gel (40 g) and eluted repeatedly with petroleum ether (60 - 90 °C)-acetone (v:v, 6 : 1, 200 mL) to give crude 5 (6 mg) and 6 (11 mg). Crude 5 was further purified by preparative TLC with petroleum ether (60 - 90 °C)-acetone (v:v, 3 : 1, R_f = 0.45) to yield pure 5 (5 mg).

1β-Angeloyloxy-6β,10β-dihydroxy-8β-methoxyeremophil-7(11)-en-8α,12-olide (1): Colorless needles, C₂₁H₃₀O₇, m. p. 180 - 181 °C; [α]_D ²⁰: + 186° (c 0.70, CHCl₃); IR (KBr): ν = 3474.1, 2948.9, 1776.0, 1704.4, 1644.9, 1459.0, 1230.9 cm^-1; UV (MeOH): λ_max (log ε) = 220 nm (5.7); HR-ESI-MS: m/z = 417.1869 [M + Na]⁺ (calcd. for C₂₁H₃₀O₇Na: 417.1884); ¹H-NMR and ¹³C-NMR, see Table [1].

1β-Angeloyloxy-6β,10α-dihydroxy-8α-methoxyeremophil-7(11)-en-8β,12-olide (2): Colorless needles, C₂₁H₃₀O₇, m. p. 170 - 171 °C; [α]_D ²⁰: -11° (c 1.12, CHCl₃); IR (KBr): ν = 3462.2, 2918.8, 1739.4, 1679.3, 1646.3, 1460.7, 1278.6 cm^-1; UV (MeOH): λ_max (log ε) = 220 nm (4.3); HR-ESI-MS: m/z = 395.2065 [M + H]⁺ (calcd. for C₂₁H₃₁O₇ : 395.2064); ¹H-NMR and ¹³C-NMR, see Tables [2] and [3].

1β,6β-Diangeloyloxy-8β,10β-dihydroxyeremophil-7(11)-en-8α,12-olide (3) and 1β,6β-diangeloyloxy-8α,10α-dihydroxyeremophil-7(11)-en-8β,12-olide (4): Colorless gum, C₂₅H₃₄O₈; HR-ESI-MS: m/z = 480.2590 [M + NH₄]⁺ (calcd. for C₂₅H₃₈O₈N: 480.2592); ¹H-NMR and ¹³C-NMR, see Tables [2] and [3].

1β-Angeloyloxy-8-oxoeremophil-6,9-dien-12-oic acid methyl ester (5): Colorless gum, C₂₁H₂₈O₅; HR-ESI-MS: m/z = 361.2015 [M + H]⁺ (calcd. for C₂₁H₂₉O₅ : 361.2010); ¹H-NMR and ¹³C-NMR, see Tables [2] and [3].

8β,10β-Dihydroxyeremophilenolide (6): Colorless gum, C₁₅H₂₂O₄; ¹H-NMR and ¹³C-NMR, see Tables [2] and [3].

X-ray crystal data and structure refinement of 1

Crystal data: C₂₁H₃₀O₇, Formula wt.: 394.46, D _cal = 1.282 g/cm³. Single-crystal X-ray diffraction data were collected by using a MAC Science DIP 2030k image plate with graphite monochrome MoKa radiation. The crystal (0.10 × 0.20 × 1.00 mm) belongs to the monoclinic system, space group P2₁. Accurate cell parameters are a = 11.044(1), b = 8.360(1), c = 11.214 2(1) Å,  β  = 99.31(1)°, V = 1021.7 (2) ų, Z = 2, There were 2200 reflections, of which 2054 (|F| ² ≥ 3δ|F|²) were observed. The position of 28 non-hydrogen atoms was obtained directly from an E-map. The structure was solved by direct methods using SHELXS-86 for the refinement. Positions of the other non-hydrogen atoms and the type of atoms was determined by using least-squares calculations and difference Fourier methods in turn. Geometric calculations and difference Fourier methods proved the positions of all hydrogen atoms. The structure was finally refined to Rf = 0.054 and R _w = 0.053.

Results and Discussion

Compound 1 was obtained as colorless needles, m. p.180 - 181 °C, [α]_D ²⁰: + 186° (c 0.70, CHCl₃), the HR-ESI-MS showed m/z = 417.1884 [M + Na]⁺ (calcd. for C₂₁H₃₀O₇Na: 417.1889). The IR (KBr) spectrum showed bands for a hydroxy group (3474.0 and 3435.4 cm^-1) and an α,β-unsaturated-γ-lactone (1644.9, 1704.4 and 1776.0 cm^-1), UV λ_max = 220 nm. Except for the typical carbon signals of an angeloyl group (Table [1]) and a methoxy group (δ_C = 51.1, δ_H = 3.25, 3H, s), the ¹³C-NMR spectrum (Table [1]) showed 15 carbons including three methyls, three methylenes, three methines, and six quaternary carbons, assigned by a DEPT experiment, which revealed that 1 was as a sesquiterpenoid. By comparison of its spectral data with those of known sesquiterpene lactones [9], [10], [11], 1 was an eremophilanoide with an angeloyl, a methoxy and two hydroxy groups. The location of the angeloyloxy moiety was established by the gHMBC correlation of H-1 (δ_H = 4.85, t, J = 2.8 Hz) with C-1′ (δ_C = 167.7), C-10 (δ_C = 76.2), C-9 (δ_C = 43.7) and C-5 (δ_C = 47.4). That the methoxy group was attached at C-8 was deduced by the gHMBC correlation of the methoxy protons (-OCH₃) with C-8 (δ_C = 105.4). The hydroxy group attached to C-10 was β-oriented because rings A and B were cis-fused from the chemical shift of the 14-CH₃ (δ_H = 1.33, 3H, s) downfield of 15-CH₃ (0.86, 3H, d) [2], [9], [10]. This was supported by a cross-peak between H-4α and H-9α in the NOESY spectrum (Fig. [2]). That the hydroxy group at C-6 was β-oriented was also shown by a cross-peak between H-6α and H-15 (Fig. [2]). The pattern and smaller J values between H-1 and H_{2 -} 2 (J _1e,2e = J _1e,2a = 2.8 Hz) indicated that the angeloyl group at C-1 was β-oriented. The structure and relative stereochemistry were confirmed by X-ray crystallography (Fig. [3]). Hence, 1 was Iβ-angeloyloxy-6β,10β-dihydroxy-8β-methoxyeremophil-7(11)-en-8α, 12-olide.

Compound 2 was obtained colorless needles, m. p. 170 - 171 °C, [α]_D ²⁰: -11° (c 1.12, CHCl₃), the HR-ESI-MS showed m/z = 395.2065 [M + H]⁺ (calcd. for C₂₁H₃₁O₇ : 395.2064). The IR (KBr) spectrum showed bands for a hydroxy group (3474.0 and 3435.4 cm^-1) and an α,β-unsaturated-γ-lactone (1646.3, 1679.3 and 1739.4 cm^-1), UV λ_max = 220 nm. The ¹H- and ¹³C-NMR spectra were very similar to those of 1 except for the 15-CH₃ (δ_H = 1.21, 3H, d, J = 7.6 Hz) that was downfield of 14-CH₃ (δ_H = 0.96, 3H, s). This suggested that 2 was a trans-eremophilane with an α-OH at C-10 and an α-OCH₃ at C-8 [9], [12]. This trans-fusion of rings A and B was supported by three cross-peaks, H-1α with H-9α, H-4α with H-6α and CH₃ - 14 with H-9β in the NOESY spectrum. Moreover, a cross-peak between H-6α and OCH₃ - 8 in the NOESY spectrum and the homoallylic coupling (J = 1.6 Hz) between H-6α and CH₃ - 13 protons also showed the OCH₃ at C-8 was α-oriented and the OH group at C-6 was β-oriented [13], [14]. The other structural information of 2 also could be concluded from the gHMBC. Then, 2 was 1β-angeloyloxy-6β, 10α-dihydroxy-8α-methoxy-eremophil-7(11)-en-8β,12-olide.

Compounds 3 and 4 were obtained as a mixture (ratio 2 : 1, by NMR), colorless gum, the HRESI-MS showed m/z = 480.2590 [M+NH₄]⁺ (calcd. 480.2592 for C₂₅H₃₈O₈N). The ¹³C-NMR experiment showed 50 signals for the mixture of two compounds that have the same formula compositions, C₂₅H₃₄O₈. Besides the typical carbon signals of four angeloyl groups (Tables [2] and [3]), the remaining thirty carbons (Tables [2] and [3]) were six methyls, six methylenes, six methines, and twelve quaternary carbons, assigned by a DEPT experiment, which revealed that 3 and 4 were sesquiterpenoids, respectively. By comparison of spectral data with those of 1 and 2, compounds 3 and 4 were an eremophilanoid with two angeloyl groups and two hydroxy groups (Tables [2] and [3]).

The locations of the two angeloyl moieties of 3 were established by the gHMBC correlations of H-1 (δ_H = 4.97, t, J = 2.8 Hz) with the C-1′ (δ_C = 167.8) and H-6 (δ_H = 5.80, s) with C-1′′ (δ_C = 166.6). Compared to 1, the OCH₃ - 8 (δ_C = 101.8) was replaced by an OH group and the missed homoallylic spinning couple between H-6 and H-13 (d_H = 1.99, 3H, s) suggested the OH-8 was β-oriented. The hydroxy group attached to C-10 (δ_C = 75.1) was β-oriented because rings A and B were cis-fused from the chemical shift of the 14-CH₃ (δ_H = 1.21, 3H, s) downfield of 15-CH₃ (δ_H = 0.96, d, J = 6.4 Hz) [13]. This was supported by a cross-peak between H-4α and H-9α in the NOESY spectrum. That angeloyl group at C-6 (δ_C = 70.8) was β-axial was also showed by a cross-peak between H-6α and H-15. The pattern and smaller J values between H-1 and H₂ - 2 (J _1e,2e = J _1e,2a = 2.8 Hz) showed that the angeloyl group at C-1 was β-oriented. Hence, 3 was 1β,6β-diangeloyloxy-8β,10β-dihydroxy eremophil-7(11)-en-8α,12-olide

The locations of the two angeloyloxy moieties of 4 were also established by the gHMBC correlation of H-1 (δ_H = 5.64, dd, J = 11.6, 5.2 Hz) with the C-1′ (δ_C = 168.2) and H-6 (δ_H = 6.14, d, J = 1.6 Hz) with C-1′′ (δ_C = 166.0). Compared to 3, the 15-CH₃ (δ_H = 1.20, 3H, d, J = 8.4 Hz) was downfield of 14-CH₃ (δ_H = 1.07, 3H, s). This suggested 4 was a trans-eremophilane with the α-OH at C-10 and C-8, respectively [14]. The homoallylic coupling (J = 1.6 Hz) between H-6α and CH₃ - 13 protons also showed that OH-8 was α-oriented. The other structural information of 4 could also be concluded from the gHMBC experiment. Then, 4 was 1β,6β-diangeloyloxy-8α,10α-dihydroxyeremophil-7(11)-en-8β,12-olide.

Compound 5 was obtained as a colorless gum, the HR-ESI-MS showed m/z = 361.2015 [M + H]⁺, calcd. for C₂₁H₂₉O₅ : 360.2385). The ¹H- and ¹³C-NMR spectra of 5 showed similarities with the reported compound, 1β-hydroxy-8-oxoeremophil-6,9-dien-12-oic acid methyl ester except for an angeloyl group (δ_C = 166.6, 128.2, 139.3,  20.7, 15.8; δ_H = 6.09, 1H, m; 1.85, 3H, m; 1.98, 3H, m) in 5 instead of a hydroxy group in the reported compound, obtained by methoxylation of 1β-hydroxy-8-oxoeremophil-6,9-dien-12-oic acid [15]. The location of the angeloyl moiety was established by the strong gHMBC correlation of H-1 (δ_H = 5.54 s) with C-1′. The methoxy group at C-12 was established by the gHMBC correlation of the protons of OCH₃ (δ_H = 3.64, 3H, s) with C-12. Thus, 5 was 1β-angeloyloxy-8-oxoeremophil-6, 9-diene-12-oic acid methyl ester. Unfortunately, we could not obtain its spectral data of UV, [α]_D and IR because it decomposed in CDCl₃.

Compound 6 was obtained as a colorless gum. Its ¹H- and ¹³C-NMR spectra were completely the same as the reported compound 8β,10β-dihydroxyeremophilenolide, which has been isolated from Hertia cheirifolia [9].

Acknowledgements

This work was supported by the National Natural Sciences Foundation of China (NSFC No. 20 021 001, 20 372 029 and 20 475 057).

References

• 1 Wu Q X, Shi Y P, Yang L. Unusual sesquiterpene lactones from Ligularia virgaurea spp. oligocephala .  Org Lett. 2004;  6 2313-6
  CrossrefPubMedSearch in Google Scholar
• 2 Wu Q X, Shi Y P, Yang L. Eremophilane sesquiterpene lactones from Ligularia virgaurea ssp. oligolephala .  Planta Med. 2004;  70 479-82
  Thieme ConnectPubMedSearch in Google Scholar
• 3 Li X Q, Gao K, Jia Z J. Eremophilenolides and other constituents from the roots of Ligularia sagitta .  Planta Med. 2003;  69 356-60
  Thieme ConnectPubMedSearch in Google Scholar
• 4 Bang Y H, Lee J H, Tae H K. Furanoligularenone, an eremophilane from Ligularia fischeri, inhibits the LPS-induced production of nitric oxide and prostaglandin E₂ in macrophage RAW264.7 cells.  Planta Med. 2004;  68 101-5
  Thieme ConnectPubMedSearch in Google Scholar
• 5 Wang W S, Gao K, Jia Z J. New Eremophilenolides from Ligulariopsis shichuana .  J Nat Prod. 2002;  65 714-7
  CrossrefPubMedSearch in Google Scholar
• 6 Asada Y, Furuya T. Studies on constituents of crude drugs. XV. New pyrrolizidine alkaloids from Ligularia dentata .  Chem Pharm Bull. 1984;  32 475-82
  PubMedSearch in Google Scholar
• 7 Zhao Y u, Hong P, Jia Z J. Six new eremophilane derivatives from two Ligularia species.  J Nat Prod. 1994;  57 1626-30
  CrossrefPubMedSearch in Google Scholar
• 8 Jiangsu College of New M edicine. A Dictionary of the Traditional Chinese Medicines. Shanghai; Shanghai Science and Technology Press 1977: p. 1152-2349
  Search in Google Scholar
• 9 Massiot G, Nuzillard J M, Le Men-Olivier L, Aclinou P, Benkouider A, Khelifa A. Eremophilenolides from Hertia cheirifolia .  Phytochemistry. 1990;  29 2207-10
  CrossrefPubMedSearch in Google Scholar
• 10 Zhang S M, Zhao G L, Lin G Q. Eremophilane sesquiterpenes from Cacalia roborowskii .  Phytochemistry. 1998;  48 519-24
  CrossrefPubMedSearch in Google Scholar
• 11 Matías R, Azucena G C, Gutiérrez C. Defensive chemistry of Senecio mister .  J Nat Prod. 2001;  64 6-11
  CrossrefPubMedSearch in Google Scholar
• 12 Mao M J, Yang Z D, Jia Z J. New eremophilane sesquiterpenes from Cacalia ainsliaeflora .  Planta Med. 2003;  69 745-9
  Thieme ConnectPubMedSearch in Google Scholar
• 13 Naya K, Manazawa R, Sawada M. The photosensitized oxygenation of furanoeremophilanes. I. The isomeric hydroperoxides from petasalbin and their transformation to lactones.  Bull Chem Soc Jpn. 1975;  48 3220-5
  PubMedSearch in Google Scholar
• 14 Moriyama Y. Takahashi T. New sesquiterpene lactones of eremophilane-type from Ligularia fauriei (Fr.) Koidz.  Bull Chem Soc Jpn. 1976;  49 3196-9
  PubMedSearch in Google Scholar
• 15 Zhao Y, Jia Z J, Yang L. Novel Sesquiterpene from Ligularia veitchiana .  Planta Med. 1994;  60 91-2
  PubMedSearch in Google Scholar

Prof. Yan-Ping Shi

Lanzhou Institute of Chemical Physics

Chinese Academy of Sciences

Lanzhou 730000

People’s Republic of China

Fax: +86-931-827-7088

Email: shiyp@lzb.ac.cn

References

• 1 Wu Q X, Shi Y P, Yang L. Unusual sesquiterpene lactones from Ligularia virgaurea spp. oligocephala .  Org Lett. 2004;  6 2313-6
  CrossrefPubMedSearch in Google Scholar
• 2 Wu Q X, Shi Y P, Yang L. Eremophilane sesquiterpene lactones from Ligularia virgaurea ssp. oligolephala .  Planta Med. 2004;  70 479-82
  Thieme ConnectPubMedSearch in Google Scholar
• 3 Li X Q, Gao K, Jia Z J. Eremophilenolides and other constituents from the roots of Ligularia sagitta .  Planta Med. 2003;  69 356-60
  Thieme ConnectPubMedSearch in Google Scholar
• 4 Bang Y H, Lee J H, Tae H K. Furanoligularenone, an eremophilane from Ligularia fischeri, inhibits the LPS-induced production of nitric oxide and prostaglandin E₂ in macrophage RAW264.7 cells.  Planta Med. 2004;  68 101-5
  Thieme ConnectPubMedSearch in Google Scholar
• 5 Wang W S, Gao K, Jia Z J. New Eremophilenolides from Ligulariopsis shichuana .  J Nat Prod. 2002;  65 714-7
  CrossrefPubMedSearch in Google Scholar
• 6 Asada Y, Furuya T. Studies on constituents of crude drugs. XV. New pyrrolizidine alkaloids from Ligularia dentata .  Chem Pharm Bull. 1984;  32 475-82
  PubMedSearch in Google Scholar
• 7 Zhao Y u, Hong P, Jia Z J. Six new eremophilane derivatives from two Ligularia species.  J Nat Prod. 1994;  57 1626-30
  CrossrefPubMedSearch in Google Scholar
• 8 Jiangsu College of New M edicine. A Dictionary of the Traditional Chinese Medicines. Shanghai; Shanghai Science and Technology Press 1977: p. 1152-2349
  Search in Google Scholar
• 9 Massiot G, Nuzillard J M, Le Men-Olivier L, Aclinou P, Benkouider A, Khelifa A. Eremophilenolides from Hertia cheirifolia .  Phytochemistry. 1990;  29 2207-10
  CrossrefPubMedSearch in Google Scholar
• 10 Zhang S M, Zhao G L, Lin G Q. Eremophilane sesquiterpenes from Cacalia roborowskii .  Phytochemistry. 1998;  48 519-24
  CrossrefPubMedSearch in Google Scholar
• 11 Matías R, Azucena G C, Gutiérrez C. Defensive chemistry of Senecio mister .  J Nat Prod. 2001;  64 6-11
  CrossrefPubMedSearch in Google Scholar
• 12 Mao M J, Yang Z D, Jia Z J. New eremophilane sesquiterpenes from Cacalia ainsliaeflora .  Planta Med. 2003;  69 745-9
  Thieme ConnectPubMedSearch in Google Scholar
• 13 Naya K, Manazawa R, Sawada M. The photosensitized oxygenation of furanoeremophilanes. I. The isomeric hydroperoxides from petasalbin and their transformation to lactones.  Bull Chem Soc Jpn. 1975;  48 3220-5
  PubMedSearch in Google Scholar
• 14 Moriyama Y. Takahashi T. New sesquiterpene lactones of eremophilane-type from Ligularia fauriei (Fr.) Koidz.  Bull Chem Soc Jpn. 1976;  49 3196-9
  PubMedSearch in Google Scholar
• 15 Zhao Y, Jia Z J, Yang L. Novel Sesquiterpene from Ligularia veitchiana .  Planta Med. 1994;  60 91-2
  PubMedSearch in Google Scholar

Prof. Yan-Ping Shi

Lanzhou Institute of Chemical Physics

Chinese Academy of Sciences

Lanzhou 730000

People’s Republic of China

Fax: +86-931-827-7088

Email: shiyp@lzb.ac.cn