Sesquiterpenoids and phenolics from Taraxacum hondoense

Fitoterapia 76 (2005) 520 – 524 www.elsevier.com/locate/fitote

Sesquiterpenoids and phenolics from Taraxacum hondoense Wanda Kisiel *, Klaudia Michalska Department of Phytochemistry, Institute of Pharmacology, Polish Academy of Sciences, Pl-31-343 Krako´w, Poland Received 12 May 2004; accepted 26 April 2005 Available online 21 June 2005

Abstract Eleven sesquiterpene lactones, including the new guaianolide 11h-hydroxydeacetylmatricarin-8O-h-glucopyranoside, along with four known phenolic glucosides were isolated from Taraxacum hondoense. The compounds were characterized by spectral methods. D 2005 Elsevier B.V. All rights reserved. Keywords: Taraxacum hondoense; Sesquiterpenoids; 11h-Hydroxydeacetylmatricarin-8-O-h-glucopyranoside; Phenolics

1. Introduction Plants of the genus Taraxacum (Asteraceae, tribe Lactuceae), including the most popular species Taraxacum officinale, have long been used as medicinal herbs. So far, only 10 representatives of this taxon have been investigated chemically [1–3], yielding a number of germacrane-, eudesmane-, and guaiane-type sesquiterpene lactones, along with some other secondary metabolites. The most common sesquiterpene lactone isolated from the plants is the germacranolide taraxinic acid h-glucopyranosyl ester (1). The compound, frequently co-occurring with its 11h,13-dihydroderivative (2) and the structurally related germacranolide ainslioside (3), has been reported from seven species. Recently, * Corresponding author. Tel.: +48 126623254; fax: +48 126374500. E-mail address: [email protected] (W. Kisiel). 0367-326X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fitote.2005.04.016

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deacetylmatricarin (4) and structurally related 2-oxo-guaianolides with cross-conjugated cyclopentadienone systems, which are more unusual sesquiterpene lactones for this taxon, have been isolated from Taraxacum platycarpum [4] and Taraxacum obovatum [3]. Compounds of this type are particularly characteristic for plants of the tribe Anthemideae of the Asteraceae, among the better known of which are Chamomilla recutita and Achillea millefolium aggregate. These non-proazulene guaianolides have been proven to contribute to anti-inflammatory and anti-ulcer activities of the plant extracts [4–7]. The present investigation is concerned with root constituents of Taraxacum hondoense Nak. et Koidz., a species originating from Japan, which reportedly contains neolupenyl and tarolupenyl acetates [8].

2. Experimental 2.1. Plant material The roots of T. hondoense were collected in May 2002 from plants growing in the Garden of Medicinal Plants, Institute of Pharmacology, Polish Academy of Sciences, Krako´w, where a voucher specimen (01/81) was deposited. Seeds of the plant were obtained from the Medicinal Plant Research Station, National Institute of Health Sciences, Tsukuba, Japan. 2.2. Extraction and isolation Dried and finely powdered plant material (335 g) was exhaustively extracted with EtOH at room temperature and the solvent was evaporated in vacuo to give a dark brown residue (34 g). The residue was chromatographed on a Si gel (Merck; Art. 7754, 186 g) column, packed in hexane, eluting with a step gradient of hexane–EtOAc (0Y50% EtOAc in hexane) followed by EtOAc and finally with 5% and 10% MeOH in EtOAc, all fractions being monitored by TLC. The relevant fractions were combined and further separated by preparative TLC (Merck; Art. 5553) and semipreparative HPLC on a DeltaPak C-18 column (particle size 15 Am, 25  100 mm) coupled to a photodiode array detector using H2O–MeOH mixtures at a flow rate of 3.0 ml min 1. Elution of the Si gel column with hexane–EtOAc (3:2) afforded crystalline 4 (4.9 mg), after purification by preparative TLC (hexane–EtOAc, 1:1). Combination of fractions eluted with EtOAc, followed by preparative TLC purification (CHCl3–MeOH, 17:3), gave three crude fractions of sesquiterpenoids containing glucosyl moieties. The less polar fraction, after separation by semipreparative HPLC (H2O–MeOH, 1:1), furnished almost pure 3 (1.1 mg), a mixture of 1 and 2 (ca. 1:1, 12.9 mg), 11 (3.9 mg), 8 (1.2 mg), and a mixture of 9 and 10 (ca. 2.5:1, 1.4 mg), in that order. The medium polar fraction was processed by semipreparative HPLC (H2O–MeOH, 3:2) to yield 6 (3.8 mg) and additional amounts of 1 (2.0 mg), 2 (1.5 mg), and 11 (2.4 mg). The more polar fraction contained a mixture (13.8 mg) of 5 and 7 as shown by 1H-NMR. From the mixture, 3.6 mg and 4.4 mg of pure 5 and 7, respectively, could be separated by semipreparative HPLC (H2O–MeOH, 7:3). Further elution of the mother column with EtOAc–MeOH (19:1) gave fractions containing

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Table 1 1 H-NMR (500.13 MHz) data of 5 and Position Aglycone moiety 1 2 3 4 5 6 7 8a 8h 9a 9h 10 11 12 13 14 15 Glucosyl moiety 1 2 3 4 5 6 6V a b c d

13

C-NMR (125.76 MHz) data of 5 and 6 in pyridine-d 5

5, yH, J (Hz)

5, yC

6, yC

– – 6.23 – 3.41 4.23 2.60 – 4.31 2.85 3.19 – – – 2.20 2.62 2.17

133.43 195.29 135.09a 170.04 51.63 81.44 63.11

132.10 195.52 135.09a 170.03 52.19 82.79 59.63

75.67

21.26

46.65 146.57 74.74 177.12 24.74 21.42 19.65

37.31 151.72 78.07 175.63 17.83 21.49 19.58

106.08 75.32 78.78d 71.63 78.66d 62.78

99.28 74.71 78.84 71.64 77.84 62.84

br s br d (10.1) dd (10.1, 10.1) mb ddd (11.0, 10.0, 1.6) dd (13.5, 11.0) dd (13.5, 1.6)

s sb s

5.16 d (7.8) 4.08 dd (8.0, 7.8) 4.18–4.27 m c 4.18–4.27 m c 3.96 m 4.38 dd (11.5, 5.6) 4.52 br d (11.5)

Partially obscured by the signal of solvent. Signals partially overlapped. Signals fully overlapped. Values interchangeable.

phenolic glucosides, which were purified by preparative TLC (CHCl3–MeOH, 17:3, two developments) and further separated by semipreparative HPLC (H2O–MeOH, 4 :1) to give 15 (1.7 mg), 14 (1.2 mg), 12 (1.5 mg), and 13 (2.5 mg). 11h-Hydroxydeacetylmatricarin8-O-h-glucopyranoside (5). Colourless gum; [a]D24 13.9 (c 1.1, MeOH); ESI-MS m/z: 441 [M + H]+, 881 [2 M + H]+; UV E max (HPLC, on-line): 260 nm; 1H- and 13C-NMR data; see Table 1.

3. Results and discussion The dried plant material was extracted with ethanol, and the extract, after sequential fractionation on silica gel followed by semipreparative HPLC, gave a total of 11 sesquiterpene lactones and four phenolic compounds (Fig. 1). Isolated were three germacranolide acids esterified with glucose (1–3), deacetylmatricarin (4), its one new (5), and two known derivatives, i.e., 11h-hydroxyleucodin-11-O-h-glucopyranoside (6) and

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CO OGlc

OGlc

CO

523

R O

O

O

O

3

1 R = CH2 2 R = H, α-Me 14

O

H 10 9

2 3

1 4 5

15

GlcO

R

8 6

7

H

R1

11

O 12

H

R

O

13

O

O

4 5 6 7

1

R = OH, R = H 1 R = OGlc, R = OH 1 R = H, R = OGlc 1 R = OGlc, R = H

8 R = CH2 9 R = H, α-Me

GlcO O

GlcO

O O

O

10

MeO

11

3

2

GlcO

1

OH

OH GlcO

R

12 R = OMe, ∆ 13 R = OMe 14 R = H

2,3

15

Fig. 1. Chemical structures (Glc = h-glucopyranosyl) of compounds 1–15.

deacetylmatricarin-8-O-h-glucopyranoside (7), vernoflexuoside (8), its 11h,13-dihydroderivative (9) in a mixture with picriside B (10), and sonchuside A (11), along with syringin (12), dihydrosyringin (13), dihydroconiferin (14), and 2-(4V-h-glucopyranosyloxy)-phenylethanol (15). Compounds 1–4, 6–8, and 11–14 and the inseparable mixture of

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9 and 10 were previously isolated from Taraxacum spp. [3,9,10] and some other members of the tribe Lactuceae [11], respectively, in our laboratory. Their chemical structures were verified by direct comparison (HPLC, UV on-line, 1H-NMR and [a]D wherever possible) with reference compounds. The hitherto unreported 13C-NMR data of 6 are given in Table 1. The identity of 15 (icariside D2) was established on the basis of its spectral data and comparison with reported values [12]. To the best of our knowledge, this is the first report on the occurrence of the glucosides 9, 10, and 15 in Taraxacum spp. Structure 5 for the new natural product was readily established when its mass, and 1HNMR and 13C-NMR spectra were directly compared with those of 7. The ESI-MS showed ion peaks at m/z 441 [M + H]+ and m/z 881 [2 M + H]+, suggesting the molecular formula C21H28O10, which was 16 mass units higher than that of 7. The 1H-NMR and 13C-NMR spectra were similar to those of 7 but there were obvious differences. The C-11 proton signal was clearly absent and the C-13 methyl doublet was replaced by a methyl singlet, which appeared downfield at y2.20. Moreover, H-6, H-8, C-7, C-11, and C-13 signals were notably deshielded. From this comparison, it became apparent that 5 differed from 7 in that a h-oriented tertiary hydroxyl group was present at the C-11 position in 5. All proton signal assignments were supported by 1H–1H COSY correlations and the relative stereochemistry was shown to be the same as that of 7 based on NOESY results and optical rotation values. The NOESY spectrum verified the proximities of H-5 to H-7, H-9a and H-13, as well as H-8 to H-9h and H-14. It also confirmed the glucosylation at the C8 position by crosspeaks from the sugar anomeric proton doublet at y5.16 ( J = 7.8 Hz) to H-8 and H-9h resonances. The sign of the optical rotation value was also negative like that of 7. Based on these data, compound 5 was characterised as 11h-hydroxydeacetylmatricarin-8-O-h-glucopyranoside. The composition of the sesquiterpene lactones found in roots of T. hondoense is similar to that reported previously from T. obovatum [3], a species native to Sicily. Roots of both plant species accumulate compounds 1, 2, 4, and 6–8 as common constituents. The production of matricarin-type guaianolides appears to distinguish T. platycarpum [4], T. obovatum [3], and T. hondoense from other Taraxacum spp. investigated so far.

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