Dandelion, plants of Taraxacum species, is a perennial herb of the Asteraceae family. They are widely distributed throughout the world and have long been used as medicinal herbs and nutritious foods due to their health promoting effects. Previous studies have reported the antioxidant, anticancer, antibacterial, and anti-inflammatory effects of Taraxacum species [1,2,3]. Phenolic compounds, inositol derivatives, and sesquiterpenes have been isolated from Taraxacum species [4,5,6,7]. Recently, the roots of Taraxacum species have been suggested as a good source of sesquiterpene lactones and as active principles of this species [4,5,6,7,8]. Phytochemical investigation on the roots of T. coreanum Nakai afforded four sesquiterpene lactones 1–4, including two newly reported ones.

Compound 2 was purified as a colorless oil, and its molecular formula was determined as C23H32O10 from the HR-ESI-MS (m/z 491.18877 [M + Na]+, calcd for C23H32NaO10, 491.1893) and 13C NMR data. The 1H and 13C NMR spectra (Table 1) suggested the presence of a glucose moiety at [δH 5.55 (1H, d, J = 8.0 Hz, H-1′), 3.35–3.45 (3H, m, H-2′, 3′, 4′), 3.59 (1H, m, H-5′), 4.20 (1H, dd, J = 12.4, 6.0 Hz, H-6′a) and 4.41 (1H, dd, J = 12.4, 2.0 Hz, H-6′b); δC 94.0 (C-1′), 72.6 (C-2′), 77.4 (C-3′), 69.8 (C-4′), 74.7 (C-5′), and 63.1 (C-6′)] and an acetyl moiety at [δH 2.06 (3H, s, COCH3); δC 19.3 (COCH3), 171.2 (COCH3)], which were supported by HMBC correlations. The configuration of the glucose moiety was determined to be β based on the coupling constant of J = 8.0 Hz. Moreover, the deshielded resonance of H-6′ [δ 4.20 and 4.41] and the HMBC correlation between H-6′ and acetyl group indicated the presence of a (6′-O-acetyl)-glucose moiety. The remaining 15 carbons in the 13C NMR spectrum together with HMBC and COSY correlations, which were similar to those of 11,13-dihydrotaraxinic acid (1), suggested that 2 is a gemacranolide-type sesquiterpenoid with a 10-membered ring. Taken together, compound 2 was suggested to be 11,13-dihydrotaraxinic acid with a β-(6′-O-acetyl)-glucose moiety. Further HMBC correlation from H-1′ to C-14 confirmed the connection of the 6′-O-acetyl-glucose moiety to 11,13-dihydrotaraxinic acid. The relative stereochemistry was determined by NOESY correlations, in which cross-peaks from H-6 to H-11 and CH3-15, and from H-5 to H-1 and H-7, confirm the β-orientation of H-6 and H-11. Thus, the structure of compound 2 was elucidated as 11β,13-dihydrotaraxinic acid β-(6-O-acetyl)-glucopyranosyl ester and named acetyldihydrotaraxinolide.

Fig. 1.
figure 1

Key COSY, HMBC, and NOESY correlations of compounds 2 and 4.

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Table 1. 1H (500 MHz) and 13C (125 MHz) NMR Data of Compounds 2 and 4 (CD3OD, δ, ppm, J/Hz)
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Compound 4 was purified as a colorless oil, and its molecular formula was determined as C23H30O10 from the HR-ESI-MS m/z 489.173118 [M + Na]+ (calcd for C23H30NaO10, 489.1737) and 13C NMR data. The 1H and 13C NMR spectra of 4 (Table 1) was similar to those of 2, which suggested that 4 is also a germacranolide glycoside. In the 1H NMR spectra of 4, methyl signals at C-13 in 2 were replaced by exomethylene signals [δ 5.63 (1H, d, J = 3.2 Hz), 6.19 (1H, d, J = 3.2 Hz)]. In addition, the downfield shift of C-15 from δ 15.7 to δ 23.3 suggested the Z-geometry of C-4/C-5 [9].

The NOESY correlations between H-1, H-6, and H-7 confirmed their existence on same side, and further correlation between H-5 and CH3-15 suggested the α-orientation of H-6 and H-7. Based on the above evidence, the structure of compound 4 was elucidated as shown and named acetyltaraxinolide.

Two known compounds were identified as 11β,13-dihydrotaraxinic acid (1) and taraxinic acid β-(6-O-acetyl)-glucopyranosyl ester (3) by comparison with literature values [7].

Experimental

General. NMR spectra were recorded on a Bruker DRX 500 MHz NMR spectrometer. ESI-mass spectra were obtained on a VG Autospec Ultima mass spectrometer. Semipreparative HPLC was performed using a Waters HPLC system equipped with Waters 600 Q-pumps, a 996 photodiode array detector, and Waters Empower software using Gemini-NX ODS-column (5 μm, 10 × 150 mm). Silica gel (70–230 mesh, Merck, Germany) and Sephadex LH-20 (25–100 μm, Amersham Biosciences, Sweden) were used for open column chromatography. Thin-layer chromatography (TLC) was performed on precoated silica gel 60 F254 (0.25 mm, Merck, Germany). All other chemicals and reagents were of analytical grade.

Isolation of Compounds. The roots of T. coreanum were obtained from the local herbal market, Chungbuk, Korea in April 2015 and were identified by the Herbarium of the College of Pharmacy at Chungbuk National University, where a voucher specimen was deposited (CBNU201504-TC). The dried roots of T. coreanum (5.0 kg) were extracted twice with 80% MeOH, which yielded the total extract (1.2 kg). The total extract was then suspended in H2O. Further successive partitioning with n-hexane, CH2Cl2, and EtOAc yielded the n-hexane, CH2Cl2, and EtOAc soluble fractions.

The CH2Cl2 fraction (TCM, 8.5 g) was subjected to silica gel column chromatography with a mixture of CH2Cl2–MeOH to give 12 fractions (TCM1–TCM12). Fraction TCM5 was subjected to silica gel column chromatography with the mixture of n-hexane–EtOAc to give 14 fractions (TCM5A–TCM5N). Fraction TCM5N was partitioned into 5 fractions (TCM5N1–TCM5N5) by Sephadex LH-20 eluting with 100% MeOH. Compound 1 (25.2 mg) was obtained from Fr. TCM5N3 by semipreparative HPLC eluting with CH3CN–H2O (30:70). The EtOAc fraction (TCE, 8.8 g) was subjected to MPLC with silica gel using a gradient mixture of n-hexane–EtOAc–MeOH to give 13 fractions (TCE1–TCE13). Fraction TCE9 was subjected to MPLC with RP-silica using a mixture of n-hexane–EtOAc to give five fractions (TCE9A–TCE9E). Compounds 2 (2.4 mg), 3 (1.7 mg), and 4 (0.9 mg) were purifed from Fr. TCE9E by semipreparative HPLC eluting with CH3CN–H2O (20:80).

Acetyldihydrotaraxinolide (2), brown syrup, \( {\left[\upalpha \right]}_{\mathrm{D}}^{25} \) –14.0° (c 0.03, MeOH). IR (νmax, cm–1): 1735, 3332. 1H NMR (500 MHz, CD3OD) and 13C NMR (125 MHz, CD3OD), see Table 1. ESI-MS (positive mode) m/z 491 [M + Na]+; HR-ESI-MS (positive mode) m/z 491.1888 [M + Na]+ (calcd for C23H32NaO10, 491.1893).

Acetyltaraxinolide (4), brown syrup, \( {\left[\upalpha \right]}_{\mathrm{D}}^{25} \) –7.33° (c 0.03, MeOH). IR (νmax, cm–1): 1673, 3370. 1H NMR (500 MHz, CD3OD) and 13C NMR (125 MHz, CD3OD), see Table 1. ESI-MS (positive mode) m/z 489 [M + Na]+; HR-ESI-MS (positive mode) m/z 489.1731 [M + Na]+ (calcd for C23H30NaO10, 489.1737).