Angelica dahurica (Hoffm.) Benth. & Hook.f. ex Franch. & Sav. roots are commonly used in traditional Chinese medicine to treat common cold and headaches [1]. A number of chemical and biological studies have been conducted on the roots of this plant [2,3,4,5,6,7,8,9,10,11]. As part of an ongoing systematic phytochemical study about bioactive compounds in plants of the Apiaceae family from Korea, a detailed phytochemical study on the stem of A. dahurica was conducted to complement the phytochemical report on the same parts of this plant [12]. In this study, the isolation and structure elucidation of a new compound from the stem of A. dahurica are described.

The UV spectrum of compound 8 showed λmax values at 209, 228, 251, 257, and 296 nm, which suggested that this compound had a chromone skeleton [13]. The molecular formula of compound 8 was determined to be C20H22O8 by HR-EI-MS, the spectrum of which showed a molecular ion peak at m/z 390.1316 (calcd for 390.1315). The 1H NMR spectrum of compound 8 exhibted two singlets at δH 6.39 and 6.29, a hydroxymethyl signal at δH 4.46, a triplet at δH 5.24 (J = 4.8 Hz), two sets of double doublets at δH 3.01 (J = 17.6, 4.9 Hz) and 2.83 (J = 17.6, 4.6 Hz), and two methyl singlets at δH 1.40 and 1.38, which correlated with the carbon signals at δC 95.9, 106.5, 61.4, 71.1, 23.5, 25.0, and 23.7 in its HSQC spectrum. Furthermore, its 13C NMR spectrum showed a γ-pyrone carbonyl signal at δC184.2, an olefinic quaternary carbon at δC 171.9, five aromatic quaternary carbon signals at δC 160.7, 160.6, 157.5, 106.5, 105.6, and 103.9, and an oxygen-bearing aliphatic quaternary carbon signal at δC 78.4. Based on its NMR signals, which included a triplet at δH 5.24, two sets of double doublets at δH 3.01 and 2.83, two methyl singlets at δH 1.40 and 1.38, and carbon signals at δC 78.4, 71.1, 25.0, 23.7, and 23.5, compound 8 has a dimethyldihydropyran ring in its chromone skeleton [14]. In the HMBC spectrum of compound 8, the hydroxymethyl signal at δH 4.46 correlated with the signal at δC 171.9, indicating a hydroxymethyl moiety at its C-2 position. In addition, the NMR spectra of compound 8 indicated the presence of an acyl moiety at δH 6.35 (1H, q, J = 7.3 Hz), 4.14 (2H, m), and 1.90 (3H, d, J = 7.3 Hz) and δC 167.3, 140.3, 133.4, 64.7, and 15.6. These NMR signals were very similar to those of the angeloyl moiety, except for the presence of a hydroxymethyl group signal (δH 4.14; δC 64.7) instead of a methyl signal in the acyl moiety [15], which was confirmed by mass fragmentations at m/z 98 (C5H6O2) and 292 (M – C5H6O2). In the HMBC spectrum of compound 8, the acyl hydroxymethyl signal at δH 4.14 was correlated with the signals at δC 167.3, 140.3, and 133.4, indicating that the hydroxymethyl signal was attached at the α-position of its angeloyl group. This finding was confirmed using a NOESY spectrum analysis, which verified the correlation between the signals at δH 4.14 and 6.35 (Fig. 1).

Fig. 1
figure 1

Important HMBC and NOESY correlations of compound 8.

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The HMBC spectrum of the compound showed a correlation between the H-3′ signal at δH 5.24 and the acyl carbonyl signal at δC 167.3, which suggested that the acyl moiety of compound 8 was located at the C-3′ position of the dihydropyranochromone skeleton. The absolute configuration of the C-3′ position of compound 8 was determined as S from the negative value (–10.4) at 227 nm and the positive value (+9.6) at 255 nm in its CD spectrum [13]. These spectral data allowed us to establish the structure of compound 8 as (S)-5-hydroxy-8-(hydroxymethyl)-2,2-dimethyl-6-oxo-3,4-dihydro-2H,6H-pyrano[3,2-g]chromen-3-yl (Z)-2-(hydroxymethyl)but-2-enoate, which could not be found in structural databases, such as SciFinder and Google Scholar. Thus, we named compound 8 of dahuricalol.

Compounds 1–7 were identified as scopoletin [12], xanthyletin [16], xanthotoxin [17], hopeyhopin [18], thamnosmonin [19], 6-[(1S,2R)-2,3-dihydroxy-1-methoxy-3-methylbutyl]-7-methoxycoumarin [12], decursidate [16], respectively, by comparing their spectral data with those reported in the literature.

Experimental

General. UV spectra were obtained using a Jasco V-530 UV/Vis spectrophotometer (Jasco, Tokyo, Japan). Optical rotations were recorded using a Jasco DIP-100 digital polarimeter (Jasco, Tokyo, Japan). NMR spectra were obtained using a Bruker Avance Neo 600 instrument (Bruker, Rheinstetten, Germany). HR-EI-MS spectra were obtained using a JMS-700 mass spectrometer (Jeol, Tokyo, Japan). Silica gel (Merck Kieselgel 60, 63–200, and 40–63 μm), reversed-phase silica gel (YMC gel ODS-A, 150 μm), and Sphadex LH-20 (Amersham Pharmacia Biotech) were used for the column chromatography (CC). Extra pure solvents (DaeJung Co., Shiheung, Korea) were used for CC. TLC was performed on a glass backed Kieselgel 60 F254 and RP F254s plates and visualized by 20% (v/v) H2SO4.

Plant Material. The stems of A. dahurica were collected from Jungseon, Gangwondo Province in August 2016. A voucher specimen (KNUH-S-1608-3) was deposited in the Herbarium of the College of Pharmacy, Kangwon National University, Korea.

Extraction and Isolation. The A. dahurica stems (2.6 kg) were air-dried, cut into small pieces, and extracted three times with MeOH (14 L) at 80°C under reflux for 4 h. All extracts were filtered and concentrated in vacuo at 40°C. The MeOH extract (311 g) was suspended in water and successively partitioned with n-hexane, CHCl3, and n-BuOH, leaving a residual water-soluble fraction. The n-hexane, CHCl3, and n-BuOH soluble fractions were evaporated in vacuo to yield the residues of the n-hexane (20 g), CHCl3 (60 g), and n-BuOH (22 g) fractions. The CHCl3 fraction (58 g) was subjected to silica gel CC with stepwise gradient elution using an CHCl3–MeOH system (19:1–1:1) to obtain five fractions (ADC-1–5). Fraction ADC-1 (1.1 g) was re-chromatographed on silica gel and ODS CC to obtain compounds 1 (105.7 mg), 2 (6.6 mg), and 3 (3.6 mg). Fraction ADC-2 (8.4 g) was fractionated using silica gel CC to obtain compound 4 (87.4 mg). The ADC-3 fraction (32 g) was re-chromatographed over ODS CC isocratic elution using MeOH–H2O (60:40) to obtain five fractions (ADC-3-1–3-5). Fraction ADC-3-1 (0.5 g) was subjected to silica gel CC with isocratic elution using n-hexane–EtOAc (1:1) to obtain compounds 5 (20.7 mg) and 6 (74.9 mg). Fraction ADC-4 (3.1 g) was further fractionated using silica gel with benzene–EtOAc (2:1) to obtain four fractions (ADC-4-1–4-4). Fraction ADC-4-2 (1.2 g) was re-chromatographed over ODS CC isocratic elution with MeOH–H2O (55:45) to obtain compound 7 (55.4 mg). Fraction ADC-4-4 (0.2 g) was purified by passing it through Sephadex LH-20 (MeOH–H2O, 50:50) to obtain compound 8 (33 mg).

Dahuricalol (8), white powder, [α]20D –21° (c 0.15, MeOH). UV (MeOH, λmax, nm): 209, 228, 251, 227, 296. CD (c 0.1, MeOH, λmax, nm) (Δε): 227 (–10.4), 255 (+9.6). 1H NMR (600 MHz, MeOH-d4, δ, ppm, J/Hz): 6.39 (1H, s, H-8), 6.35 (1H, q, J = 7.3, β-H), 6.29 (1H, s, H-3), 5.24 (1H, t, J = 4.8, H-3′), 4.46 (2H, s, H-11), 4.14 (2H, m, H-9′), 3.01 (1H, dd, J = 17.6, 4.9, Hb-4′), 2.83 (1H, dd, J = 17.6, 4.6, Ha-4′), 1.90 (3H, d, J = 7.3, H-8′), 1.40 (3H, s, CH3-2′), 1.38 (3H, s, CH3-2′). 13C NMR (150 MHz, MeOH-d4, δ, ppm): 184.2 (C-4), 171.9 (C-2), 167.3 (5′-C=O), 160.7 (C-7), 160.6 (C-5), 157.5 (C-9), 140.3 (C-7′), 133.4 (C-6′), 106.5 (C-3), 105.6 (C-10), 103.9 (C-6), 95.9 (C-8), 78.4 (C-2′), 71.1 (C-3′), 64.1 (C-9′), 61.4 (C-11), 25.0 (CH3-2′), 23.7 (CH3-2′), 23.5 (C-4′), 15.6 (C-8′). HR-EI-MS m/z 390.1316 (calcd for C20H22O8, 390.1315); EI-MS m/z (Irel., %): 390 [M]+ (2.2), 292 (4.5), 274 (46.4), 259 (100), 257 (75), 252 (14), 250 (8.8), 243 (9.5), 221 (16.9), 203 (5.1), 129 (3.6), 115 (3.5), 98 (5.3), 83 (3.0).