A New Arylnaphthalide Lignan From Oxalis corniculata

Abstract

A new arylnaphthalide lignan, corniculin (1), was isolated from 75% EtOH extract of Oxalis corniculata. Its chemical structure was determined by mass spectrometric analysis as well as 1D and 2D nuclear magnetic resonance (NMR).

Oxalis corniculata L. (Oxalidaceae) is a commonly used herb with a range of biological activities including the effects of anti-oxidation, antitumor, and anti-inflammation. ^{1 -5} The previous studies show that flavonoids, phenolic acids, and alkaloids were major constituents of O. corniculata. ^{6 -11} In our efforts toward the clarification of the chemical constituents of O. corniculata, a new arylnaphthalide lignan named corniculin (1) (Figure 1) was isolated. Here, we report its isolation, structure identification, and cytotoxicity activity.

Figure 1

The structure of 1.

Corniculin (1) was isolated as a yellowish amorphous solid. The molecular formula was determined to be C₂₁H₁₈O₆ by the high resolution electrospray ionization mass spectrometry (HR-ESI-MS) spectrum (Figure S1) (quasi-molecular ion peak at m/z 367.1204, calcd for [M+H]⁺ 367.1182 in the positive mode). The ¹H- and ¹³C-NMR spectra (Figures S2 and S3) showed the typical signals of the arylnaphthalene lignan. The ¹H-NMR spectrum (Figure S2) showed the presence of a 1,2,4-trisubstituted benzene ring [δ _H 7.08 (1H, d, J = 8.4 Hz), 6.89 (1H, dd, J = 8.4, 1.6 Hz), and 6.85 (1H, d, J = 1.6 Hz)], 3 isolated aromatic proton [δ _H 8.28 (1H, s), 7.28 (1H, s), and 7.10 (1H, s)], a γ-lactone methylene [(δ _H 5.23 (1H, d, J = 14.8 Hz) and 5.19 (1H, d, J = 14.8 Hz)], and 3 methoxy groups [δ _H (4.03, 3H, s), (3.89, 3H, s), and (3.82, 3H, s)]. These observations suggested that 1 was likely a 4-aryl-2,3-naphthalide lignan. The ¹³C-NMR spectrum (Figure S3) showed 21 signals, including a lactone carbonyl (δ _c 171.7), a γ-lactone methylene (δ 69.6), 16 aromatic signals (δ _c 151.9, 150.0, 146.9, 145.6, 137.9, 132.3, 131.7, 129.9, 128.0, 124.0, 122.3, 121.3, 115.1, 111.4, 107.6, and 104.1), and 3 methoxy groups (δ 56.1, 56.1, and 55.9). ¹H- and ¹³C-NMR data of 1 (Table 1) were similar to those of detetrahydroconidendrin reported in the literature. ¹² The slight difference only lies in the presence of an additional methoxy group in 1. The positions of the methoxy groups were further confirmed by the ¹H detected heteronuclear multiple bond connectivity (HMBC) correlations (Figure S5) of 6-OCH₃ (δ _H 4.03)/C-6(δ _C 150.0), H-5(δ _H 7.10)/C-6, 7-OCH₃(δ _H 3.82)/C-7(δ _C 151.9), H-8(δ _H 7.28)/C-7, 3′-OCH₃(δ _H 3.89)/C-3′(δ _C 146.9), H-5′(δ _H 7.08)/C-3′, and H-6′(δ _H 6.89)/C-4′(δ 145.6) (Figure 2), which suggested that 3 methoxy groups were connected at C-6, C-7, and C-3′, respectively. Their ¹H- and ¹³C-NMR data (Table 1) were assigned explicitly by ¹H detected heteronuclear multiple-quantum coherence (HMQC) (Figure S4) and HMBC (Figure S5). Therefore, 1 was determined to be 4-(3′-methoxyl-4′-hydroxyl)-phenyl-6,7-dimethoxy-2,3-naphthalide, namely corniculin.

Figure 2

Key HMBC (H→C) correlations of 1.

Table 1

¹H (400 MHz, CDCl₃) and ¹³C (100 MHz, CDCl₃) NMR Data for 1.

Position	δ H	δ C
1	8.28 (1H, s)	124.0
2		121.3
3		137.9
4		132.3
5	7.10 (1H, s)	104.1
6		150.0
7		151.9
8	7.28 (1H, s)	107.6
9		129.9
10		131.7
11		171.7
12	5.23 (1H, d, J = 14.8 Hz)	69.6
	5.19 (1H, d, J = 14.8 Hz)
1′		128.0
2′	6.85 (1H, d, J = 1.6 Hz)	111.4
3′		146.9
4′		145.6
5′	7.08 (1H, d, J = 8.4 Hz)	115.1
6′	6.89 (1H, dd, J = 8.4, 1.6 Hz)	122.3
6-OCH3	4.03 (3H, s)	56.1
3′-OCH3	3.89 (3H, s)	56.1
7-OCH3	3.82 (3H, s)	55.9

In vitro cytotoxicity of corniculin was then evaluated by the methy thiazolyl tetrazolium (MTT) assay against 4 cancer cell lines (SMMC-7721, A549, HCT-15, and MCF-7). The new compound exhibited moderate cytotoxicity against SMMC-7721, MCF-7, HCT-15, and A549 cell lines, with IC₅₀ values of 29.0, 35.6, 31.3, and 25.7 µM, respectively, which were comparable to that of the positive control cisplatin (Table 2).

Table 2

Comparison of IC₅₀ Values of Compound 1 and Cisplatin on SMMC-7721, MCF-7, HCT-15, and A549.

Compound	SMMC-7721	MCF-7	HCT-15	A549
1	29.0 ± 0.5	35.6 ± 5.9	31.3 ± 1.7	25.7 ± 10.3
Cisplatin	34.9 ± 4.3	32.6 ± 2.6	35.7 ± 0.1	34.4 ± 11.0

Experimental

General

HR-ESI-MS was measured on a Bruker Daltonics microTOF-Q II mass spectrometer (Bruker Daltoniks GmbH, Germany). NMR was recorded on a JEOL ECS 400 NMR spectrometer, with δ in ppm and tetramethyl silane (TMS) and solvent residue (for CDCl₃) as internal reference (JEOL, Japan). D-101 macroporous resin (Tianjin Haiguang Chemical Co., Ltd., China), normal-phase silica gel (200-300 and 300-400 mesh, Qingdao Haiyang Chemical Co., Ltd., China), and Sephadex LH-20 (Pharmacia Biotech, Switzerland) were used. Thin layer chromatography (TLC) was run on a prefabricated silica gel GF₂₅₄ (Qingdao Haiyang Chemical Co., Ltd., China).

Plant Material

The whole plant of O. corniculata was collected in June and supplied by Guizhou Wei Kang Zi Fan Pharmaceutical Co., Ltd., which was identified by Professor Qing-De Long of Guizhou Medical University. A voucher specimen (No. 20170601) was stored in the Herbarium of the Department of Guizhou Provincial Key Laboratory of Pharmaceutics, Guizhou Medical University.

Extraction and Isolation

The air-dried whole plant of O. corniculata (15 kg) was chopped and refluxed with 75% EtOH (120 L, 1.5 hours × 3). The combined EtOH extract was concentrated to yield a residue (2.1 kg) under reduced pressure. The resulting residue was then subjected to D-101 macroporous resin column (22 × 80 cm) and eluted successively by H₂O, 50% EtOH, and 95% EtOH. The 50% EtOH eluate and 95% EtOH eluate were concentrated to give a brown syrup, respectively. The combined brown syrup (130 g) was applied to a normal-phase silica gel column (10 × 80 cm, 200-300 mesh) and eluted accordingly with chloroform-methanol (50:1, 20:1, 15:1, 10:1, 8:1, 6:1, 4:1, 2:1, and 0:1, v/v) to give 12 fractions (Fr.1-12). Fr.9 was purified successively by the first normal-phase silica gel column (4 × 60 cm, 300-400 mesh, chloroform-methanol, 9:1-1:1, v/v), the secondary normal-phase silica gel column (3 × 40 cm, 300-400 mesh, petroleum ether-acetone, 8:2, v/v) and the final Sephadex LH-20 column (1.5 × 120 cm, methanol) to give 1 (5.4 mg).

Corniculin (1)

Yellowish amorphous solid.

¹H- and ¹³C-NMR (CDCl₃): Table 1.

HR-ESI-MS m/z: [M+H]⁺ calcd. for C₂₁H₁₉O₆: 367.1204; found: 367.1182.

Supplemental Material

Supplemental material - Supplemental material for A New Arylnaphthalide Lignan From Oxalis corniculata

Supplemental material, Supplemental material, for A New Arylnaphthalide Lignan From Oxalis corniculata by Bao Zhang, Li Jiang, Xue Ma, Ai-Min Wang, Ting Liu, Meng Zhou, Shang-Gao Liao, Yong-Lin Wang, Yong Huang and Yong-Jun Li in Natural Product Communications