A New Anthraquinone From the Roots of Senna Obtusifolia

Abstract

Background

Senna obtusifolia is a widely used medicinal plant rich in anthraquinones, yet the chemical constituents of its roots remain insufficiently characterized compared with its seeds and aerial parts. To expand the phytochemical understanding of this species, the ethyl acetate extract of naturally grown roots was investigated.

Methods

The EtOAc extract of S. obtusifolia roots was subjected to repeated chromatographic separation, and the structures of the isolated compounds were elucidated using NMR and MS techniques.

Results

One new anthraquinone, 6-dehydroxy-9-O-methylviocristin (1), together with 13 known anthraquinones (2–14) and two known diterpenoids (15 and 16), was isolated. Compound 1 possesses a 2-methyl-1,4-anthracenedione skeleton and is characterized by O-methylation at C-8 and C-9, representing a structurally plausible O-methylated derivative of related anthraquinones previously reported from S. tora.

Conclusions

This study provides the systematic characterization of anthraquinones from the roots of S. obtusifolia, revealing distinct oxidation and methylation patterns compared with aerial organs. These findings contribute to the chemotaxonomic understanding of the Senna genus and offer new insights into the biosynthetic diversity of root-derived anthraquinones.

Keywords

anthraquinones roots polyketides

1. Introduction

Senna obtusifolia (L.) H.S.Irwin & Barneby (syn. Cassia obtusifolia, Fabaceae) is widely used in traditional medicine, and its seeds (“Ketsumeishi”) are listed in the Japanese Pharmacopoeia, Chinese Pharmacopoeia, and South Korean pharmacopoeia.^1,2 S. obtusifolia is a leguminous annual herb that grows in various warm temperate regions including Asia.³ Numerous phytochemical studies have been conducted on the seeds, leaves, and sprouts, leading to the identification of diverse anthraquinones, naphthopyrones, and related phenolic metabolites.^2,4,5 These constituents have been associated with various biological activities, including α-glucosidase inhibitory,⁵ anti-inflammatory,⁶ hepatoprotective effects.⁶ In contrast to the extensive investigations of the aerial parts, chemical studies on the roots of S. obtusifolia are extremely limited.^7,8 Although these studies provided important initial insights, their scope was restricted by extraction methods, culture conditions, and analytical limitations of the time. Consequently, the anthraquinone diversity of naturally grown roots remains far from fully characterized, particularly with respect to oxygenation and methylation patterns.⁹ Moreover, plant organs often exhibit distinct metabolic pathways, and anthraquinone-producing species are known to generate organ-specific oxidative or methoxylated derivatives.¹⁰

In an attempt to fill this gap, we investigated the methanolic extract of S. obtusifolia roots and report here the isolation and structural elucidation of a new anthraquinone, 6-dehydroxy-9-O-methylviocristin (1), together with 13 known anthraquinones (2–14), two known diterpenoids (15 and 16).

2. Materials and Methods

2.1. General Experimental Procedures

Specific rotations were measured using a P-2200 digital polarimeter (l = 5 cm; JASCO, Tokyo, Japan). Fourier-transform infrared (FTIR) spectra were recorded on a JASCO FT/IR-4600 spectrometer. Ultraviolet–visible (UV-vis) spectra were acquired on a Shimadzu UV-1850 spectrophotometer (Shimadzu, Kyoto, Japan). High-resolution electrospray-ionization mass spectrometry (HR-ESI-MS) was performed using a JMS-T100LP AccuTOFLC-Plus 4G instrument (JEOL, Tokyo, Japan). ¹H (600 MHz), ¹³C (150 MHz), and 2D nuclear magnetic resonance (NMR) spectra were recorded using a JEOL JNM-ECZ 600R spectrometer. High-performance liquid chromatography (HPLC) was performed using an SPD-10Avp UV-vis detector (Shimadzu, Kyoto, Japan), an LC-10ADvp pump (Shimadzu), an SCL-10Avp system controller (Shimadzu), and LabSolutions LC software (ver. 1.25, Shimadzu) along with two COSMOSIL 5C₁₈-AR-II columns (Nacalai Tesque), 250 × 4.6 mm i.d., and 250 × 10 mm i.d., and two Luna Phenyl-Hexyl columns (Phenomenex, CA, USA), 250 × 4.6 mm i.d., and 250 × 10 mm i.d., for analytical and preparative purposes, respectively. Solvent ratios are based on volume. IR, UV, and MS spectra were recorded using spectroscopic-grade methanol (Wako Pure Chemical, Japan).

2.2. Plant Material

S. obtusifolia seeds were purchased from KANEKO SEEDS (Gunma, Japan; voucher specimen number: SOCU-COS-1).⁵ The seeds were sown in the herbarium of the Department of Pharmacognosy, Faculty of Pharmaceutical Sciences, Sanyo-Onoda City University, Yamaguchi, Japan in May 2023, and the roots used in this study were harvested in February 2024. Voucher specimens were deposited in the herbarium of the Department of Pharmacognosy, Faculty of Pharmaceutical Sciences, Sanyo-Onoda City University. The plant material was examined and identified by Prof. Hiroyuki Tanaka (PhD), Department of Pharmacognosy and Kampo, Faculty of Pharmaceutical Sciences, Sanyo-Onoda City University. The deposited voucher specimens (voucher specimen number: SOCU-2024-1), including roots, flowers, seeds, and leaves, were shade-dried and stored (Figure S1.1. 2, and 3).

2.3. Extraction and Isolation

Dried roots of S. obtusifolia (6.0 kg) were extracted one time with ethyl-acetate (EtOAc) for 24 h at room temperature. The EtOAc-soluble extract (47.0 g, 0.8%) was subjected to normal-phase silica-gel column chromatography [1.5 kg, n-hexane:CHCl₃ (1:0→ 1:1→ 1:1) → CHCl₃→ CHCl₃:MeOH (100:1→ 50:1→ 30:1→ 20:1→ 10:1→ 5:1→ 0:1)]. S. obtusifolia EtOAc-soluble fraction number five (SOEA3) (1.6 g) was further separated via reverse-phase silica-gel column chromatography [100.0 g, MeOH:H₂O (5:5→ 6:4→ 7:3→ 8:2→ 9:1→ 1:0)] to give eight fractions (SOEA3-1–3-7). Subfraction SOEA3-5 (60.0 mg) was purified via HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O/MeCN/AcOH (60:40:0.3)] to obtain four subfractions (Fr. SOEA3-5-1–3-5-4). Compound 16 was obtained as a yellow amorphous solid after purifying subfraction SOEA3-5-3 (3.5 mg) by HPLC [Luna Phenyl-Hexyl column, H₂O:MeCN:AcOH (40:60:0.3)]. Compounds 2 and 3 were obtained as yellow and orange amorphous solids after purifying subfraction SOEA3-7 (280.0 mg) by HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O:MeCN:AcOH) (30:70:0.3)]. Compounds 4 and 5 were obtained as red amorphous solids after purifying subfraction SOEA3-7-2 (20.6 mg) by HPLC [Luna Phenyl-Hexyl column, H₂O:MeCN:AcOH) (30:70:0.3)]. Compound 15 was obtained as yellow amorphous solids after purifying fraction SOEA4-8 (100.0 mg) by HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O:MeCN:AcOH) (40:60:0.3)]. Subfraction SOEA5-4 (177.1 mg) was purified via HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O/MeCN/AcOH (70:30:0.3)] to obtain four subfractions (Fr. SOEA5-4-1–5-4-10). Compound 11 was obtained as a yellow amorphous solid after purifying subfraction SOEA5-4-2 (78.0 mg) by HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O:MeCN:AcOH (70:30:0.3)]. Subfraction SOEA5-4-2 was purified via HPLC to obtain four subfractions (Fr. SOEA5-4-2-1–5-4-2-9). Compounds 10 and 14 were obtained as red and yellow amorphous solids after purifying subfraction SOEA5-4-2-3 (29.4 mg) by HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O:MeCN:AcOH (65:35:0.3)]. Compounds 1, 6, 8, 9, 12 and 13 were obtained as red and yellow amorphous solids after purifying subfraction SOEA5-4. Compound 7 was obtained as yellow amorphous solids after purifying fraction SOEA5-5 (502.7 mg) by HPLC [COSMOSIL 5C₁₈-AR-II column, H₂O:MeCN:AcOH) (55:45:0.3)]. See Figure 1 for the structures of the isolated compounds (1–16). These fractionation and chromatography processes were performed by slightly modifying the reported protocol.¹¹

Figure 1.

Skeletal (shorthand) structures of compounds isolated from Senna obtusifolia roots

2.4. 6-Dehydroxy-9-O-Methylviocristin (1)

Red amorphous solid; FTIR (ATR, CH₃CN) v_max 1555, 1638, 2025, 2152, 2200, and 2360 cm⁻¹; UV (MeOH) λ_max (logε) 244.0 nm (4.19), 439.0 nm (3.21); ¹H NMR (chloroform-d, 600 MHz) and ¹³C NMR (chloroform-d, 150 MHz) (Table 1); HR-ESI-MS m/z: 305.0792 (Calcd. for C₁₇H₁₄O₄Na [M+Na]⁺ m/z: 305.0784.

Table 1.

¹H (600 MHz) and ¹³C (150 MHz) NMR Data of 6-Dehydroxy-9-O-Methylviocristin (1) in Chloroform-d

Position	1
Position	δ_C	δ_H (J in Hz)
1	185.4
2	139.8	6.83 (q-like, 1.2)
3	146.5
4	183.5
4a	129.9
5	109.7	7.05 (dd, 2.1, 7.2)
5a	138.2
6	130.3	7.58 (t-like, 7.2)
7	123.1	7.60 (m)
8	158.6
8a	123.2
9	160.5
9a	120.0
10	125.5	8.38 (s)
3-Me	16.0	2.18 (d, 1.2)
8-OMe	56.5	4.04 (s)
9-OMe	62.8	3.99 (s)

3. Results

3.1. Isolating Anthraquinones and Diterpenoids From S. obtusifolia

An EtOAc extract was obtained from the roots of S. obtusifolia. The EtOAc soluble fraction was subjected to normal- and reversed-phase silica gel column chromatography, followed by HPLC, affording a new anthraquinone, 6-dehydroxy-9-O-methylviocristin (1, 3.3 mg). Furthermore, 13 known anthraquinones were also isolated: chrysophanol (2, 125.7 mg),¹² physcion (3, 18.7 mg),¹³ helminthosporin (4, 1.8 mg),¹⁴ islandicin (5, 3.1 mg),¹⁵ 2-methoxy chrysophanol (6, 4.3 mg),¹⁶ 8-O-methylchrysophanol (7, 61.2 mg),¹⁷ 1,8-di-O-methylchrysophanol (8, 16.9 mg),¹⁸ obtusifolin (9, 1.9 mg),¹⁹ 1, 7, 8-methoxyl-2-hydroxyl-3-methyl-anthraquinone (10, 2.1 mg),²⁰ aurantioobtusin (11, 54 mg),²¹ chrysoobtusin (12, 3.6 mg),²¹ obtusin (13, 4 mg),²¹ 8-hydroxy 1,2,6,7-tetramethoxy-3-methylanthraquinone (14, 1.8 mg),²² and two known diterpenoids were also isolated: suginol methyl ether (15, 1.6 mg)²³ and cryptojaponol (16, 2.1 mg)²⁴ (see Figure 1 for compound structures).

3.2. Structure of 6-Dehydroxy-9-O-Methylviocristin (1)

6-Dehydroxy-9-O-methylviocristin (1) was isolated as a red amorphous solid. Its molecular formula (C₁₇H₁₄O₄) was determined by HR-ESI-MS; m/z: 305.0792 (Calcd. for C₁₇H₁₄O₄Na [M+Na]⁺ m/z: 305.07843 and ¹³C NMR spectroscopy. The 1D NMR data (Table 1) showed that 1 possesses a carbon skeleton similar to that of viocristin, with some differences in the substitution pattern.²⁵ The ¹H and ¹³C NMR spectra (chloroform-d) exhibit characteristic signals for a anthraquinone group (Table 1), including five aromatic groups [δ_H 6.83 (q-like, H-2), 7.05 (dd, H-5), 7.58 (t-like, H-6), 7.60 (m, H-7), and 8.38 (s, H-10)], two quinone carbons [δ_C 185.4 (C-1) and 183.5 (C-4)], one methyl group [δ_H 2.18 (d, 3-Me) and δ_C 16.0 (3-Me)], two methoxy groups [δ_H 4.04 (s, 8-OMe), 3.99 (s, 9-OMe), δ_C 56.5 (8-OMe), and 62.8 (s, 9-OMe)]. The position of each functional group was determined using correlation spectroscopy (COSY) and heteronuclear multiple bond correlation (HMBC) NMR spectroscopy (Figure 2). In particular, long-range correlations were observed between the following proton and carbon pairs in 1: H-2/C-1 and C-9; H-5/C-7, and C-8a; H-6/C-8 and C-5a; H-7/C-5 and C-8; H-10/C-4a, C-5a, C-8a, and C-9a; 3-Me/C-2, C-3, and C-4; 8-OMe/C-8; and 9-OMe/C-9. The chemical structure of 1 was established based on the evidence provided above (Figure 1).

Figure 2.

Important 2D NMR correlations for 6-dehydroxy-9-O-methylviocristin (1)

4. Discussion

Plants of the Senna genus are widely distributed across tropical and subtropical regions worldwide, including warm temperate areas of Asia, Africa, the Americas, and Australia.³ Plants of the Senna genus produce a diverse array of secondary metabolites, including anthraquinones, naphthopyrones, naphthalene derivatives, flavones, and other phenolic constituents, with anthraquinones representing the most abundant and chemotaxonomically important class of compounds in S. obtusifolia and S. tora.² S. obtusifolia is closely related to S. tora, and both species accumulate abundant anthraquinones in their seeds, whereas chemical studies on the roots remain extremely limited.^2,7,8 Elucidating the oxidation and methylation patterns of root-derived anthraquinones is therefore essential for understanding the chemical diversity and biosynthetic features of the Senna genus.

In this study, one new anthraquinone, 6-dehydroxy-9-O-methylviocristin (1), together with thirteen known anthraquinones (2–14) and two known diterpenoids (15 and 16), was isolated from the EtOAc extract of S. obtusifolia roots, and their chemical structures were elucidated. The new compound 1 is characterized by O-methylation at C-8 and C-9 and possesses a 2-methyl-1,4-anthracenedione skeleton,²⁶ a structural feature shared with viocristin-type anthraquinones. Moreover, its structure is closely related to 10-hydroxy-5-methoxy-2-methyl-1,4-anthracenedione, which has been reported from the seeds of S. tora (Figure 3).²⁷ This suggests that compound 1 may represent a further O-methylated derivative formed through the same biosynthetic pathway. Such structural relationships are consistent with recent findings that anthraquinone biosynthesis in Senna species proceeds via type III polyketide synthases (PKSs), followed by oxidative and O-methylation steps that diversify the final metabolites.⁹ This suggests that compound 1 may represent a further O-methylated derivative formed through the same biosynthetic pathway (Figure 3).

Figure 3.

The proposed biosynthetic pathway of the new compound 1 and related compounds. A linear polyketide derived from acetyl-CoA and malonyl-CoA undergoes alternative cyclization and subsequent CYP- and OMT-mediated tailoring reactions, leading to 1,4- and 9,10-anthraquinone derivatives

The anthraquinone profile obtained in this study includes major constituents commonly found in S. tora and S. occidentalis seeds, such as chrysophanol, physcion, obtusifolin, and aurantioobtusin, while also revealing O-methylated derivatives such as 1,8-di-O-methylchrysophanol and 2-methoxy chrysophanol, which are considered characteristic of S. obtusifolia.^3,9,10 These compounds, together with naphthopyrones such as rubrofusarin reported in Senna species, belong to biosynthetically related originating from PKS-derived polyketides.⁹ The present results indicate that S. obtusifolia roots exhibit oxidation and methylation patterns distinct from those of aerial parts, supporting the presence of organ-specific metabolic pathways.

A limitation of this study is that the small quantities of isolated compounds precluded more detailed biological evaluations. Nevertheless, this work represents the systematic characterization of anthraquinones from the roots of S. obtusifolia, providing new insights into the chemotaxonomy and biosynthetic diversity of the Senna genus. Future studies examining variations in anthraquinone composition across developmental stages and plant organs (roots, seeds, leaves, sprouts), as well as gene expression analyses of PKSs and O-methyltransferases, will further clarify the chemical characteristics and biosynthetic pathways of S. obtusifolia.

5. Conclusion

In summary, a new anthraquinone, 6-dehydroxy-9-O-methylviocristin (1), was isolated from the roots of S. obtusifolia, along with 13 known anthraquinones and two diterpenoids. Structural elucidation revealed that compound 1 possesses a 2-methyl-1,4-anthracenedione core and distinctive O-methylation at C-8 and C-9, suggesting its formation through a biosynthetic pathway shared with related anthraquinones in Senna species. The present findings demonstrate that S. obtusifolia roots harbor a unique anthraquinone profile that differs from that of seeds and aerial parts, highlighting organ-specific metabolic diversification. This work expands the phytochemical knowledge of S. obtusifolia and provides a foundation for future studies on anthraquinone biosynthesis, chemotaxonomy, and organ-dependent metabolic specialization within the Senna genus.

Supplemental Material

Supplemental material - A New Anthraquinone From the Roots of Senna Obtusifolia

Supplemental material for A New Anthraquinone From the Roots of Senna Obtusifolia by Daisuke Imahori, Asuka Yabu, Takuya Muraoka, Hiroyuki Tanaka in Natural Product Communications

Footnotes

ORCID iDs

Daisuke Imahori

Takuya Muraoka

Hiroyuki Tanaka

Ethical Considerations

Ethical Approval is not applicable for this article.

Consent to Participate

There are no human subjects in this article and informed consent is not applicable.

CRediT Authorship Contribution Statement

Daisuke Imahori: Writing–original draft, Project administration, Conceptualization. Asuka Yabu: Data curation, Formal analysis. Takuya Muraoka: Data curation. Hiroyuki Tanaka: Writing – review & editing, Supervision, Conceptualization.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Japan Society for the Promotion of Science (JSPS): KAKENHI Grant Numbers 22K20720 and 23K16308.

Declaration of Conflicting Interests

The authors declare no potential conflicts of interest with respect to the research, authorship, or publication of this article.

Author Agreement Statement

We declare that this manuscript is original, has not been published previously, and is not currently under consideration for publication elsewhere. We confirm that the manuscript has been read and approved by all named authors, and that there are no other persons who satisfy the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all authors. We understand that the Corresponding Author is the sole contact for the editorial process. He is responsible for communicating with the other authors about progress, submissions of revisions, and final approval of proofs.

Statement of Human and Animal Rights

This article does not contain any studies with human or animal subjects.

Supplemental Material

Supplemental material for this article is available online.

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