A New Norsesquiterpenoid from the Rhizomes of Curcuma longa

A new norsesquiterpenoid, 4-(1′,5′-dimethyl-3′-oxo-4′-hexenyl)-2-cyclohexen-1-one (1), was isolated from the rhizomes of Curcuma longa (Zingiberaceae). The structure of the new norsesquiterpenoid was elucidated by chemical and physical evidence.

The traditional system of medicinals consists of a large number of plants with various medicinal and pharmacological importances and hence represents a priceless resource of new bioactive molecules. Curcuma (family Zingiberaceae) is a genus containing 70 known species that has been historically used as a spice, food preservative and coloring material. Curcuma longa L. is distributed throughout tropical and subtropical regions of the world. It is used in traditional medicine as a household remedy for various diseases. Also, it has been reported that C. longa possesses multiple pharmacological activities, including antioxidant, antimicrobial, anti-inflammatory, anticancer, anticoagulant, antidiabetic, and immunological [1, 2]. In continuation of some studies of the chemotaxonomy and biologically active metabolites from Zingiberaceous plants [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20], a methanol extraction of the rhizomes of C. longa afforded one new norsesquiterpenoid, 4-(1′,5′-dimethyl-3′-oxo-4′-hexenyl)- 2-cyclohexen-1-one (1). In this paper, we report the isolation and structural elucidation of this new compound (1).

Norsesquiterpenoid (1), a yellow oil, was deduced as C₁₄H₂₀O₂ by HR-MS-ESI (m/z 243.1354 [M + Na]+; calcd 243.1361). IR bands at 1685 (C=O) and 1620 (C=C) cm^–1 and a signal appearing at δ 199.7 in the ¹³C NMR spectrum indicate the characteristic of the turmerone homologues [21]. The ¹H NMR spectrum of 1 showed two aromatic protons at δ 6.83 (1H, dt, J = 10.0, 2.0 Hz, H-2) and 6.03 (1H, ddd, J = 10.0, 2.0, 1.2 Hz, H-3), three methylene protons at δ 2.32/2.51 (H-6), 2.31/2.48 (H-2′), and 1.78/1.96 (H-5), three methine protons at δ 6.07 (1H, m, H-4′), 2.50 (1H, m, H-4), and 2.36 (1H, m, H-1′), and three methyl protons at δ 2.15 (3H, s, H-7′), 1.90 (3H, s, H-6′), and 0.92 (3H, d, J = 6.8 Hz, H-8′), indicating that 1 was probably a turmerone. The carbons of 1 were assigned, from ¹³C NMR and DEPT experiments, to three methyls at δ 16.5 (C-8′), 20.8 (C-7′), and 27.7 (C-6′), three methylenes at δ 24.2 (C-5), 37.4 (C-6), and 48.2 (C-2′), five methines at δ 32.7 (C-1′), 40.6 (C-4), 123.8 (C-4′), 130.0 (C-3), and 154.4 (C-2), and three quaternary carbons at δ 156.2 (C-5′), 199.7 (C-3′, C=O), and 199.9 (C-1, C=O). Complete unambiguous assignments for the ¹H and ¹³C NMR signals were made by a combination of the COSY, HETCOR, long-range HETCOR, and NOESY spectrum (Table 1). COSY correlations were observed between H-2, H-3, H-4, H-5 and H-6, and between H-1′, H-2′, H-4, and H-8′. The HETCOR experiment showed that the carbon/proton signals at δ 16.5/0.92 for C-8′, 20.8/2.15 for C-7′, 24.2/1.78, 1.96 for C-5, 27.7/1.90 for C-6′, 32.7/2.36 for C-1′, 37.4/2.32, 2.51 for C-6, 40.6/2.50 for C-4, 48.2/2.31, 2.48 for C-2′, 123.8/6.07 for C-4′, 130.0/6.03 for C-3, and 154.4/6.83 for C-2.

Table 1. ¹H (400 MHz) and ¹³C (100 MHz) NMR Data of 1 (CDCl₃, δ, ppm, J/Hz)

The observation of the NOESY correlations between H-6′ and H-7′, between H-1′, H-2′, H-8′, H-3, and H-4, between H-2 and H-3, and between H-5 and H-6 established the connective site as shown in structure 1. Thus, the structure of this compound was determined to be 4-(1′,5′-dimethyl-3′-oxo-4′-hexenyl)-2-cyclohexen-1-one, which was further confirmed by HMBC experiments (Table 1).

The compound was dehydrogenated to give (+)-ar-turmerone, which was indicative that the absolute configuration at C-1′ was S. In the ¹H NMR spectrum, an intramolecular NOESY was observed between the secondary methyl hydrogens at C-8′ and the olefinic hydrogen at C-3, showing that these hydrogens were situated in a spatially close relationship. Provided that the hydrogens at C-4 and C-1′ adopted the thermodynamically most stable anti-arrangement, the above observation of the NOESY allowed the absolute configuration at C-4 to be S. The combined evidence pointed to the stereostructure 1 for 4S-(1′S,5′-dimethyl-3′-oxo-4′-hexenyl)-2-cyclohexen-1-one.

Experimental

General. UV spectra were obtained in MeCN, IR spectra were measured on a Hitachi 260-30 spectrophotometer. ¹H NMR (400 MHz), ¹³C NMR (100 MHz), HETCOR, HMBC, COSY, and NOESY spectra were obtained on a Varian (Unity Plus) NMR spectrometer. Low-resolution ESI-MS spectra were obtained on an API 3000 (Applied Biosystems), and high-resolution ESI-MS spectra on a Bruker Daltonics APEX II 30e spectrometer. Silica gel 60 (Merck, 70–230 mesh, 230–400 mesh) was used for column chromatography. Precoated Silica gel plates (Merck, Kieselgel 60 F-254), 0.20 mm and 0.50 mm, were used for analytical TLC and preparative TLC, respectively, visualized with 50% H₂SO₄.

Plant Material. The rhizomes of Curcuma longa were collected from Chiayi County, Taiwan, in April 2017. Plant material was identified by Dr. Fu-Yuan Lu (Department of Forestry and Natural Resources, College of Agriculture, National Chiayi University). A voucher specimen was deposited at the Department of Medical Technology, School of Medical and Health Sciences, Fooyin University, Kaohsiung, Taiwan.

Extraction and Isolation. The rhizomes (0.1 kg) of C. longa were extracted repeatedly with MeOH (3 L × 2) at room temperature for 24–48 h. The MeOH extract was dried and evaporated to leave a viscous residue (21.2 g). The residue was placed on a silica gel column (4.8 kg, 70–230 mesh) and eluted with CH₂Cl₂ gradually enriched with MeOH to afford five fractions. Part of fraction 2 (2.1 g) was subjected to silica gel chromatography (0.9 kg, 70–230 mesh) by eluting with n-hexane–acetone (100:1) and enriched with acetone to furnish five fractions (2-1–2-5). Part of fraction 2-2 (0.4 g) was further purified on a silica gel column using n-hexane–acetone mixtures to obtain 4-(1′,5′-dimethyl-3′-oxo-4′-hexenyl)-2- cyclohexen-1-one (12.2 mg).

4S-(1′S,5′-Dimethyl-3′-oxo-4′-hexenyl)-2-cyclohexen-1-one (1). Yellow oil, ${\left[\alpha \right]}_{\mathrm{D}}^{25}$ –0.21° (c 0.51, CH₂Cl₂). UV/Vis (CH₃CN, λ_max, nm) (log ε): 233 (4.11), 238 (3.65). IR (neat, ν_max, cm^–1): 1685 (C=O), 1620 (aromatic C=C), 870. ESI-MS m/z 243 [M + Na]⁺; HR-ESI-MS m/z 243.1354 [M + Na]⁺ (calcd for C₁₄H₂₀O₂Na, 243.1361). For ¹H and ¹³C NMR, see Table 1.