1,1,4,7-Tetramethyldecahydro-1H-cyclopropa[e]azulen-7-ol from the Stembark Chisocheton pentandrus

Abstract

A new aromadendrane-type sesquiterpenoid, namely dehydrosphatulenol (1), has been isolated from the stembark of Chisocheton pentandrus. The chemical structure of 1 was characterized on the basis of spectroscopic evidences including mainly one dimension and two dimension Nuclear Magnetic Resonance, and Mass Spectroscopy as well as through a comparison with those related compounds previously reported.

1. Introduction

Chisocheton plants have been known to be a rich source of secondary metabolites including various sterols, limonoids, terpenoids, and alkaloids with biologically properties such as antifungal, antibacterial, antiviral, anti-inflammatory, cytotoxic, and antiplasmodial agents [1,2,3,4]. In our previous research for novel cytotoxic constituents from Indonesia Chisocheton, we isolated and described limonoids, dysobinol from the seed C. macrophyllus [5], pentandricine from stem bark C. pentandrus [6], four new apo-euphane-type triterpenoid from the bark of C. patens [1] and a triterpenoid from C. cumingianus and C. celebicus [7,8]. In the further search for anticancer candidate compounds from C. pentandrus, we found a new aromadendrane-type sesquiterpenoid, namely dehydrospatulenol (1) from the stembark of C. pentandrus. In this communication, the isolation and structural determination of the new aromadendrane-type sesquiterpenoid are described.

2. Results

Extraction and Isolation

The dried stem bark of C. pentandrus (3.8 kg) was extracted with MeOH at room temperature to give a crude MeOH extract (560 g) after solvent was removed. The crude MeOH extract (560 g) was partitioned between n-hexane and water to give the n-hexane fraction (96.6 g) after evaporation of the solvent. The n-hexane soluble fraction was separated by column chromatography (CC) using gradient n-hexane/EtOAc to give eight fractions (A–H). Fraction A (3.3 g) was separated by medium pressure liquid chromatography (MPLC) on silica using isocratic of MeOH:H₂O (8:2) to give 12 subfractions (A1–12). Subfraction A9 (1.5 g) was subjected to column chromatography (CC) using CH₂Cl₂ to give three subfractions (A9.1–9.3). Compound 1 (335 mg) (Figure 1) was obtained by further purification of subfraction A9.3 (0.6 g) on silica gel eluted with n-hexane as a mobile phase.

Dehydrosphatulenol (1), colorless oil, ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{23}$ +7.2 (c, 0.17, CH₃OH), ¹H NMR (CDCl₃, 500 MHz), see

Table 1. Nuclear Magnetic Resonance data for compound 1 (500 MHz for ¹H and 125 MHz for ¹³C in CDCl₃).

C	δC	δH (ΣH., mult., J = Hz)
1	39.7	1.72 (1H, m)
2	29.1	1.15 (1H, m) 1.68 (1H, m)
3	37.8	1.45 (1H, m) 1.59 (1H, m)
4	76.6	-
5	58.2	1.69 (1H, m)
6	22.3	0.10 (1H, t, 9.3)
7	28.6	0.51 (1H, ddd, 6.0, 9.6)
8	18.8	1.29 (1H, m) 1.52 (1H, m)
9	25.8	1.49 (1H, m) 1.54 (1H, m)
10	38.5	1.85 (1H, m)
11	18.4	-
12	16.3	0.88 (3H, s)
13	28.7	0.92 (3H, s)
14	16.1	0.84 (3H, d, 6.8)
15	32.1	1.04 (3H, s)

. ¹³C NMR (CDCl₃, 125 MHz), δ_C (ppm), see Table 1. HR-TOFMS m/z 223.2064 [M + H]⁺ (calcd. for C₁₅H₂₆O, m/z 222.2084).

3. Discussion

Compound 1 was obtained as a colorless oil with ${\left[\mathsf{\alpha }\right]}_{\mathrm{D}}^{23}$ +7.2 (c, 0.17, CH₃OH) and the High Resolution Time of Flight-Mass Spectroscopy (HRTOF-MS) spectra showed a pseudomolecular ion peak at 223.2064 [M + H]⁺, corresponding to the molecular formula C₁₅H₂₆O (calculated m/z 222.2084). The ¹H NMR spectrum of 1 showed four methyls at δ_H 0.82 (3H, d, J = 6.82 Hz, Me-14), 0.88 (3H, s, Me-12), 0.92 (3H, s, Me-13), and 1.04 (3H, s, Me-15), each 3H, four methylene proton at δ_H 1.15 and 1.68 (2H, m, H-2), 1.29 and 1.52 (2H, m, H-8), 1.49 and 1.54 (2H, m, H-9), 1.45 and 1.59 (2H, m, H-3), five methine protons at δ_H 0.01 (1H, t, J = 9.3 Hz, H-6), 0.51 (1H, ddd, J = 6.05 and 9.6 Hz, H-7), 1.69 (1H, m, H-5), 1.72 (1H, m, H-1), and 1.85 (1H, m, overlap, H-10). The ¹³C NMR (Table 1) and Distortionless Enhancement by Polarization Transfer (DEPT) spectra revealed 15 carbon resonances due to two sp³ quaternary carbons at δ_C 18.4 (C-11) and 76.6 (C-4) and five sp³ methines at δ_C 22.3 (C-6), 28.6 (C-7), 38.5 (C-10), 39.7 (C-1), and 58.2 (C-5). In addition, there were four sp³ methylene at δ_C 18.8 (C-8), 25.8 (C-9), 29.1 (C-2), and 37.8 (C-3) and four methyls at δ_C 16.1 (C-14), 16.3 (C-12), 28.7 (C-13), and 32.1 (C-15). Among them, one sp³ quaternary carbon (δ_C 74.60) was ascribed bearing an oxygen atom.

A comparison of the NMR data of 1 with a ledol isolated from Renealmia chrysotrycha [9] revealed that the structures of the compound are closely related. The main difference was the position of an oxygenated sp³ quaternary carbon. In order to clarify the position of the hydroxyl group, Heteronuclear Multiple Bond Correlation (HMBC) and ¹H-¹H Corelated Spectroscopy (COSY) experiments were conducted and the results are shown in Figure 2 and Supplementary Materials. The HMBC spectrum of 1 showed correlation from the proton signal of Me-15 (δ_H 1.04) and methylene proton at δ_C 1.45 to oxygenated sp³ quaternary carbon C-14 (δ_C 74.60), indicating that a tertiary alcohol was located at C-4. The HMBC spectrum also showed correlations of proton methine H-6 (δ_H 0.10), proton methine H-7 (δ_H 0.51), Me-12 (δ_H 0.88), and Me-13 (δ_H 0.92) to sp³ quaternary carbon C-11 (δ_C 18.4), suggesting that a cyclopropane ring is located at C-6, C-7, and C-11, respectively. Furthermore, in the HMBC spectrum, a proton methyl with doublet multiplicity signal at δ_H 0.82 (H-14) was correlated with methine carbon C-10, indicating a secondary methyl located at C-10. The ¹H-¹H COSY spectrum of the isolated compound showed correlation in H1–H2, H1–H10, H5–H6, H6–H7, H7–H8, H8–H9, and H14–H10, supporting the presence of an aromadendrane structure in 1.

The ring-junction between cycloheptane and cyclopropane is cis. This was confirmed by the ~9 Hz vicinal coupling constant (³J_HH) of H-6 and H-7 from the experimental data and literature [9]. In the Nuclear Overhauser Effect-one dimension (NOE-1D) spectrum, there was correlation between H-6 with H-7. In addition, there are also correlation between H-6 with CH₃-14 and CH₃-13 when the signal H-6 was irradiated. As there was no correlation signal in the NOE-1D spectrum between CH₃-14 with H-1 and H-5 with H-5, this indicates that the configuration of methine H-1 and H-5 is cis to each other. The proton CH₃-15 showed no NOE interaction with H-5, this indicates the stereochemistry of CH₃-15 at the β-side of the molecule. Based on the literature, another aromadendrane-type sesquiterpenoid was isolated from Chisocheton penduliflorus [10], compound 1 was determined as a new aromadendrane-type sesquiterpenoid, 1,1,4,7-tetramethyldecahydro-1H-cyclopropa[e]azulen-7-ol, namely dehydrospathulenol (1).

4. Materials and Methods

4.1. General Experimental Procedures

The optical rotation was measured with an Autopol IV automatic polarimeter. The mass spectra was measured with a Water Xevo QTOFMS (Waters, Milford, MA, USA). NMR data were recorded on a Bruker Topspin spectrometer at 600 MHz for ¹H and 150 MHz for ¹³C using Tetramethylsilane (TMS) as an internal standard (Bruker, Billerica, MA, USA). Medium performance liquid chromatography was undertaken using a Buchi Pump Controller C-610, Buchi Pump Modules C-605 with FLH-R10030B SiliCycle column-ISO04 (Siliasep^TM, Buchi, Swizerland). Silica gel 60 was used for column chromatography (Merck, Darmstadt, Germany). Thin layer chromatography plates were precoated with silica gel GF₂₅₄ (Merck, Darmstadt, Germany, 0.25 mm) and detection was achieved by spraying with 10% H₂SO₄ in EtOH, followed by heating and irradiation under UV–Vis light at wavelengths of 254 and 364 nm.

4.2. Plant Material

The stem bark of C. pentandrus was collected in Halimun Salak Mountain National Park, Sukabumi, West Java Province, Indonesia. The plant was identified by the staff of the Bogoriense Herbarium, Bogor, Indonesia. A voucher specimen (MSF-G01) was deposited at the herbarium.

5. Conclusions

A new aromadendrane-type sesquiterpenoid, namely, dehydrospathulenol (1), was isolated from the stembark of Chisocheton pentandrus. This examination confirms that Chisocheton pentandrus is capable of producing sesquiterpenoid-type compounds.