Yan-Qiong Guo,
Gui-Hua Tang,
Zhen-Zhen Li,
Shu-Ling Lin and
Sheng Yin*
School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, China. E-mail: yinsh2@mail.sysu.edu.cn
First published on 25th November 2015
Chlojapolactone A (1), a novel lindenane sesquiterpenoid dimer with an unprecedented 1,3-dioxolane linkage, was isolated from Chloranthus japonicus. Its structure and absolute configuration was elucidated by combined spectral, computational, and chemical approaches. Compound 1 exhibited potential inhibitory effects on nitric oxide production in RAW 264.7 cells.
C. japonicus Sieb., a perennial herbaceous plant growing in southern China, has been applied in the Traditional Chinese Medicine (TCM) for the treatment of rheumatic arthralgia, bone fracture, pulmonary tuberculosis, and neurasthenia.12 Previous investigations on this plant have proved that it was a rich source of structurally diverse lindenane sesquiterpenoid dimers.3,4 In our efforts toward novel nitric oxide (NO) inhibitors from medicinal plants,13 a fraction of the ethanolic extract of C. japonicus showed an inhibitory activity of 61.21% at a concentration of 10 μg ml−1 against the lipopolysaccharide (LPS) induced NO production in RAW 264.7 macrophages. Subsequent chemical investigation led to the isolation of chlojapolactone A (1), a lindenane dimer with a unique 1,3-dioxolane linkage and comprising a rare dimaleate featured 8,9-seco-lindenane monomer (Fig. 1). Compound 1 represents a novel dimerization pattern of the lindenane dimer class, which is proposed to be biosynthetically formed by an aldol condensation of a rare 8,9-seco-lindenane and a lindenane sesquiterpenoids. Bioassay verified that compound 1 had inhibition against NO production in LPS-stimulated RAW 264.7 macrophages with IC50 value of 14.87 μM, being comparable to the positive control quercetin (IC50 = 15.90 μM). Herein, the isolation, structure elucidation, biogenetic origin, and NO inhibitory activity of 1 are described.
The air-dried powder of the whole plant of C. japonicus (1.0 kg) was extracted with 95% EtOH at room temperature to give a crude extract, which was suspended in H2O and successively partitioned with petroleum ether, EtOAc, and n-BuOH. Various column chromatographic separations of the EtOAc extract afforded compound 1 (15 mg).
Chlojapolactone A (1) was obtained as colorless oil. The HRESIMS displayed a pseudo-molecular ion at m/z 535.2331 [M − H]− (calcd for 535.2337), which was consistent with a molecular formula of C31H36O8, corresponding to 14 degrees of unsaturation. The IR spectrum exhibited absorption bands for carbonyl (1751 and 1711 cm−1) functionalities. The 1H NMR spectrum showed signals for a terminal double bond [δH 5.03 (1H, d, J = 2.0 Hz) and 4.73 (1H, brs)], five methyl singlets [δH 1.96, 1.85, 1.62, 1.14, and 0.51 (each, 3H)], an O-methyl [δH 3.75 (3H, s)], two oxygenated methines [δH 5.15 (1H, s) and 4.30 (1H, s)], and four highly upfield-shifted protons [δH 0.85, 0.72, 0.69, and 0.58 (each, 1H, m)] characterized for two cyclopropane methylenes.6,7 The 13C NMR spectrum, in combination with DEPT experiments, resolved 31 carbon resonances attributable to three carbonyls (δC 171.0, 170.1, 167.1), three double bonds (one exocyclic and two tetrasubstituted), six methyls (one O-methyl), four sp3 methylenes, eight sp3 methines (two oxygenated), and four sp3 quaternary carbons (two oxygenated). The aforementioned information (Table 1) implied that compound 1 consisted of two lindenane sesquiterpenoid units.1,6
No. | δHa | δCb | No. | δHa | δCb |
---|---|---|---|---|---|
a Recorded at 400 MHz.b Recorded at 100 MHz, chemical shifts are in ppm, coupling constant J is in Hz. | |||||
1 | 1.70, m | 27.4 | 1′ | 1.81, m | 23.6 |
2α | 0.72, m | 7.7 | 2′α | 0.85, m | 15.6 |
2β | 0.58, m | 2′β | 0.69, m | ||
3 | 1.61, m | 29.3 | 3′ | 1.95, m | 23.5 |
4 | 92.5 | 4′ | 151.1 | ||
5 | 2.33, dd (12.5, 6.9) | 48.1 | 5′ | 3.43, m | 50.3 |
6α | 2.45, dd (12.5, 6.9) | 27.2 | 6′α | 2.51, dd (18.0, 6.7) | 22.1 |
6β | 2.03, dd (12.5, 12.5) | 6′β | 2.26, dd (18.0, 12.5) | ||
7 | 128.2 | 7′ | 154.1 | ||
8 | 167.1 | 8′ | 109.5 | ||
9 | 5.15, s | 109.7 | 9′ | 4.30, s | 85.2 |
10 | 48.9 | 10′ | 42.6 | ||
11 | 137.9 | 11′ | 127.8 | ||
12 | 170.1 | 12′ | 171.0 | ||
13 | 1.96, s | 16.2 | 13′ | 1.85, s | 8.7 |
14 | 1.14, s | 16.4 | 14′ | 0.51, s | 17.1 |
15 | 1.62, s | 31.1 | 15′a | 4.73, brs | 106.6 |
OMe | 3.75, s | 52.6 | 15′b | 5.03, d (2.0) |
Detailed 2D NMR studies (HSQC, 1H–1H COSY, and HMBC experiments) afforded the gross structures of these two units (a and b) as depicted in Fig. 2. Unit b (in blue) was readily determined to be the typical lindenane-type sesquiterpenoid, chloranthalactone E,14,15 by direct comparison of their 1D NMR data. This was further supported by 1H–1H COSY and HMBC correlations (Fig. 2). As for unit a (in red), two structural fragments [a cyclopropane ring (C-1–C-2–C-3) and C-5–C-6] were first established by the 1H–1H COSY correlations (Fig. 2). The connectivities of these fragments, quaternary carbons, and other functional groups were mainly achieved by analysis of the HMBC spectrum (Fig. 2). The HMBC correlations from H3-14 to C-1, C-5, C-9, and C-10 allowed the connection of C-1, C-5, C-9, and C-14 to the quaternary carbon C-10, while the HMBC correlations from H3-15 to C-3, C-4, and C-5 linked C-3, C-5, and C-15 to the quaternary carbon C-4. Thus, ring A of unit a (Fig. 2) was established. The linkages of C-6–C-7–C-8 were revealed by HMBC correlations of H2-6/C-7 and C-8, while the correlations from H3-13 to C-7, C-11, and C-12 and from H2-6 to C-7 and C-11 further attached C-7, C-12, and C-13 to C-11. The upfield-shifted carbonyl of C-8 (δC 167.1) and still “loose end” oxygenated quaternary carbon of C-4 (δC 92.5) required the existence of a lactone bridge between C-4 and C-8, which generated ring B of unit a (Fig. 2). Finally, the methoxy group was located at C-12 by the correlation from the O-methyl to C-12. Unit a represented a rare 8,9-seco-lindenane skeleton.1
As above-mentioned structure elucidation already accounted for 13 out of the 14 degrees of unsaturation, the remaining one thus required the presence of an additional ring to link units a and b. In HMBC spectrum, a strong correlation from H-9 (δH 5.15) to the ketal carbon at C-8′ was observed, suggesting the presence of an oxygen bridge between C-9 and C-8′. Based on the acetal nature of C-9 (δC 109.7) and still “loose end” oxygenated methine C-9′ (δC 85.2), the existence of the other oxygen bridge between C-9 and C-9′ was proposed to form the additional 1,3-dioxolane ring. However, in regular HMBC experiment [delay time = 62.5 ms, J(C,H) = 8 Hz], even recorded in different deuterated solvents, the expected correlations between CH-9 and CH-9′ were not observed (ESI S15 and S20†). Thus, a modified HMBC experiment (delay time = 250 ms, J(C,H) = 2 Hz) was employed,16 which generated the correlations of H-9/C-9′ and H-9′/C-9 (ESI S9†). This was also collaborated by a long rang 1H–1H COSY correlation17 between H-9 and H-9′ (ESI S6†). Thus, the gross structure of 1 was elucidated as depicted, with a 1,3-dioxolane ring linking the two lindenane units.
The relative configuration of 1 was established by ROESY experiment and by comparison of its 1D NMR data with those of known lindenane monomers. In unit a, the correlations of H-2β/H-6β and H3-14/H-2β and H-6β indicated that these protons were co-facial and were arbitrarily assigned as β-orientation (Fig. 3). In consequence, the ROESY cross-peaks of H-5/H-6α and H3-15 revealed that these protons were α-oriented, which was further supported by the large coupling constant between H-5 and H-6β (J = 12.5 Hz). As for unit b, the ROESY correlations of H3-14′/H-2′β, H-6′β, and H-9′ indicated that they were co-facial and were arbitrarily assigned as β-oriented (Fig. 3). Consequently, the ROESY cross-peaks of H-5′/H-6′α indicated that these protons were α-oriented, which was further supported by the large coupling constant between H-5′ and H-6′β (J = 12.5 Hz). In addition, the correlation of H-9/H-9′ indicated that these protons were co-facial on the 1,3-dioxolane ring (Fig. 4). The 1D NMR data of most of the chiral centers in units a and b were consistent with those reported in their corresponding monomers.1,2
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Fig. 4 Internuclear distances of H-5′/H-5 and H-5′/H3-14 in isomers 1a (C-8′–O–C-12′ in β-orientation) and 1b (C-8′–O–C-12′ in α-orientation). |
The configuration of C-8′ could not be directly assigned owing to its ketal nature, resulting in two possible isomers of 1 (1a with C-8′–O–C-12′ in β-orientation and 1b with C-8′–O–C-12′ in α-orientation, in Fig. 4). Thus, a Chem3D molecular modeling study considering the rotatable C-9–C-10 bond was employed to rationalize the ROESY correlations crossing the two lindenane units. In the energy minimized conformation of 1a, after optimization of the conformation around C-9–C-10 bond, the internuclear distances between H-5′ and H-5 or H3-14 could reach the range within 3.0 Å, while these distances were greater than 4.0 Å in 1b no matter how to rotate the two units around this bond (ESI S32†). As the strong cross-peaks of H-5′/H-5 and H-5′/H3-14 were observed in the ROESY spectrum, 1a was selected with favorable stereochemistry.
The absolute configuration of 1 was determined by the exciton chirality method.18 The UV spectrum of 1 showed a strong absorption at λmax 223 nm (logε 4.20) corresponding to two moieties of α,β-unsaturated ketones. Consistent with this UV maximum, the CD spectrum of 1 showed a positive Cotton effect at 251 nm (Δε + 0.52, n → π* transition) and a negative Cotton effect at 222 nm (Δε −1.60, π → π* transition) due to the transition interaction between two α,β-unsaturated ketone chromophores (Fig. 5), indicating a positive chirality for 1. The positive chirality of 1 revealed that the transition dipole moments of two chromophores were oriented in clockwise direction, and the absolute stereochemistry of 1 was assigned as depicted.
To verify the absolute configuration assigned by the CD exciton chirality method, ECD calculations using time dependent density functional theory (TDDFT) were carried out on compound 1 (ESI S33†). The experimental ECD spectrum of 1 showed first positive and second negative Cotton effects at 251 and 222 nm respectively, which matched the calculated ECD curve for 1a (Fig. 5), an isomer with 1R, 3S, 4S, 5R, 9S, 10S, 1′R, 3′S, 5′S, 8′S, 9′S, 10′S configuration, indicating that 1 possessed the same absolute configuration.
This assignment was consistent with the biogenetic origin of lindenane-type sesquiterpenoids from the genus Chloranthus. Thus, compound 1 was assigned as depicted.
Based on the co-isolated known lindenanes 2–4,14,15,19 a plausible biosynthesis of chlojapolactone A was proposed in Scheme 1. Chloranthalactone A (2), a major component isolated in the study, was considered as the precursor. 2 was modified by several steps of transformations, involving epoxidation of Δ,8 epoxy ring-opening, and oxidative cleavage of C-8–C-9 diol, to generate intermediate i and ii. Intermediate ii underwent hydrolysis and hydration then followed by intramolecular esterification to produce chloranerectuslactone V (5). Finally, the aldehyde 5 was acetalized with diol i to afford 1. To support the proposed biosynthetic pathway and to further secure the structure of unit a, compound 5 was biomimetically semisynthesized from 3 (Scheme 2). Interestingly, just after we finished the synthetic work, chloranerectuslactone V (5) was reported as a novel natural product from C. erectus by other group.20 The efforts to semisynthesize i for further condensation with 5 was failed, as i was very labile, which could only be detected in the acid-catalyzed epoxy ring-opening solution of 3 and quickly isomerized to 4 during the purification process. The existence of i was demonstrated by isolation of its 8-O-methyl derivative in a TsOH/MeOH/H2O reaction system (ESI S2.5†). Above chemical evidences together with the absence of 5 in the crude extract of this plant suggested that 1 was a genuine natural product.
Chlojapolactone A (1) was evaluated for its inhibitory effect on nitric oxide (NO) production of LPS-activated RAW 264.7 macrophages. Under the subtoxic concentration (100 μM), 1 exhibited potential inhibition against NO production with IC50 value of 14.87 μM, being comparable to the positive control quercetin (IC50 = 15.90 μM).
Footnote |
† Electronic supplementary information (ESI) available: NMR spectra of 1–5, detail information for ECD calculation, isolation, purification, semisynthesis, and bioassay protocols. See DOI: 10.1039/c5ra22145e |
This journal is © The Royal Society of Chemistry 2015 |