Rearranged Phloroglucinol-Monoterpenoid Adducts from Callistemon

Article pubs.acs.org/jnp

Cite This: J. Nat. Prod. 2018, 81, 57−62

Rearranged Phloroglucinol-Monoterpenoid Adducts from Callistemon rigidus Jia-Qing Cao,†,‡,§ Hai-Yan Tian,†,‡,§ Man-Mei Li,†,‡ Wei Zhang,†,‡ Ying Wang,†,‡ Lei Wang,*,†,‡ and Wen-Cai Ye*,†,‡ †

Institute of Traditional Chinese Medicine & Natural Products and ‡Guangdong Province Key Laboratory of Pharmacodynamic Constituents of TCM and New Drugs Research, Jinan University, Guangzhou 510632, People’s Republic of China S Supporting Information *

ABSTRACT: Callisretones A (1) and B (2), two rearranged phloroglucinol-monoterpenoid adducts featuring an unprecedented isopropylcyclopenta[b]benzofuran backbone, together with their postulated biosynthetic precursors (3−9), were isolated from Callistemon rigidus. The previously assigned absolute configurations of viminalins H (7), L (8), and N (9) were revised and unequivocally established by X-ray diffraction data. A putative biosynthetic pathway toward callisretones A and B involving the rearrangement of the terpenoid motif is proposed. In addition, 1 and 2 showed inhibitory effects on nitric oxide production with IC50 values of 15.3 ± 1.0 and 17.7 ± 1.1 μM, respectively.

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inhibitory effects on nitric oxide (NO) production. In addition, the configurations of the known viminalins H (7), L (8), and N (9)14 were revised. Herein, the structural identification, biosynthetic route, and anti-inflammatory activities of these compounds are reported. Compound 1 was obtained as colorless crystals. The molecular formula was established as C22H30O5 based on its HRESIMS data (m/z 375.2184 [M + H]+, calcd for C22H31O5 375.2166). The IR spectrum suggested the presence of aromatic (1629 and 1600 cm−1), hydroxy (3417 cm−1), and carbonyl groups (1721 cm−1). The 1H NMR spectrum of 1 showed signals for an α-methylbutanoyl moiety [δH 3.33 (1H, m, H-2′), 1.72 (1H, m, H-4′a), 1.29 (1H, m, H-4′b), 0.86 (3H, t, J = 7.4 Hz, H-5′), and 1.09 (3H, d, J = 6.8 Hz, H-3′)], an isopropyl unit [δH 0.85 (6H, d, J = 6.8 Hz, H-9′/H-10′) and 1.61 (1H, m, H-8′)], a tertiary methyl (δH 1.56, 3H, s, H-6′), a methoxy (δH 3.82, 3H, s, H-11′), an aromatic proton (δH 6.01, 1H, s, H-2), a formyl proton (δH 9.73, 1H, d, J = 3.6 Hz, H-7′), and a phenolic proton (δH 13.71, 1H, s, HO-3). The 13C NMR and DEPT spectra exhibited 22 signals including those of a phloroglucinol ring, a ketocarbonyl, a formyl carbon, and 14 aliphatic carbons. The above data implied that 1 could be a phloroglucinol-monoterpenoid adduct. A comparison of the NMR data of 1 (Table 1) with those of aspidinol D15 suggested the presence of an α-methylbutanoylphloroglucinol moiety (fragment 1a) in 1 (Figure 1), which was confirmed by the HMBC correlations from H3-5′ to C-2′, H33′/H2-4′ to C-1′, HO-3 to C-2/C-4, and H3-11′ to C-1. The

hloroglucinol derivatives are prevalent in the families Myrtaceae, Guttiferae, and Euphorbiaceae1,2 and have shown various appealing biological effects, such as antimicrobial,3 anti-inflammatory,4 antiproliferative,5 and antiviral activities.6 Among these phloroglucinol derivatives, phloroglucinol-terpenoid adducts have diverse structures that are produced from polyketide and terpenoid precursors that are controlled by terpene cyclases and polyketide synthases.7 The terpenoid part of phloroglucinol-terpenoid adducts could be monoterpene,8 sesquiterpene,6,9 or diterpene10 derived to construct a library of complex natural products. These compounds have become attractive targets for organic chemists and pharmacologists.11 In a previous study of the medicinal plants of the family Myrtaceae, a series of novel phloroglucinol-terpenoid adducts with cytotoxic or antibacterial activities were isolated.12 During ongoing work on the bioactive constituents from the leaves of Callistemon rigidus (family Myrtaceae), two new phloroglucinolmonoterpenoid hybrids, callisretones A (1) and B (2), with an unprecedented carbon skeleton were isolated. The co-isolation of their putative biosynthetic intermediates (3−6) revealed that compounds 1 and 2 could be formed via rearrangement of the terpene moiety. As an important class of natural products, more than 70 phloroglucinol-terpenoid hybrids had been isolated from plants and fungi.13 Although there are various differences in their skeletal types, all the reported hybrids possess the original terpene framework such as β-phellandrene, pinene, and β-caryophyllene. Compounds 1 and 2 are the first examples of phloroglucinol-terpenoid hybrids with a rearranged terpene moiety, in which the unusual isopropylcyclopenta[b]benzofuran backbone is present. These compounds showed © 2017 American Chemical Society and American Society of Pharmacognosy

Received: July 19, 2017 Published: December 20, 2017 57

DOI: 10.1021/acs.jnatprod.7b00606 J. Nat. Prod. 2018, 81, 57−62

Journal of Natural Products

Article

Chart 1

Table 1. 1H (500 MHz) and 13C (125 MHz) NMR Data of 1 and 2 in CDCl3 (δ in ppm, J in Hz) 1 position

δC

1 2 3 4 4a 5a 6 7 8

161.7 93.1 167.3 101.4 160.5 102.0 65.7 46.3 35.2

8a 8b 1′ 2′ 3′ 4′

49.4 108.5 208.6 45.3 16.0 27.1

5′ 6′ 7′ 8′ 9′ 10′ 11′ OH

12.0 25.3 203.8 30.6 21.9 19.7 55.8

2 δH

6.01 s

2.36 dd (11.8, 3.6) 2.33 m α 1.98 ddd (13.0, 5.7, 0.7) β 1.69 m 3.42 dd (8.9, 0.7)

3.33 m 1.09 d (6.8) 1.72 m 1.29 m 0.86 t (7.4) 1.56 s 9.73 d (3.6) 1.61 m 0.85 d (6.8) 0.85 d (6.8) 3.82 s 13.71 br s

δC 161.7 93.0 167.2 100.9 160.4 102.0 65.6 46.3 35.2

49.3 108.5 208.8 38.6 19.0 19.2 25.3 203.8 30.5 21.9 19.6 55.8

δH 6.01 s

Figure 1. Key 1H−1H COSY and HMBC correlations of 1 and 2.

2.38 dd (11.8, 3.6) 2.33 m α 1.97 ddd (13.0, 5.7, 0.7) β 1.69 m 3.41 dd (8.9, 0.7)

Furthermore, the HMBC correlations from H3-6′ to C-6/C-8a, H3-6′/H2-8 to C-5a, H-7′ to C-6, and H-9′/H-10′ to C-7 s u p p o r t e d t h e c o n s t r u c t i o n o f a 2 - i s o p r o py l - 5 methylcyclopentanecarbaldehyde unit (fragment 1b). In addition, the HMBC correlations from H-8a to C-4a/C-6/C7 and the upfield shift of C-4a (δC 160.5) and downfield shift of C-5a (δC 102.0) indicated the linkage of fragments 1a and 1b via C-4a−O−C-5a and C-8a−C-8b. Thus, the 2D structure was established. In the NOESY spectrum, the correlations of H3-6′ with H35′/H-6/H-8a, H-6 with H-8a/H3-9′, H-8β with H3-9′, and H8α with H-7 established the relative configurations of C-5a, C6, C-7, and C-8a (Figure 2). However, the configuration of C-2′ could not be deduced. However, crystals acquired from MeOH were suitable for X-ray diffraction analysis, in which the anomalous dispersion of Cu Kα was applied. The absolute parameter [−0.07(5)] and the refinement of the Hooft parameter [−0.01(4)] allowed the definition of the absolute configurations of 1 as (5aS, 6R, 7R, 8aR, and 2′S) (Figure 3). Furthermore, quantum-chemical electronic circular dichroism (ECD) calculations also revealed that the experimental ECD spectrum of 1 was similar to the calculated spectrum of the

3.47 m 1.09 d (6.8) 1.11 d (6.8) 1.56 9.73 1.61 0.84 0.84 3.82

s d (3.6) m d (6.8) d (6.8) s

13.61 br s

remaining 10 carbon atoms should be attributable to a monoterpene moiety. The 1H−1H COSY data of 1 revealed the presence of consecutive connections as shown in Figure 1. 58



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Figure 2. Optimized structures and key NOE correlations of 1 and 2.

The NMR data of 2 were similar to those of 1, except for the presence of signals of an isobutanoyl unit [δH 3.47 (1H, m, H2′), 1.11 (3H, d, J = 6.8 Hz, H-4′), 1.09 (3H, d, J = 6.8 Hz, H3′); δC 208.8 (C-1′), 38.6 (C-2′), 19.0 (C-3′), 19.2 (C-4′)] in 2 rather than the α-methylbutanoyl moiety in 1 (Table 1). The HMBC correlations from HO-3 to C-2/C-4 and H-2 to C-1/C3, as well as the NOE correlation between H3-10′ and H-2, suggested that the isobutanoyl group was attached at C-4. The relative configuration of 2 was the same as that of 1 by analysis of the NOE correlations (Figure 2). The experimental ECD spectrum of 2 showed a positive Cotton effect at 248.3 (Δε +0.4) nm and negative Cotton effects at 333.1 (Δε −4.5), 288.3 (Δε −8.9), and 223.2 (Δε −18.2) nm, which were similar to the calculated spectrum for the (5aS, 6R, 7R, 8aR) diastereomer (Figure 4). Therefore, the structure of callisretone B (2) was defined as shown. Together with 1 and 2, seven other phloroglucinolmonoterpenoid hybrids (3−9) were also isolated from C. rigidus. The known compounds were identified as callisalignene B (3),16 2-methyl-1-[(5aR,8R,9aR)-5a,8,9,9a-tetrahydro-3-hydroxy-1-methoxy-5a-methyl-8-(1-methylethyl)-4-dibenzofuranyl]-1-propanone (4),12b viminalin C (5),14 and viminalin B (6).14 However, the configuration of C-2′ of 5 had not been determined previously.14 In this study, the (5aS, 6R, 7R, 8R, 9aR, 2′S) absolute configuration of 5 was determined by X-ray diffraction with Cu Kα radiation (Figure 5). For compounds 7, 8, and 9, single crystals were obtained for X-ray diffraction analysis. The Cu Kα data of 7−9 resulted in small Flack parameters [(−0.02(5) for 7; 0.00(11) for 8; −0.04(8) for 9)], allowing the assignment of (5aR,8R,9aR), (5aS,8R,9aS), and (5aR,9R,9aS) absolute configurations for 7, 8, and 9, respectively (Figure 5). Recently, Wu et al. reported the isolation and identification of viminalins A−O from C. viminalis.14 The absolute configurations of viminalins H, L, and N were established by comparison of experimental ECD data but without chemical calculation or X-ray crystallographic analysis. Since the MS, NMR, specific rotation, and ECD data of viminalins H, L, and N are identical to those of 7, 8, and 9, respectively (Tables S1−S3, Supporting Information), the proposed absolute configurations for viminalins H, L, and N should be revised as shown. The occurrence of intermediates 3−6 inspired us to propose a putative biosynthetic pathway of compounds 1 and 2 (Scheme 1). It has been reported that acylphloroglucinols and monoterpenes are major components of plants from the genus Callistemon.17,18 The intermediates 3 and 4 would be

Figure 3. X-ray ORTEP drawing of 1.

(5aS, 6R, 7R, 8aR, 2′S) diastereomer (Figure 4). Hence, the structure of callisretone A (1) was established. Compound 2 was obtained as a colorless gum. The molecular formula C21H28O5 was deduced by the protonated molecular ion peak at m/z 361.2016 [M + H]+ (calcd for C21H29O5 361.2010). The IR bands at 1719, 1629, and 1604 cm−1 suggested the presence of aromatic and carbonyl groups.

Figure 4. Calculated and experimental ECD spectra of 1 and 2.. 59



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Figure 5. X-ray ORTEP drawings of 5−9.

Scheme 1. Putative Biosynthetic Pathways toward the Formation of 1 and 2

μM, respectively (indomethacin was used as a positive control, IC50 = 21.1 ± 1.4 μM). In contrast, compounds 3−9, with the intact terpene units, exhibited no inhibitory activity (IC50 > 50 μM). The above results suggested that the rearranged terpene moieties play important roles in the NO inhibitory activity of these phloroglucinol-monoterpenoid adducts. In summary, two new phloroglucinol-monoterpenoid adducts (1 and 2) with new carbon skeletons were isolated from C. rigidus. Biosynthetically, compounds 1 and 2 could be formed from common acylphloroglucinol-phellandrene meroterpenoids (3−6) via a pinacol rearrangement reaction. Finally, spectroscopic and X-ray diffraction data analysis

formed from acylphloroglucinols and the monoterpene phellandrene through radical addition and intramolecular cyclization processes.12b Further oxidation of 3 and 4 would form the epoxy intermediates 5 and 6.19 The hydrolysis of 5 and 6 would lead to the formation of the vicinal diol i.20 Compounds 1 and 2 would be produced by a pinacol rearrangement process via intermediates ii → iv.21 The levels of NO play important roles in inflammatory disorders such as sepsis and tissue damage. Therefore, compounds 1−9 were tested for their inhibitory activities on NO production. The results showed that 1 and 2 suppressed NO production with IC50 values of 15.3 ± 1.0 and 17.7 ± 1.1 60



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= 1.541 78 Å) was used for X-ray diffraction data collection. The structures were solved and refined using the SHELXL-97 program. These data can be acquired from the Cambridge Crystallographic Data Centre (CCDC 1532708 for 1, CCDC 1553923 for 5, CCDC 1550573 for 6, CCDC 1550575 for 7, CCDC 1550576 for 8, CCDC 1550574 for 9). Biological Assay. The macrophage RAW 264.7 cells were cultivated in DMEM in a humidified atmosphere with 5% CO2 at 37 °C. Ten thousand cells per well grown in 96-well plates were used. Cells were preincubated for 1 h with or without compounds before the addition of lipopolysaccharide for 24 h. Then, 100 μL of the culture supernatant was incubated with a Griess reagent (100 μL, Sigma) at room temperature for 10 min. The absorbance was measured at 550 nm against a calibration curve with NaNO2 standards. The experiments were carried out in triplicate.

permitted revision of the absolute configurations of viminalins H (7), L (8), and N (9).

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EXPERIMENTAL SECTION

General Experimental Procedures. Melting points were recorded with an X-5 melting point instrument (Fukai, Beijing, China) without correction. Optical rotations were measured in MeOH on a P-1020 polarimeter (Jasco, Tokyo, Japan). UV spectra were acquired on a V-550 UV/vis spectrometer (Jasco, Tokyo, Japan). ECD spectra were measured in CH3CN on a J-810 spectropolarimeter (Jasco, Tokyo, Japan). IR spectra were acquired on an FT/IR-480 Fourier transform spectrometer (Jasco, Tokyo, Japan). NMR data were measured with Bruker AV-600, AV-500, and AV-300 spectrometers (Bruker, Fällanden, Switzerland). HRESIMS spectra were obtained using an Agilent 6210 TOF-MS spectrometer (Agilent Technologies, CA, USA). Column chromatography was performed using Sephadex LH-20 (Pharmacia, Uppsala, Sweden), ODS (YMC, Kyoto, Japan), or silica gel (Qingdao Marine, China). HPLC was carried out using an Agilent 1260 instrument with YMC-Pack ODS-A columns (250 × 4.6 mm; 250 × 10 mm). X-ray crystallographic analysis was performed on a Gemini S Ultra CCD diffractometer (λ = 1.541 78 Å). All solvents were of analytical or chromatographic grade. The RAW264.7 cells were purchased from the Chinese Academy of Sciences. Plant Material. The leaves of C. rigidus were collected from Guangzhou, Guangdong Province, China, in June 2015, and were authenticated by Prof. G.-X. Zhou (Jinan University, Guangzhou, China). A voucher specimen (No. 2015062305) was deposited in the College of Pharmacy, Jinan University, China. Extraction and Isolation. The leaves of C. rigidus (12.5 kg) were percolated using 95% EtOH (50 L × 4). The concentrated crude extract (985 g) was partitioned between H2O and petroleum ether. The petroleum ether part (256 g) was subjected to silica gel column chromatography eluting by a gradient cyclohexane−EtOAc mixture (100:0 → 0:100) to give Frs. A−H. Fr. C (45.5 g) was further separated on a silica gel column (petroleum ether−EtOAc, 100:0 → 0:100) to give subfractions Frs. C1−C5. Fr. C4 (1.3 g) was purified by a Sephadex LH-20 column (CHCl3−MeOH, 6:4) followed by preparative RP-HPLC (70% CH3CN−H2O, 3 mL/min, 280 nm) to afford compounds 1 (15.7 mg) and 2 (10.2 mg). Fr. F (11.2 g) was separated on Sephadex LH-20 (CHCl3−MeOH, 6:4) to afford subfractions Frs. F1−F4. Fr. F3 (6.3 g) was rechromatographed by ODS using CH3OH−H2O (60:40 → 100:0) as eluent to yield subfractions Frs. F3a−F3e. Fr. F3c was purified to obtain compounds 3 (8.7 mg), 4 (14.5 mg), and 7 (30.5 mg) by preparative RP-HPLC (75% CH3CN, 280 nm, 3 mL/min). Fr. D (6.5 g) was separated on an ODS column eluted with CH3OH−H2O (60:40 → 100:0) to give subfractions Frs. D1−D5. Fr. D3 was purified to obtain compounds 5 (12.7 mg) and 6 (12.5 mg) by preparative RP-HPLC (65% CH3CN, 280 nm, 3 mL/min). Fr. D4 was purified by preparative RP-HPLC (70% CH3CN, 280 nm, 3 mL/min) to give 8 (18.7 mg) and 9 (17.5 mg). Callisretone A (1): colorless crystals (CH3OH); mp 110−112 °C; [α]25D +28 (c 0.1, CH3OH); HRESIMS m/z 375.2184 [M + H]+ (calcd for C22H31O5 375.2166); UV (CH3OH) λmax (log ε) 204 (4.14), 230 (4.18), 284 (4.33) nm; IR (KBr) νmax 3417, 2965, 2930, 2871, 1721, 1629, 1600, 1458, 1375, 1238, 813 cm−1; ECD (CH3CN) 333.1 (Δε −4.5), 288.3 (Δε −8.9), 223.2 (Δε −18.2) nm; NMR data in Table 1. Callisretone B (2): colorless gum (CH3OH); [α]25D +98 (c 0.1, CH3OH); HRESIMS m/z 361.2016 [M + H]+ (calcd for C21H29O5 361.2010). UV (CH3OH) λmax (log ε) 204 (4.10), 230 (4.15), 284 (4.28) nm; IR (KBr) νmax 3420, 2966, 2933, 2873, 1719, 1629, 1604, 1458, 1375, 1238, 811 cm−1; ECD (CH3CN) 327.1 (Δε −3.2), 285.7 (Δε −6.8), 224.0 (Δε −11.3) nm; NMR data in Table 1. Characterization of 3−9. The physicochemical data of 3−9 are provided in the Supporting Information. X-ray Crystallographic Analysis of 1 and 5−9. Single crystals of 1 and 5−9 were obtained from MeOH solution. Cu Kα radiation (λ

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.7b00606. Detailed HRESIMS, NMR, UV, and IR spectra of 1−9; physicochemical data of 3−9; chemical calculation details for 1 and 2; bioassay results for 1−9 (PDF) X-ray crystallography data for 1 and 5−9 (CIF) (CIF) (CIF) (CIF) (CIF) (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lei Wang: 0000-0001-9242-1109 Wen-Cai Ye: 0000-0002-2810-1001 Author Contributions §

J.-Q. Cao and H.-Y. Tian contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for the National Natural Science Foundation of China (Nos. 81273391, 81573307, U1401225), the Guangdong Natural Science Foundation for Distinguished Young Scholars (No. 2015A030306022), and the Science and Technology Planning Project of Guangdong Province (No. 2016B030301004).

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REFERENCES

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Rearranged Phloroglucinol-Monoterpenoid Adducts from Callistemon

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