Letter Cite This: Org. Lett. 2018, 20, 3124−3127
pubs.acs.org/OrgLett
Heliojatrones A and B, Two Jatrophane-Derived Diterpenoids with a 5/10 Fused-Ring Skeleton from Euphorbia helioscopia: Structural Elucidation and Biomimetic Conversion Zhen-Peng Mai, Gang Ni, Yan-Fei Liu, Li Li, Jia-Yuan Li, and De-Quan Yu* State Key Laboratory of Bioactive Substance and Function of Natural Medicines, Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, P. R. China S Supporting Information *
ABSTRACT: Heliojatrones A and B (1 and 2), two jatrophane-derived diterpenoids with an unprecedented trans-bicyclo[8.3.0]tridecane core, were isolated from the whole plants of Euphorbia helioscopia. Their structures and absolute configurations were unequivocally determined by a combination of spectroscopic data, electronic circular dichroism calculations, biomimetic conversion, and X-ray diffraction analyses. Their plausible biosynthetic pathways were also proposed. Compounds 1 and 2 were biomimetically synthesized from 3 and 4 through a photochemical rearrangement reaction of β,γ-unsaturated ketone, respectively. Compound 2 showed significant Pglycoprotein inhibitory activity compared with the positive control (cyclosporine A).
D
iterpenoids occurring in plants of the genus Euphorbia are significant to natural product drug discovery due to their broad range of bioactivities and their great structural diversity, including ingenol 3-angelate, prostratin, and resiniferatoxin.1 Among Euphorbia diterpenes, jatrophane diterpenoids with multidrug resistance-reversing activities are interesting components.2 These macrocyclic diterpenes based on a transbicyclo[10.3.0]pentadecane skeleton serve as the precursors of presegetane, segetane, paraliane, and peluane diterpenes.3 Additionally, 12,17-cyclojatrophane, 1(15 → 14)-abeo-jatrophane, and 9(10 → 18)-abeo-jatrophane skeletons are also derived from jatrophanes.4 Euphorbia helioscopia L., belonging to the genus Euphorbia (Euphorbiaceae), is an annual herb widely distributed in most parts of China and is used in traditional medicine to treat warts, bacillary dysentery, and tumors.5 In a previous chemical investigation, 1 novel jatropholane diterpenoid with a 5/6/7/ 7 fused-ring system, 4 unprecedented diterpenoids possessing 5/6/4/6 fused-ring and 7,8-seco-jatrophane skeletons, and 17 new jatrophane diterpenoids, together with 15 known jatrophane diterpenoids, were characterized in several fractions isolated from an EtOH extract of the whole plants.6 Some of these compounds showed antiviral (HSV-1), neuroprotective, and permeability glycoprotein (P-glycoprotein) inhibitory activities. As a continuing investigation for the bioactive diterpenoids, two novel diterpenoids, heliojatrones A (1) and B (2), possessing an unusual trans-bicyclo[8.3.0]tridecane core (Figure 1) were obtained. Compounds 1 and 2 were successfully prepared from biological precursors 3 (euphoscopin F)7 and 4 (2-epi-euphornin I)6a through a photochemical rearrangement reaction of β,γ-unsaturated ketone,8 respectively. Herein, the details of the structural elucidation, chemical © 2018 American Chemical Society
Figure 1. Structures of compounds 1 and 2.
conversion, hypothetical biosynthetic pathways, and P-glycoprotein inhibitory activities of 1−4 are described. Heliojatrone A (1) was isolated as colorless prisms. Its molecular formula, C31 H 38 O 8, was determined by the (+)-HRESIMS of a Na+ adduct of a molecule at m/z 561.2445 [M + Na]+ (calcd for C31H38O8Na, 561.2459), corresponding to 13 indices of hydrogen deficiency. The IR spectrum showed absorption bands assignable to carbonyl (1739 and 1718 cm−1) and aromatic ring (1603 and 1453 cm−1) functionalities. The 1H and 13C NMR resonances (Table S1, SI) of 1 were indicative of one benzoyloxy group [δH 8.06 × 2 (dd, J = 7.8, 1.2 Hz, H-2′ and H-6′), 7.57 (t, J = 7.8 Hz, H4′), and 7.45 × 2 (t, J = 7.8 Hz, H-3′ and H-5′); δC 165.7 (C7′), 133.2 (C-4′), 130.5 (C-1′), 129.6 × 2 (C-2′ and C-6′), and 128.6 × 2 (C-3′ and C-5′)] and two acetoxy groups [δH 2.36 (s, H3-2‴) and 1.66 (s, H3-2″); δC 171.2 (C-1‴), 169.6 (C-1″), 21.9 (C-2′′′), and 20.7 (C-2′′)]. The 1H NMR spectrum also Received: April 17, 2018 Published: May 7, 2018 3124
DOI: 10.1021/acs.orglett.8b01215 Org. Lett. 2018, 20, 3124−3127
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Organic Letters contained signals for two olefinic protons [δH 5.70 (brd, J = 11.4 Hz, H-5) and 4.50 (brd, J = 10.8 Hz, H-11)], two oxygenated methine protons [δH 5.11 (dd, J = 5.4, 3.0 Hz, H-7) and 5.10 (dd, J = 7.2, 3.0 Hz, H-3)], three tertiary methyl groups [δH 1.80 (H3-17) and 1.76 × 2 (H3-18 and H3-19)], and two secondary methyl groups [δH 1.28 (d, J = 7.2 Hz, H3-16) and 0.87 (d, J = 7.2 Hz, H3-20)]. In addition to the resonances of one benzoyloxy group and two acetoxy groups, the 13C and HSQC NMR data revealed 20 carbon resonances corresponding to five quaternary carbons (two carbonyls, two olefinic carbons, and one oxygenated carbon), eight methines (two olefinic methines and two oxygenated methines), two methylenes, and five methyls. The 1H−1H COSY correlations of H2-1/H-2(H3-16)/H-3/ H-4/H-5, H-7/H2-8, H-11/H-12/H-13(H3-20), and H-2′(H6′)/H-3′(H-5′)/H-4′ highlighted four key fragments, as shown in Figure 2 (thick blue lines). The HMBC correlations from
and 15R based on X-ray crystallography (Figure 3) using Cu Kα radiation [Flack Parameters 0.02(12)].
Figure 3. Single-crystal X-ray structure of 1.
The molecular formula of heliojatrone B (2) was determined to be C31H40O8 by HRESIMS (m/z 563.2604 [M + Na]+, calcd for C31H40O8Na, 563.2615), indicating 12 indices of hydrogen deficiency. The IR spectrum showed absorption bands assignable to hydroxyl (3482 cm−1), carbonyl (1744 and 1717 cm−1), and aromatic ring (1602 and 1452 cm−1) functionalities. On a detailed comparison of 1D and 2D NMR data (Table S1, SI), compound 2 showed the same planar structure as that of 1, except for the presence of an additional hydroxyl group and the concomitant absence of a keto-carbonyl group. In the HMBC spectrum of 2 (Figure 4),
Figure 2. Key 1H−1H COSY, HMBC (H → C), and NOESY correlations for 1.
H2-1 to C-3, C-4, and C-15; from H3-16 to C-1, C-2, and C-3; from H-3 to C-1 and C-15; and from H-4 to C-1, C-3, C-5, C6, and C-14 constructed a five-membered ring that was fused to the macrocycle at the C-4 and C-15, which was substituted with a methyl group (CH3-16) at C-2. HMBC correlations from H-5 to C-7, from H3-17 to C-5, C-6, and C-7; from H-7 to C-8 and C-9; from H2-8 to C-6 and C-12; from H-12 to C-14; and from H3-20 to C-12, C-13, and C-14 revealed the presence of a tenmembered macrocyclic skeleton, with two methyl groups (CH3-17 and CH3-20) at C-6 and C-13, respectively, a double bond at C-5 and C-6, and two keto-carbonyls at C-9 and C-14. HMBC correlations from H3-18 and H3-19 to C-10 and C-11 and from H-12 to C-10 and C-11 demonstrated the presence of an isobutenyl fragment at C-12. In addition, HMBC correlations from H-3 to C-7′ and from H-7 to C-1″ indicated the presence of one benzoyloxy group at C-3 and one acetoxy group at C-7. At last, the remaining one acetoxy group could only be attached at C-15. The planar structure of 1, therefore, was established, as depicted in Figure 1. The relative configuration of 1 was elucidated by analyzing the correlations detected in the NOESY spectrum. The NOESY correlations (Figure 2) of H3-16/H-4, H-4/H3-17, H3-17/H8α, H-8α/H-12, and H-12/H3-20 suggested that these protons were cofacial and were randomly assigned as α-oriented. On the other hand, the NOESY correlations from H-5/H-2′, H-5/H13, H-2′/H-2″, and H-2′/H-2‴ indicated that these protons were β-oriented. Moreover, the geometry of the Δ5 double bond was assigned as E based on the NOESY correlations of H4/H3-17 and H-5/H-7. Fortunately, the absolute configuration of compound 1 was confirmed to be 2R, 3S, 4S, 7R, 12R, 13R,
Figure 4. Key 1H−1H COSY, HMBC (H → C), and NOESY correlations for 2.
correlations from H-5 to C-7, from H3-17 to C-7, from H-12 to C-14, from H3-20 to C-14, and from H-14 to C-1″ revealed the presence of a hydroxyl group at C-7 and an acetoxy group at C14, which were different from 1. Moreover, apart from the relative configurations of C-12 and C-13 in compound 2, the relative configuration of 2 was found to be the same as that of 1. As shown in Figure 4, the NOESY correlations of H-12/H14, H-14/H3-20, and H-12/H3-2‴ indicated that H-12 is βoriented, while the NOESY correlation of H-4/H-13 indicated that H-13 is α-oriented. Finally, the absolute configuration of 2 was determined by comparison of experimental and theoretical ECD spectra of compound 2. The CD spectrum of 2 displayed two Cotton effects at 227 (Δε 3.70) and 283 (Δε 2.86) nm, corresponding to π−π* and n−π* transitions of the benzoyloxy 3125
DOI: 10.1021/acs.orglett.8b01215 Org. Lett. 2018, 20, 3124−3127
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7 and 8, respectively. Then, the intermediates 7 and 8 subsequently involved an oxidative cleavage between C-9 and C-10 as well as the formation of a double bond at C-10 and C11 to give 1 and 2. Obviously, the configurations of 3 and 4 are important and reasonable to propose the same absolute configurations of 1 and 2, except for that of C-12. In order to exclude the possibilities of 1 and 2 being formed by heating and irradiating during the isolation procedure, compounds 1 and 2 were detected in the crude extract prepared at room temperature and under dark condition by LC-MS (Figures S27−S31, SI). These results suggested that 1 and 2 are not artificial products and are presumed to be generated in the physiological process of plant growth. In order to further define the absolute configurations of 1 and 2 as well as validate their postulated biosynthetic pathways (shown as black arrows in Scheme 1), the biomimetic conversions from 3 and 4 to 1 and 2 were carried out, respectively. Guided by the biosynthetic hypothesis (Scheme 1), a photochemical rearrangement reaction of β,γ-unsaturated ketone was performed to afford 1 from the speculative precursor 3 that is a known jatrophane diterpenoid.7,8 Irradiating 3 in dry toluene using a 500 W high-pressure mercury lamp under an argon atmosphere for 12 h produced 3a in 15% yield (Scheme 2). The 1D NMR spectrum and specific
and the isolated carbonyl chromophores, respectively. Using time-dependent density functional theory (TDDFT) calculations at the B3LYP/6-31g(d) level, the experimental ECD spectrum of 2 is in excellent agreement with the theoretically calculated ECD spectrum of 2a (2R, 3S, 4S, 7R, 12S, 13S, 15R), supporting assignment of the absolute configuration (Figure 5).
Figure 5. Experimental ECD spectrum of 2 and calculated ECD spectra of 2a and 2b in MeOH.
Scheme 2. Photochemical Rearrangement Reactions of 3 and 4
Heliojatrones A (1) and B (2) have been identified as the first representatives of diterpenoids featuring a novel transbicyclo[8.3.0]tridecane core. Two hypothetical biogenetic pathways for 1 and 2 were proposed in Scheme 1 (shown as Scheme 1. Hypothetical Biosynthetic Pathways for 1 and 2
rotation of 3a (Figures S2 and S3, SI) are identical to those of natural heliojatrone A (1). Compound 4, named 2-epieuphornin I,6a is also a known jatrophane diterpenoid, of which the absolute configuration was determined by X-ray crystallographic analysis. Irradiating 4 in dry toluene using a 500 W high-pressure mercury lamp under an argon atmosphere for 12 h produced 4a in 12% yield (Scheme 2). The NMR spectrum and specific rotation of 4a (Figures S4 and S5, SI) are identical to those of natural heliojatrone B (2). Accordingly, the absolute configurations of 1 and 2 were established as depicted. The P-glycoprotein inhibitory activities of compounds 1−4 were evaluated by measuring the change in the intracellular adriamycin accumulation in the ADM-resistant human breast adenocarcinoma cell line (MCF-7), and cyclosporine A (CsA) was used as positive control.9 As a result, compound 2 inhibited P-glycoprotein significantly in a concentration-dependent manner, which showed similar P-glycoprotein inhibitory activity compared with CsA and exhibited stronger P-glycoprotein inhibitory activity than its precursor (4) (Table 1 and Figure
black solid and blue dashed arrows, respectively). Starting from the jatrophane skeleton, two intermediates 3 (euphoscopin F) and 4 (2-epi-euphornin I) were generated by oxidation and esterification reactions. Then, a primary Norrish-type I cleavage leading to the 9,10-biradical intermediates 5 and 6, accompanied by bonding at C-12 of the allylic radical moiety and allylic rearrangement, generated 1 and 2, respectively.8 Alternatively, a double bound of 3 and 4 at C-11 and C-12 first attacked C-9 followed by the new C-9−C-12 linkage to obtain 3126
DOI: 10.1021/acs.orglett.8b01215 Org. Lett. 2018, 20, 3124−3127
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Moreno, S.; Appendino, G.; Munoz, E. Curr. Drug Targets 2011, 12, 348−356. (e) Kissin, I.; Szallasi, A. Curr. Top. Med. Chem. 2011, 11, 2159−2170. (f) Yu, S. S.; Liu, Y. F. J. Asian Nat. Prod. Res. 2017, 19, 1047−1072. (2) (a) Schnabel, C.; Sterz, K.; Müller, H.; Rehbein, J.; Wiese, M.; Hiersemann, M. J. Org. Chem. 2011, 76, 512−522. (b) Rinner, U. Eur. J. Org. Chem. 2015, 15, 3197−3219. (3) (a) Wan, L. S.; Shao, L. D.; Fu, J. B.; Xu, J.; Zhu, G. L.; Peng, X. R.; Li, X. N.; Li, Y.; Qiu, M. H. Org. Lett. 2016, 18, 496−499. (b) Barile, E.; Lanzotti, V. Org. Lett. 2007, 9, 3603−3606. (c) Jakupovic, J.; Morgenstern, T.; Bittner, M.; Silva, M. Phytochemistry 1998, 47, 1601−1609. (4) (a) Hohmann, J.; Evanics, F.; Dombi, G.; Szabó, P. Tetrahedron Lett. 2001, 42, 6581−6584. (b) Macro, J. A.; Sanz-Cervera, J. F.; Yuste, A.; Jakupovis, J.; Jeske, F. Phytochemistry 1998, 47, 1621−1630. (c) Hohmann, J.; Evanics, F.; Dombi, G.; Molnár, J.; Szabó, P. Tetrahedron 2001, 57, 211−215. (5) Hua, Y. X.; Liu, S. F.; Yang, Q. Chinese Bencao; Shanghai Science and Technology Press: Shanghai, 1999; Vol. 4, pp 782−785. (6) (a) Mai, Z. P.; Ni, G.; Liu, Y. F.; Li, Y. H.; Li, L.; Li, J. Y.; Yu, D. Q. J. Org. Chem. 2018, 83, 167−173. (b) Mai, Z. P.; Ni, G.; Liu, Y. F.; Li, L.; Shi, G. R.; Wang, X.; Li, J. Y.; Yu, D. Q. Sci. Rep. 2017, 7, 4922− 4929. (c) Mai, Z. P.; Ni, G.; Liu, Y. F.; Zhang, Z.; Li, L.; Chen, N. H.; Yu, D. Q. Acta Pharm. Sin. B 2018, DOI: 10.1016/j.apsb.2018.03.011. (7) (a) Yamamura, S.; Shizuri, Y.; Kosemura, S.; Ohtsuka, J.; Tayama, T.; Ohba, S.; Ito, M.; Saito, Y.; Terada, Y. Phytochemistry 1989, 28, 3421−3436. (b) Tao, H. W.; Hao, X. J.; Liu, P. P.; Zhu, W. M. Arch. Pharmacal Res. 2008, 31, 1547−1551. (8) (a) Paquette, L. A.; Eizember, R. F.; Cox, O. J. Am. Chem. Soc. 1968, 90, 5153−5159. (b) Paquette, L. A.; Eizember, R. F. J. Am. Chem. Soc. 1967, 89, 6205−6208. (c) Paquette, L. A.; Meehan, G. V. J. Org. Chem. 1969, 34, 450−454. (d) Kärkäs, M. D.; Porco, J. A. J.; Stephenson, C. R. J. Chem. Rev. 2016, 116, 9683−9747. (9) Zhao, X. R.; Huo, X. K.; Dong, P. P.; Wang, C.; Huang, S. S.; Zhang, B. J.; Zhang, H. L.; Deng, S.; Liu, K. X.; Ma, X. C. J. Nat. Prod. 2015, 78, 1868−1876.
S1, SI), while weak P-glycoprotein inhibitory activities were observed for 1, 3, and 4. Table 1. Inhibitory Effects of P-Glycoprotein-Mediated DAM Efflux by Compounds 1−4 in MCF-7/ADM Cells samples CsA 1 2 3 4
IC50 (μM) 0.84 12.03 0.58 8.65 3.57
± ± ± ± ±
0.03 4.14 0.05 3.35 1.46
In summary, heliojatrones A (1) and B (2) bear a previously undescribed 5/10-fused bicyclic carbon skeleton formed through jatrophane diterpenoids with a rearrangement reaction of β,γ-unsaturated ketone. Their structures were unambiguously confirmed by extensive spectroscopic analyses, especially by biomimetic conversion and X-ray crystallography. Remarkably, heliojatrone B (2) showed P-glycoprotein inhibitory activity against MCF-7/ADR and provided a new structural template for the development of potential MDR reversal agents from natural products.
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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.orglett.8b01215. Detailed experimental procedures; CD, UV, IR, MS, and NMR spectra for compounds 1 and 2 (PDF) Accession Codes
CCDC 1831599 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
De-Quan Yu: 0000-0003-4774-6419 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the CAMS Innovation Fund for Medical Science (CIFMS: 2016-I2M-1-010). The authors are grateful to the Department of Instrumental Analysis of Peking Union Medical College for the spectroscopic measurements.
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REFERENCES
(1) (a) Vasas, A.; Hohmann, J. Chem. Rev. 2014, 114, 8579−8612. (b) Wang, H. B.; Wang, X. Y.; Liu, L. P.; Qin, G. W.; Kang, T. G. Chem. Rev. 2015, 115, 2975−3011. (c) Vasas, A.; Redei, D.; Csupor, D.; Molnar, J.; Hohmann, J. Eur. J. Org. Chem. 2012, 27, 5115−5130. (d) Sanchez-Duffhues, G.; Vo, M. Q.; Perez, M.; Calzado, M. A.; 3127
DOI: 10.1021/acs.orglett.8b01215 Org. Lett. 2018, 20, 3124−3127
