Letter Cite This: Org. Lett. 2018, 20, 3682−3686
pubs.acs.org/OrgLett
Magterpenoids A−C, Three Polycyclic Meroterpenoids with PTP1B Inhibitory Activity from the Bark of Magnolia of ficinalis var. biloba Chuan Li, Chuang-Jun Li, Jie Ma, Fang-You Chen, Li Li, Xiao-Liang Wang, Fei Ye, and Dong-Ming Zhang* 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, People’s Republic of China
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S Supporting Information *
ABSTRACT: Magterpenoid A (1), possessing a rare 4,6,11trioxatricyclo[5.3.1.01,5]undecane framework with an irregular monoterpenoid moiety, magterpenoid B (2), with an unprecedented 6/6/6/6 polycyclic skeleton, and magterpenoid C (3), a novel terpenoid quinone with a C6−C3 unit, were isolated from the bark of Magnolia off icinalis var. biloba. Plausible biogenetic pathways of 1-3 are presented. Compounds 1 and 3 exhibited significant PTP1B inhibitory activities with IC50 values of 1.44 and 0.81 μM, respectively.
T
he bark of Magnolia off icinalis Rehd. et Wils (Magnolia of ficinalis) and Magnolia off icinalis Rehd. et Wills var. biloba (Magnolia off icinalis var. biloba), important traditional Chinese medicines,1,2 are widely used in many prescriptions for the treatment of pain from abdominal distension, dyspepsia, and phlegm. Previous studties2,3 of M. off icinalis showed the presence of neolignans, terpenoids, alkaloids, and phenylethanoid glycosides. Biological investigations4−9 on the main chemical constitutions of M. off icinalis have been widely reported, and the components have shown biological activities, including antitumor, antihyperglycemia, antihypertrophic, neurotropic, memory enhancing, and antiosteoclast differentiation activities. Their structural diversity and biological activities have attracted the interest of both natural products1 and synthetic chemists.10 Protein tyrosine phosphatase 1B11 (PTP1B) regulates many cellular responses relating to the important target of diabetes. In our study, 10 μg of the ethanolic extract of the bark of M. off icinalis var. biloba showed 97.7% inhibition of PTP1B,12 and the bioassay-guided separation of this extract led to the isolation of three novel compounds, magterpenoids A−C (1− 3) (Figure 1); compound 1 possesses a 4,6,11-trioxatricyclo[5.3.1.01,5]undecane framework with an irregular monoterpenoid moiety, and 2 possesses an unprecedented 6/6/6/6 polycyclic ring system. Biogenetically, compounds 2 and 3 were biosynthesized from a common precursor via a [4 + 2] Diels− Alder cycloaddition.13 Detailed information regarding the isolation, structure elucidation, biogenetic pathway, and bioactivity evaluation of 1−3 is reported. Compound 1, [α]25 D −26.3 (c 0.10, MeOH), was obtained as colorless needles (MeOH). The molecular formula, C28H32O4, was deduced from the (+)-HRESI-MS ions at m/z 455.2202 © 2018 American Chemical Society
Figure 1. Chemical structures of compounds 1−3.
[M + Na]+, calcd for C28H32O4Na (455.2193), revealing 13 degrees of unsaturation. The IR spectrum showed absorption bands for hydroxyl moieties (at 3428 cm−1) and a carbon− carbon double bond (at 1640 cm−1). The 1H NMR spectrum (Table 1) of 1 showed proton signals for a 1,2,3,5tetrasubstituted benzene ring [δH 7.12 (1H, d, J = 3.0 Hz), 7.09 (1H, d, J = 3.0 Hz)], a 1,2,4-trisubstituted benzene ring with one ABX coupling pattern [δ H 6.97 (1H, d, J = 2.4 Hz), 6.93 (1H, dd, J = 8.4 Hz, 2.4 Hz), 6.79 (1H, d, J = 8.4 Hz)], and a downfield signal at δH 9.21 (4′-OH). In combination with the HSQC data, two dioxygenated carbon signals [δc 108.0 (C10) and 111.7 (C-15)], four olefinic carbon signals [δc 116.0 (C-8) and 138.5 (C-9), δc 115.7 (C-8′) and 138.6 (C-9′)], and one oxygenated quaternary carbon signal at δc 93.9 (C-11) were observed in the 13C NMR (Table 1) and DEPT spectrum of 1; the 12 aromatic carbons were attributed to two benzene rings. Moreover, two double bonds and two benzene rings accounted for 10 degrees of unsaturation, and the remaining Received: May 10, 2018 Published: June 4, 2018 3682
DOI: 10.1021/acs.orglett.8b01476 Org. Lett. 2018, 20, 3682−3686
Letter
Organic Letters Table 1. 1H (600 MHz) and 13CNMR (150 MHz) Data of Compounds 1 and 2 in DMSO-d6a 1 no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ a
δH (J in Hz)
2 δc
δH (J in Hz)
δc
1-OH 7.79, 6.45, dd (10.2, 2.4) 5.98, d (10.2)
7.12, d (3.0) 3.32, brd (6.6)
131.8 133.0 121.9 156.1 123.7 124.7 39.3
96.7 143.2 129.0 196.1 48.7 41.7 27.7
5.94, m 5.01, m 6.20, s
138.5 116.0 108.0
7.09, d (3.0)
2.38, m H-α, 1.54, m H-β, 1.71, m H-α, 1.63, m H-β, 1.74, m 1.37, 1.94, 0.67, 0.88,
s m d (6.6) d (7.2)
6.97, d (2.4) 4′-OH, 9.21, 6.79, d (8.4) 6.93, dd (8.4, 2.4) 3.24, brs 5.90, m 4.97, m
and that the second oxygen atom bridged C-4 and C-10. The above-mentioned information indicated the presence of a seven-membered oxygen heterocycle (rings A and B). The HMBC correlations from H-10 to C-5 and from H-6 to C-11 suggested C-11 was located at C-5. Thus, ring C (C-5−C-4− O−C-10−C-11) was established. Obviously, the tricyclic system in the structure of 1 was satisfied by the 11,15-oxide bridge, which was determined by the chemical shifts of oxygenated carbon C-11 (δc93.9) and dioxygenated carbon C-15 (δc 111.7). Along with the 1,2,4-trisubstituted benzene and the 1,2,3,5-tetrasubstituted benzene proton signals, the 1 H−1H COSY correlations of H-7/H-8/H-9 and H-7′/H-8′/ H-9′ and the HMBC correlations from H-6 to C-7 and C-2 and from H-7′ to C-1′ constructed two allylbenzene moieties (C1−C-9, C-1′−C-9′). The correlation from H-2′ to C-3 suggested two C6−C3 fragments conjugated through the carbon−carbon bond between C-3 and C-3′. The relative configuration of 1 was determined from its NOESY spectrum, which showed correlations of H-10/Hα-13, and H-12/Hβ-14, which suggested that H-10, Hα-13, Hα-14 were on the same face and H-12, Hβ-13, and Hβ-14 were on the opposite face. The X-ray diffraction experiment (Figure 3) using Cu Kα radiation with a Flack parameter of 0.0(3) confirmed the absolute configuration of compound 1 to be (10R,11R,12S,15S).
93.9 40.5 18.1
2.27, m H-α 1.74, m H-β 2.15, m 5.38, brd (3.6) H-α 3.28, brd (5.4) H-β 1.87, m 1.89, m 1.96, m
34.8 111.7 23.7 27.8 17.9 22.1 130.0 131.3 123.6 153.2 116.2 129.0 39.1 138.6 115.7
135.3 119.2 29.7 37.2 26.2 124.2 131.2
1.50, s 1.59, s
6.87, d (2.4)
6.78, d (6.4) 6.99, dd (6.4, 2.4) 3.23, brd (7.2) 5.86, m 5.03, m
18.0 25.9
132.9 126.3 123.5 151.3 117.5 129.7 39.1 138.2 116.1
Figure 3. ORTEP drawing of 1.
Data were recorded on a Bruker AV-600 spectrometer.
three degrees of unsaturation hinted at the presence of three additional rings in this molecule. Interpretation of the HMBC and 1H−1H COSY spectra (Figure 2) constructed the planar structure of compound 1.
Compound 2, a colorless oil, possessed a molecular formula of C25H28O3, which was deduced from the (+)-HRESI-MS ions at m/z 399.1927 [M + Na]+ [calcd for C25H28O3Na (399.1931)], which indicates 12 degrees of unsaturation. Its IR spectrum exhibited absorption bands indicative of hydroxy
Figure 2. Key 2D NMR correlations of 1.
The 1H−1HCOSY correlations of H-14/H-13/H-12/H-17/H18 established the linkage of C-14−C-13−C-12−C-17−C-18, and the key HMBC correlations from H-10 to C-4, C-11, and C-15 in combination with their chemical shift values [C-10 (δc 108.0), C-15 (δc 111.7) and C-4 (δc 156.1)] confirmed the connection between C-10 and C-15 through an oxygen atom
Figure 4. Key 2D NMR correlations of 2 and 3. 3683
DOI: 10.1021/acs.orglett.8b01476 Org. Lett. 2018, 20, 3682−3686
Letter
Organic Letters (3340 cm−1) and carbonyl (1686 cm−1) functionalities. The 1H NMR spectrum of 2 (Table 1) showed the presence of a hydroxy proton δH7.79 (1H, s) and three aromatic protons [δH 6.78 (1H, d, J = 6.4 Hz), δH 6.87 (1H, d, J = 2.4 Hz), and δH 6.99 (1H, dd, J = 2.4, 6.4 Hz)]. In conjunction with the HSQC spectrum, the 13C NMR spectrum of 2 indicated the presence of two methyls, five methylenes, one methine, eight olefinic carbons, six aromatic carbons, and three quaternary carbons (including one ketocarbonyl and one hemiketal carbonyl). The shifts of δc 196.1, δc 143.2, and δc 129.0 showed the presence of an α,β-unsaturated ketone fragment. The HMBC correlations from 1-OH to C-1, C-2, and C-6; from H-3 to C-4 and C-5; from H-6 to C-1, C-4, and C-5; and from H-10 to C-8 along with the 1H−1H COSY cross peaks of H-2/H-3, H-6/H-7, and H-9/H-10 supported the establishment of A/B rings, and the 1H−1H COSY correlations of H11/H-12/H-13 in conjunction with the HMBC correlations between H-15 and C-13 and C-14 and between H-16 and C-14 confirmed the presence of a 2-methyl-2-pentenyl side chain, which was attached at C-8 by the HMBC correlations of H-11 with C-7 and C-8. The ABX coupling system in the 1H NMR spectrum indicated a 1,2,4-trisubstituted benzene ring (ring D), which when combined with the HMBC correlations from H-7′ to C-1′ and C-6′; from H-5′ to C-1′, C-3′, and C-4′ and the 1 H−1H COSY correlations of H-7′/H-8′/H-9′, defined the structure of the allylbenzene unit (C-1′−C-9′). For ring C, the HMBC correlations from H-6 to C-5 and C-3′ and from H-2′ to C-5 indicated the connectivity of C-5 and C-3′ through a C− C bond and the presence of an oxygen bridge between C-1 and C-4′, which was confirmed by the chemical shifts of C-1 and C4′ shifts (δc 96.7 and 151.3, respectively) and an additional degree of unsaturation. Thus, the planar structure of 2 was constructed. The relative configuration of 2 was determined by analysis of its NOESY spectrum. The correlations of H-6/Hβ-10, 1-OH/ H-6, and 1-OH/Hβ-7 suggested that H-6, Hβ-7, Hβ-10, and 1OH were β-oriented; the Hα-7, Hα-10, and C6−C3 fragment were α-oriented. The electronic circular dichroism (ECD) spectrum of 2 showed no Cotton effect, indicating that 2 was a racemic mixture. Subsequently, an AD-H column yielded (+)-2 and (−)-2 (Figure 5), and the theoretically calculated ECD (Figure 6) of (1S, 5R, 6R)-2 and (1R, 5S, 6S)-2 well matched with the experimental ECD of (+)-2 and (−)-2, respectively. Thus, the absolute structures of (+)-2 and (−)-2 were defined. Compound 3 was found to have a molecular formula of C25H24O3 based on its (+)-HRESI-MS spectrum. Its 13C NMR
Figure 6. Experimental ECD spectra of (+)-2 and (−)-2 and calculated ECD spectra of (1S,5R,6R)-2 and (1R,5S,6S)-2.
spectrum (see the Supporting Information Table S1) showed two quinoid carbonyl groups at δc 185.4 and 183.8. Its 1H NMR spectrum showed four aromatic signals at δH 6.97 (s), δH 8.01 (d, J = 7.8 Hz), δH 7.63 (dd, J = 1.8, 7.8 Hz), and δH 7.88 (d, J = 1.8 Hz), indicating the presence of a 2,9-disubstituted naphthoquinone and three aromatic signals indicating the presence of a 1,2,4-trisubstituted benzene ring [δH 6.80 (d, J = 8.4 Hz), δH 7.09 (dd, J = 1.8, 8.4 Hz), and δH 7.01 (d, J = 1.8 Hz)]. The structure of the 2-methyl-2-pentenyl side chain was confirmed by the HMBC (Figure 4) crosspeaks from H-16 to C-14 and C-13 and from H-15 to C-14 and the 1H−1H COSY correlations of H-11/H-12/H-13. Along with a 2,9-disubstituted naphthoquinone moiety, the HMBC spectrum showed correlations from H-11 to C-9 and from H-10 to C-11, which established the linkage between C-9 and C-11. Combined with the 1,2,4-trisubstituted benzene ring, the HMBC crosspeaks from H-2′ to C-2 and from H-2′ to C-2 indicated a C−C bond between C-2 and C-3′. Additionally, the structure of compound 3 was confirmed by the single-crystal X-ray diffraction experiment (Figure 7).
Figure 7. ORTEP drawing of 3.
Compounds 1−3 possess different types of monoterpenoid moieties, which are generated from the progenitor GPP. Plausible biogenetic pathways for compounds 1−3 are presented in Scheme 1. Intermediate i is formed by the addition reaction of the presumed terpinyl cation and magnolol precursors; key intermediate iv is produced from intermediate i through a Hock cleavage, hydrolysis, oxidation sequence; intermediate iv is generated from compound 1 by acetal and ketal reactions. A pair of enantiomers (±)-2 is produced by ketal reactions of intermediate vi in different orientations, and iv was derived from the putative β-myrcene and randaiol precursors through a Diels−Alder cycloaddition reaction, which takes place between β-myrcene and the carbon−carbon double bond on the substituted side of the benzoquinone of intermediate v. The D−A reaction to produce intermediate vii involves β-myrcene and the CC bond on the
Figure 5. HPLC separation chromatogram of 2 on the chiral AD-H column. 3684
DOI: 10.1021/acs.orglett.8b01476 Org. Lett. 2018, 20, 3682−3686
Organic Letters
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Scheme 1. Plausible Biogenetic Pathways for 1−3
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01476. Experimental section; UV, IR, MS, 1D, 2D NMR, and ECD spectra of compounds 1−3 (PDF) Accession Codes
CCDC 1838527−1838528 contain 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
[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
unsubstituted side of the benzoquinone. Intermediate vii is converted to compound 3 through a dehydrogenation reaction. In this work, the isolated compounds were evaluated for their inhibitory effects against PTP1B, and compounds 1 and 3 had significant PTP1B inhibitory activities with IC50 values of 1.44 and 0.81 μM, respectively. In addition, 10 μM of compounds (+)-2 and (−)-2 exhibited 30.6% and 40.5% PTP1B inhibitory activities. Moreover, the neuroprotective effects of compounds 1−3 against glutamic acid and oxygen glucose deprivation14 (OGD)-induced SK-N-SH cell injury indicated compounds 1, (+)-2, and (−)-2 at a concentration of 10 μM (Figure 8),
Dong-Ming Zhang: 0000-0003-2016-8639 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Sciences Foundation of China (No. 81730093), the CAMS Innovation Fund for Medical Sciences (No. 2016-I2M-2-003), and the CAMS Initiative for Innovative Medicine Project (CAMS-I2M1-010).
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REFERENCES
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Figure 8. Neuroprotective effects of 1−3 on ODG- and glutamic acid induced injury of SK-N-SH cells (10 μM, means ± SD, n = 3). ##p < 0.001 versus control, *p < 0.005 versus model. Positive controls: donepezil and tetraethylammonium (TEA).
increased glutamic acid-induced SK-N-SH cell injury from 62.5% to 76.0%, 64.7%, and 70.0%, and increased OGDinduced SK-N-SH cell injury from 47.2% to 54.5%, 49.1%, and 48.6%, respectively. They are more powerful than positive control drugs donepezil (45.3% and 46.2%) and TEA (62.8% and 60.8%). In conclusion, we have reported three novel compounds, magterpenoids A−C (1−3), from the bark of M. off icinalis var. biloba. Compound 1 possesses an irregular meroterpenoid with a rare 4,6,11-trioxatricyclo[5.3.1.01,5]undecane skeleton. Compound 2 features a 6/6/6/6 polycyclic skeleton, and 3 is a novel terpenoid quinone with a C6−C3 unit. Their relative and absolute configurations were determined by 1D and 2D NMR spectroscopy, X-ray diffraction, and ECD calculations. The significant PTP1B inhibitory effects of compounds 1 and 3 highlighted their value as templates for the exploration of antitype 2 diabetes mellitus agents. 3685
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DOI: 10.1021/acs.orglett.8b01476 Org. Lett. 2018, 20, 3682−3686
