- Email: [email protected]
Citrinovirin with a new norditerpene skeleton from the marine algicolous fungus Trichoderma citrinoviride
Bioorganic & Medicinal Chemistry Letters 26 (2016) 5029–5031
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl
Citrinovirin with a new norditerpene skeleton from the marine algicolous fungus Trichoderma citrinoviride Xiao-Rui Liang a,b,c, Feng-Ping Miao a, Yin-Ping Song a,c, Xiang-Hong Liu a, Nai-Yun Ji a,⇑ a
Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China Naval Aeronautical and Astronautical University, Yantai 264001, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 26 May 2016 Revised 26 August 2016 Accepted 30 August 2016 Available online 31 August 2016 Keywords: Marine algicolous fungi Trichoderma citrinoviride Norditerpene Citrinovirin
a b s t r a c t Citrinovirin (1), a novel norditerpene with an unprecedented carbon skeleton along with three known compounds, cyclonerodiol (2), 3-(2-hydroxypropyl)-4-(hexa-2E,4E-dien-6-yl)furan-2(5H)-one (3), and 5-hydroxy-3-hydroxymethyl-2-methyl-7-methoxychromone (4), was isolated from the culture of a marine brown alga-endophytic strain (cf-27) of Trichoderma citrinoviride. The structure and relative configuration of 1 were identified by spectroscopic methods, including 1D/2D NMR and MS. Its absolute configuration was established by analysis of ECD spectrum, aided by quantum chemical calculations. A plausible biogenetic pathway is proposed for 1, and it was evaluated to be active against Staphylococcus aureus. Ó 2016 Elsevier Ltd. All rights reserved.
Marine-derived fungi have attracted a great attention for natural product researchers since the end of the last century, which afforded a series of candidates for the developments of new drugs and functional agents.1 Marine alga-derived fungi made up a large group of them, and they were even the main contributors of new compounds of marine fungal origin.2 More than 360 new compounds, involving polyketides, terpenoids, polyketide-terpenoids, peptides, alkaloids, and shikimate derivatives, were produced by marine algicolous fungi.3,4 Especially, the structures featuring novel or rare scaffolds were often encountered in them, represented by varioxepine A,5 aspeverin,6 harzianone,7 aspewentins A and B.8 During our continuing investigation towards the structurally unique and biologically active metabolites from marine algicolous fungi, a brown alga-endophytic strain (cf-27) of Trichoderma citrinoviride was chemically examined. As a result, a novel norditerpene, citrinovirin (1), with an unprecedented skeleton along with three known compounds, including cyclonerodiol (2),9 3-(2-hydroxypropyl)-4-(hexa-2E,4E-dien-6-yl)furan-2(5H)-one (3),10 and 5-hydroxy-3-hydroxymethyl-2-methyl-7-methoxychromone (4),11 was isolated and identified (Fig. 1). Herein, the isolation, structure elucidation, bioactivity evaluation, and plausible biogenetic pathway of compound 1 are described. Compound 1 was obtained as a colorless oil with a specific optical rotation ([a]24 10.9 (c 0.18, MeOH). The positive D ) value of ⇑ Corresponding author. Tel.: +86 535 2109176; fax: +86 535 2109000. E-mail address: [email protected] (N.-Y. Ji). http://dx.doi.org/10.1016/j.bmcl.2016.08.093 0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.
electrospray ionization mass spectrum (ESI+MS) showed an array of peaks at m/z 340, 345, 662, 667, and 990, corresponding to [M +NH4]+, [M + Na]+, [2M+NH4]+, [2M+Na]+, and [3M+Na]+. Thus, a molecular weight of 322.2 was assigned to 1. Additionally, the fragmental ion peaks at m/z 305 [M H2O+H]+ and 287 [M 2H2O+H]+ suggested the presence of two hydroxy groups. A molecular formula of C19H30O4 was determined by interpretation of high-resolution ESI+MS (m/z 345.2040 [M+Na]+, calcd for C19H30O4Na, 345.2042), requiring five degrees of unsaturation. The 1H NMR spectrum (Table 1) of compound 1 showed one methyl singlet, two methyl doublets, one methyl triplet, two doublets of double doublets attributable to two oxygenated methines, one doublet of double doublets and one double doublet ascribable to two olefinic protons, and two doublets assignable two exchangeable protons. According to MS data, the exchangeable protons should be derived from two hydroxy groups. The 13C NMR spectrum (Table 1) along with DEPT and HSQC data demonstrated the presence of four methyls, three methylenes, ten methines, and two nonprotonated carbons. Based on 1H–1H COSY correlations (Fig. 1), all the hydrogen-bearing groups except for Me-19 made up a large spin system, C-2 to C-10 and C-15. Especially, C-6 was bonded to C-10 to form ring A by the COSY correlations between H-6/H-10. C-8 and C-9 were hydroxylated by COSY correlations between H-8/OH-8 and between H-9/OH-9, and C-7 and C-11 were methylated by those between H-7/H3-18 and between H-11/H3-16. Moreover, the connectivity at C-1 was established by the HMBC correlations from H3-19 to C-1, C-2, and C-10,
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X.-R. Liang et al. / Bioorg. Med. Chem. Lett. 26 (2016) 5029–5031 HO
15
H
13
H
19 11
H1
16 3
17
H OH
OH
9
O
A
O
OH
2
7 5
H 18
1
OH O O 3
OH O O
OH
COSY HMBC
O
O
OH O 4
OH
Figure 1. Structures of 1–4 and key 1H–1H COSY and HMBC correlations of 1.
Table 1 H and 13C NMR data for 1 (at 500 MHz for 1H and 125 MHz for
1
Pos. 1 2 3 4 5 6 7 8 9 10 11 12a 12b 13a 13b 14 15 16 17 18 19 OH-8 OH-9
dH, mult. (J in Hz) 2.21, 3.36, 5.74, 5.85, 2.09, 1.39, 3.31, 3.81, 2.19, 1.98, 1.28, 1.47, 1.19, 1.46, 1.32, 0.92, 0.89,
dd (10.6, 5.8) m ddd (11.0, 7.6, 2.6) dd (11.1, 1.7) br t (12.2) m ddd (7.1, 7.1, 7.1) ddd (8.8, 8.2, 6.6) dd (12.8, 9.4) m m m m m m t (7.2) d (6.3)
1.07, 1.74, 4.78, 4.54,
d (6.6) s d (6.4) d (6.4)
13
C, in DMSO-d6) dC 91.6, C 61.0, CH 47.4, CH 125.6, CH 134.1, CH 43.8, CH 46.3, CH 75.8, CH 72.4, CH 62.4, CH 31.8, CH 34.9, CH2 29.3, CH2 23.0, CH2 14.9, CH3 20.2, CH3 175.5, C 17.3, CH3 26.7, CH3
Figure 2. Energy-minimized conformer and key NOE correlations (arrows, the solid on b face and the dashed on a face) of 1.
In order to determine the absolute configuration of compound 1, we did our best to crystallize it for an X-ray diffraction analysis but failed under various conditions. Then, we turned to the electronic circular dichroism (ECD) technique, which was also reliable to confirm the absolute configurations of organic molecules aided by quantum chemical calculations.7,12 The ECD spectrum of 1 was determined, and it exhibited a positive Cotton effect at 223 nm. The conformers were generated by the Dreiding force field in MarvinSketch13 and further optimized using density function theory (DFT) at the gas-phase B3LYP/6-31G(d) level via Gaussian 09 software.14 The energy-minimized conformer matching NOE correlations (Fig. 2) was submitted to ECD computation with time-dependent DPT method at the same level, and the spectrum was produced by SpecDis software.15 The accordance between calculated ECD spectrum and experimental one (Fig. 3) suggested the absolute configuration to be 1R, 2R, 3S, 6S, 7S, 8R, 9S, 10R, and 11R, which could also be supported by the exciton chirality method.16 Compound 1 possesses a perhydroazulene ring system, and it looks like a cage due to the presence of c-lactone. Although pachydictyol and dictyol derivatives from the marine brown algae have a carbon skeleton similar to that of 1,17,18 the positions attached by side chains and methyl groups are quite different. The framework of pachydictyols and dictyols has been deduced to be formed by two-step cyclization of geranylgeranyl diphosphate (GGDP).18
exptl
12
calcd 8
Δε/M -1cm-1
and a carbonyl group (C-17) was attached to C-3 by the HMBC correlation from H-3 to C-17. To match the molecular formula, C-1 should be bonded to C-17 through an oxygen atom, which was also supported by their chemical shifts. The above data evidenced the gross structure of 1, which was further verified by the other HMBC correlations from H-3 to C-1, C-2, C-4, and C-5, from H3-18 to C-6, C-7, and C-8, from HO-8 to C-7, C-8, and C-9, from OH-9 to C-8, C-9, and C-10, H3-16 to C-2, C-11, and C-12, and H3-15 to C-13 and C-14 (Fig. 1). The relative configuration of compound 1 was established by analysis of NOE correlations (Fig. 2). H-6, H-8, H-9, and Me-18 were located on the same face of ring A by the NOE correlations of H-6 with H-8, H-9, and H3-18 and of H-8 with H3-18, whereas H-10 was oriented on the opposite face by its NOE correlation with H-7. Me-19 was positioned on the same face of H-9 based on their NOE correlation, and then the ester group was placed on the other face. Additionally, C-11 was syn and vicinal to H-6 by the NOE correlation between H-11 and H-6, which along with the NOE correlations between H-3 and H3-16 and between H2-12 and H3-19 confirmed the relative configuration at C-11. Thus, the rotation of single bond between C-2 and C-11 was proposed to be restricted by the surrounding groups. The large coupling constant (12.2) of H-6 also suggested it to be opposite to H-7 and H-10, which further corroborated the relative configuration of 1.
4
0 200
220
240
-4
-8
Wavelength/nm Figure 3. Experimental and calculated ECD spectra of 1.
260
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X.-R. Liang et al. / Bioorg. Med. Chem. Lett. 26 (2016) 5029–5031
OPP H [H]
rearrangement
H cyclization
Bu
cyclization
-Me
GGDP
H 1b
1a
H 1c cyclization
Bu 1
Bu
H OH
SN2 reaction H
H
[O]
COOH
OH
H 1f
Bu
H OH H
O O- H
H [O]
H
OH
H 1e
H 1d
Figure 4. Proposed biosynthetic pathway for 1.
However, this process is almost impossible to yield 1, even if reasonable rearrangements are considered. Thus, 1 may be synthesized by a different biogenetic pathway (Fig. 4), which is proposed to contain the demethylation (GGDP to 1a), three-step cyclization (1a to 1d), oxidation at C-8/C-9/C-17and at C-4/C-5 (1d to 1f), and SN2 reaction with Walden inversion (1f to 1). It is worth to mention that the bicyclo[5.2.0]nonane ring system in 1d is also present in the harziane diterpenes from Trichoderma spp., which signifies the possibility of this pathway to some extent.7 Moreover, the tension of cyclobutyl moiety may play an important role in the formation of c-lactone ring. To evaluate the biological activity of citrinovirin (1), it was assayed for growth inhibition against two bacteria (Staphylococcus aureus and Escherichia coli), two phytopathogenic fungi (Colletotrichum lagenarium and C. gloeosporiodes), one marine zooplankton (Artemia salina), and four marine phytoplankton species (Chattonella marina, Heterosigma akashiwo, Prorocentrum donghaiense, and Scrippsiella trochoidea).7,19 The results (Table S1) showed that 1 inhibited the growth of S. aureus with an MIC value of 12.4 lg/mL, and it was also toxic to A. salina with an LC50 value of 65.6 lg/mL. Additionally, 1 exhibited 14.1–37.2% inhibition of C. marina, H. akashiwo, and P. donghaiense at 100 lg/mL, but it could promote the growth of S. trochoidea. Acknowledgments Financial support from the National Natural Science Foundation of China (31670355) and Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11020702) is gratefully acknowledged. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2016.08. 093.
References and notes 1. Blunt, J. W.; Copp, B. R.; Keyzers, R. A.; Munro, M. H. G.; Prinsep, M. R. Nat. Prod. Rep. 2015, 32, 116. 2. Rateb, M. E.; Ebel, R. Nat. Prod. Rep. 2011, 28, 290. 3. Ji, N.-Y.; Wang, B.-G. Fungal Divers. 2016. http://dx.doi.org/10.1007/s13225016-0358-9. 4. Zhang, P.; Li, X.-M.; Wang, J.-N.; Li, X.; Wang, B.-G. Phytochem. Lett. 2015, 11, 85. 5. Zhang, P.; Mándi, A.; Li, X.-M.; Du, F.-Y.; Wang, J.-N.; Li, X.; Kurtán, T.; Wang, B.G. Org. Lett. 2014, 16, 4834. 6. Ji, N.-Y.; Liu, X.-H.; Miao, F.-P.; Qiao, M.-F. Org. Lett. 2013, 15, 2327. 7. Miao, F.-P.; Liang, X.-R.; Yin, X.-L.; Wang, G.; Ji, N.-Y. Org. Lett. 2012, 14, 3815. 8. Miao, F.-P.; Liang, X.-R.; Liu, X.-H.; Ji, N.-Y. J. Nat. Prod. 2014, 77, 429. 9. Laurent, D.; Goasdoue, N.; Kohler, F.; Pellegrin, F.; Platzer, N. Magn. Reson. Chem. 1990, 28, 662. 10. Almassi, F.; Ghisalberti, E. L.; Narbey, M. J.; Sivasithamparam, K. J. Nat. Prod. 1991, 54, 396. 11. Takenaka, Y.; Tanahashi, T.; Nagakura, N.; Hamada, N. Heterocycles 2000, 53, 1589. 12. Li, K.; Brant, C. O.; Huertas, M.; Hur, S. K.; Li, W. Org. Lett. 2013, 15, 5924. 13. MarvinSketch with Calculator Plugins for Structure Property Prediction and Calculation. Marvin, Version 6.2.2; ChemAxon (http://www.chemaxon.com), 2014. 14. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T., ; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian Inc: Wallingford, CT, 2010. 15. Bruhn, T.; Hemberger, Y.; Schaumlöffel, A.; Bringmann, G. SpecDis, Version 1.51; University of Wuerzburg: Germany, 2011. 16. Berova, N.; Bari, L. D.; Pescitelli, G. Chem. Soc. Rev. 2007, 36, 914. 17. Abou-El-Wafa, G. S. E.; Shaaban, M.; Shaaban, K. A.; El-Naggar, M. E. E.; Maier, A.; Fiebig, H. H.; Laatsch, H. Mar. Drugs 2013, 11, 3109. 18. Hirschfeld, D. R.; Fenical, W.; Lin, G. H. Y.; Wing, R. M.; Radlick, P.; Sims, J. J. J. Am. Chem. Soc. 1973, 95, 4049. 19. Chen, C.; Imamura, N.; Nishijima, M.; Adachi, K.; Sakai, M.; Sano, H. J. Antibiot. 1996, 49, 998.
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl
Citrinovirin with a new norditerpene skeleton from the marine algicolous fungus Trichoderma citrinoviride Xiao-Rui Liang a,b,c, Feng-Ping Miao a, Yin-Ping Song a,c, Xiang-Hong Liu a, Nai-Yun Ji a,⇑ a
Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, China Naval Aeronautical and Astronautical University, Yantai 264001, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 26 May 2016 Revised 26 August 2016 Accepted 30 August 2016 Available online 31 August 2016 Keywords: Marine algicolous fungi Trichoderma citrinoviride Norditerpene Citrinovirin
a b s t r a c t Citrinovirin (1), a novel norditerpene with an unprecedented carbon skeleton along with three known compounds, cyclonerodiol (2), 3-(2-hydroxypropyl)-4-(hexa-2E,4E-dien-6-yl)furan-2(5H)-one (3), and 5-hydroxy-3-hydroxymethyl-2-methyl-7-methoxychromone (4), was isolated from the culture of a marine brown alga-endophytic strain (cf-27) of Trichoderma citrinoviride. The structure and relative configuration of 1 were identified by spectroscopic methods, including 1D/2D NMR and MS. Its absolute configuration was established by analysis of ECD spectrum, aided by quantum chemical calculations. A plausible biogenetic pathway is proposed for 1, and it was evaluated to be active against Staphylococcus aureus. Ó 2016 Elsevier Ltd. All rights reserved.
Marine-derived fungi have attracted a great attention for natural product researchers since the end of the last century, which afforded a series of candidates for the developments of new drugs and functional agents.1 Marine alga-derived fungi made up a large group of them, and they were even the main contributors of new compounds of marine fungal origin.2 More than 360 new compounds, involving polyketides, terpenoids, polyketide-terpenoids, peptides, alkaloids, and shikimate derivatives, were produced by marine algicolous fungi.3,4 Especially, the structures featuring novel or rare scaffolds were often encountered in them, represented by varioxepine A,5 aspeverin,6 harzianone,7 aspewentins A and B.8 During our continuing investigation towards the structurally unique and biologically active metabolites from marine algicolous fungi, a brown alga-endophytic strain (cf-27) of Trichoderma citrinoviride was chemically examined. As a result, a novel norditerpene, citrinovirin (1), with an unprecedented skeleton along with three known compounds, including cyclonerodiol (2),9 3-(2-hydroxypropyl)-4-(hexa-2E,4E-dien-6-yl)furan-2(5H)-one (3),10 and 5-hydroxy-3-hydroxymethyl-2-methyl-7-methoxychromone (4),11 was isolated and identified (Fig. 1). Herein, the isolation, structure elucidation, bioactivity evaluation, and plausible biogenetic pathway of compound 1 are described. Compound 1 was obtained as a colorless oil with a specific optical rotation ([a]24 10.9 (c 0.18, MeOH). The positive D ) value of ⇑ Corresponding author. Tel.: +86 535 2109176; fax: +86 535 2109000. E-mail address: [email protected] (N.-Y. Ji). http://dx.doi.org/10.1016/j.bmcl.2016.08.093 0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.
electrospray ionization mass spectrum (ESI+MS) showed an array of peaks at m/z 340, 345, 662, 667, and 990, corresponding to [M +NH4]+, [M + Na]+, [2M+NH4]+, [2M+Na]+, and [3M+Na]+. Thus, a molecular weight of 322.2 was assigned to 1. Additionally, the fragmental ion peaks at m/z 305 [M H2O+H]+ and 287 [M 2H2O+H]+ suggested the presence of two hydroxy groups. A molecular formula of C19H30O4 was determined by interpretation of high-resolution ESI+MS (m/z 345.2040 [M+Na]+, calcd for C19H30O4Na, 345.2042), requiring five degrees of unsaturation. The 1H NMR spectrum (Table 1) of compound 1 showed one methyl singlet, two methyl doublets, one methyl triplet, two doublets of double doublets attributable to two oxygenated methines, one doublet of double doublets and one double doublet ascribable to two olefinic protons, and two doublets assignable two exchangeable protons. According to MS data, the exchangeable protons should be derived from two hydroxy groups. The 13C NMR spectrum (Table 1) along with DEPT and HSQC data demonstrated the presence of four methyls, three methylenes, ten methines, and two nonprotonated carbons. Based on 1H–1H COSY correlations (Fig. 1), all the hydrogen-bearing groups except for Me-19 made up a large spin system, C-2 to C-10 and C-15. Especially, C-6 was bonded to C-10 to form ring A by the COSY correlations between H-6/H-10. C-8 and C-9 were hydroxylated by COSY correlations between H-8/OH-8 and between H-9/OH-9, and C-7 and C-11 were methylated by those between H-7/H3-18 and between H-11/H3-16. Moreover, the connectivity at C-1 was established by the HMBC correlations from H3-19 to C-1, C-2, and C-10,
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X.-R. Liang et al. / Bioorg. Med. Chem. Lett. 26 (2016) 5029–5031 HO
15
H
13
H
19 11
H1
16 3
17
H OH
OH
9
O
A
O
OH
2
7 5
H 18
1
OH O O 3
OH O O
OH
COSY HMBC
O
O
OH O 4
OH
Figure 1. Structures of 1–4 and key 1H–1H COSY and HMBC correlations of 1.
Table 1 H and 13C NMR data for 1 (at 500 MHz for 1H and 125 MHz for
1
Pos. 1 2 3 4 5 6 7 8 9 10 11 12a 12b 13a 13b 14 15 16 17 18 19 OH-8 OH-9
dH, mult. (J in Hz) 2.21, 3.36, 5.74, 5.85, 2.09, 1.39, 3.31, 3.81, 2.19, 1.98, 1.28, 1.47, 1.19, 1.46, 1.32, 0.92, 0.89,
dd (10.6, 5.8) m ddd (11.0, 7.6, 2.6) dd (11.1, 1.7) br t (12.2) m ddd (7.1, 7.1, 7.1) ddd (8.8, 8.2, 6.6) dd (12.8, 9.4) m m m m m m t (7.2) d (6.3)
1.07, 1.74, 4.78, 4.54,
d (6.6) s d (6.4) d (6.4)
13
C, in DMSO-d6) dC 91.6, C 61.0, CH 47.4, CH 125.6, CH 134.1, CH 43.8, CH 46.3, CH 75.8, CH 72.4, CH 62.4, CH 31.8, CH 34.9, CH2 29.3, CH2 23.0, CH2 14.9, CH3 20.2, CH3 175.5, C 17.3, CH3 26.7, CH3
Figure 2. Energy-minimized conformer and key NOE correlations (arrows, the solid on b face and the dashed on a face) of 1.
In order to determine the absolute configuration of compound 1, we did our best to crystallize it for an X-ray diffraction analysis but failed under various conditions. Then, we turned to the electronic circular dichroism (ECD) technique, which was also reliable to confirm the absolute configurations of organic molecules aided by quantum chemical calculations.7,12 The ECD spectrum of 1 was determined, and it exhibited a positive Cotton effect at 223 nm. The conformers were generated by the Dreiding force field in MarvinSketch13 and further optimized using density function theory (DFT) at the gas-phase B3LYP/6-31G(d) level via Gaussian 09 software.14 The energy-minimized conformer matching NOE correlations (Fig. 2) was submitted to ECD computation with time-dependent DPT method at the same level, and the spectrum was produced by SpecDis software.15 The accordance between calculated ECD spectrum and experimental one (Fig. 3) suggested the absolute configuration to be 1R, 2R, 3S, 6S, 7S, 8R, 9S, 10R, and 11R, which could also be supported by the exciton chirality method.16 Compound 1 possesses a perhydroazulene ring system, and it looks like a cage due to the presence of c-lactone. Although pachydictyol and dictyol derivatives from the marine brown algae have a carbon skeleton similar to that of 1,17,18 the positions attached by side chains and methyl groups are quite different. The framework of pachydictyols and dictyols has been deduced to be formed by two-step cyclization of geranylgeranyl diphosphate (GGDP).18
exptl
12
calcd 8
Δε/M -1cm-1
and a carbonyl group (C-17) was attached to C-3 by the HMBC correlation from H-3 to C-17. To match the molecular formula, C-1 should be bonded to C-17 through an oxygen atom, which was also supported by their chemical shifts. The above data evidenced the gross structure of 1, which was further verified by the other HMBC correlations from H-3 to C-1, C-2, C-4, and C-5, from H3-18 to C-6, C-7, and C-8, from HO-8 to C-7, C-8, and C-9, from OH-9 to C-8, C-9, and C-10, H3-16 to C-2, C-11, and C-12, and H3-15 to C-13 and C-14 (Fig. 1). The relative configuration of compound 1 was established by analysis of NOE correlations (Fig. 2). H-6, H-8, H-9, and Me-18 were located on the same face of ring A by the NOE correlations of H-6 with H-8, H-9, and H3-18 and of H-8 with H3-18, whereas H-10 was oriented on the opposite face by its NOE correlation with H-7. Me-19 was positioned on the same face of H-9 based on their NOE correlation, and then the ester group was placed on the other face. Additionally, C-11 was syn and vicinal to H-6 by the NOE correlation between H-11 and H-6, which along with the NOE correlations between H-3 and H3-16 and between H2-12 and H3-19 confirmed the relative configuration at C-11. Thus, the rotation of single bond between C-2 and C-11 was proposed to be restricted by the surrounding groups. The large coupling constant (12.2) of H-6 also suggested it to be opposite to H-7 and H-10, which further corroborated the relative configuration of 1.
4
0 200
220
240
-4
-8
Wavelength/nm Figure 3. Experimental and calculated ECD spectra of 1.
260
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X.-R. Liang et al. / Bioorg. Med. Chem. Lett. 26 (2016) 5029–5031
OPP H [H]
rearrangement
H cyclization
Bu
cyclization
-Me
GGDP
H 1b
1a
H 1c cyclization
Bu 1
Bu
H OH
SN2 reaction H
H
[O]
COOH
OH
H 1f
Bu
H OH H
O O- H
H [O]
H
OH
H 1e
H 1d
Figure 4. Proposed biosynthetic pathway for 1.
However, this process is almost impossible to yield 1, even if reasonable rearrangements are considered. Thus, 1 may be synthesized by a different biogenetic pathway (Fig. 4), which is proposed to contain the demethylation (GGDP to 1a), three-step cyclization (1a to 1d), oxidation at C-8/C-9/C-17and at C-4/C-5 (1d to 1f), and SN2 reaction with Walden inversion (1f to 1). It is worth to mention that the bicyclo[5.2.0]nonane ring system in 1d is also present in the harziane diterpenes from Trichoderma spp., which signifies the possibility of this pathway to some extent.7 Moreover, the tension of cyclobutyl moiety may play an important role in the formation of c-lactone ring. To evaluate the biological activity of citrinovirin (1), it was assayed for growth inhibition against two bacteria (Staphylococcus aureus and Escherichia coli), two phytopathogenic fungi (Colletotrichum lagenarium and C. gloeosporiodes), one marine zooplankton (Artemia salina), and four marine phytoplankton species (Chattonella marina, Heterosigma akashiwo, Prorocentrum donghaiense, and Scrippsiella trochoidea).7,19 The results (Table S1) showed that 1 inhibited the growth of S. aureus with an MIC value of 12.4 lg/mL, and it was also toxic to A. salina with an LC50 value of 65.6 lg/mL. Additionally, 1 exhibited 14.1–37.2% inhibition of C. marina, H. akashiwo, and P. donghaiense at 100 lg/mL, but it could promote the growth of S. trochoidea. Acknowledgments Financial support from the National Natural Science Foundation of China (31670355) and Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11020702) is gratefully acknowledged. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.bmcl.2016.08. 093.
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