Chinese Chemical Letters  2019, Vol. 30 Issue (1): 51-54   PDF    
Dimericbiscognienynes B and C: New diisoprenyl-cyclohexene-type meroterpenoid dimers from Biscogniauxia sp.
Huan Zhaoa,b,1, Meizhi Wangc,1, Guodong Chena,*, Dan Hua, Enqing Lib, Yibo Quc, Libing Zhouc, Liangdong Guod, Xinsheng Yaoa, Hao Gaoa,*    
a Institute of Traditional Chinese Medicine and Natural Products, College of Pharmacy/Guangdong Province Key Laboratory of Pharmacodynamic Constituents of TCM and New Drugs Research, Jinan University, Guangzhou 510632, China;
b College of Traditional Chinese Medicine, Jinan University, Guangzhou 510632, China;
c Guangdong-Hongkong-Macau Institute of CNS Regeneration, Joint International Research Laboratory of CNS Regeneration, Jinan University, Guangzhou 510632, China;
d State Key Laboratory of Mycology, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100190, China
Abstract: Dimericbiscognienynes B and C (1 and 2), two new diisoprenyl-cyclohexene-type meroterpenoid dimers, were isolated from Biscogniauxia sp. 71-10-1-1. Their structures, including the absolute configurations, were determined by spectroscopic analyses and ECD experiments. Meroterpenoids are special natural products that originate from mixed terpenoid-nonterpenoid pathway. As a member of meroterpenoid family, diisoprenyl-cyclohexene/ane-type meroterpenoids composed of two isoprenyl chains (C5 unit) and a cyclohexene/ane moiety (C6 unit), featuring diverse skeleton structures with wide range of bioactivities. In these reported diisoprenyl-cyclohexene/ane-type meroterpenoids, only three dimers were identified. The discovery of the two new dimers added members of this rare class of meroterpenoids.
Keywords: Meroterpenoid     Dimer     Diisoprenyl-cyclohexene/ane-type     meroterpenoid     Diels-Alder reaction     Biscogniauxia sp.    

Meroterpenoids are broadly defined as compounds of mixed terpenoid-nonterpenoid origin, which have attracted broad attention due to their structural complexity and remarkably wide range of bioactivities [1, 2], such as pyripyropene A (cholesterol acyltransferase-2 inhibitor) [3], territrem B (acetylcholinesterase inhibitor) [4], ascofuranone (trypanosome alternative oxidase inhibitor) [5], berkeleyacetal C (anti-inflammatory activity) [6], and so on. Diisoprenyl-cyclohexene/ane-type meroterpenoids, generally possessing two isoprenyl chains (C5 unit) attached to a cyclohexene/ane moiety (C6 unit) at ortho or meta-position, originate from the hybrid terpenoid-shikimate biosynthesis. To date, more than 50 diisoprenyl-cyclohexene/anes have been reported from fungi (such as Pestalotiopsis sp. [7-13], Isariopsis sp. [14-16], and Truncatella sp. [17]) and plants (such as Ophryosporus lorentzii [18] and Cephalozia otaruensis [19]).

In our previous investigation on metabolites of a fungal strain of Biscogniauxia sp. 71-10-1-1 fermented with rice, a novel diisoprenyl-cyclohexene-type meroterpenoid dimer (dimericbiscognialkyne A) with short-term memory enhancement activity was isolated (Fig. 1), along with three new biscognienynes [20]. Dimericbiscognialkyne A is the first meroterpenoid dimer featuring hexadecahydrobenzo[kl]xanthene ring system, which could derive from two same monomeric diisoprenyl-cyclohexene-type meroterpenoids (biscognienyne B) via a unique intermolecular redox coupling Diels-Alder adduct and a nucleophilic addition reaction. In addition, the dimericbiscognialkyne A and biscognienynes possess aesthetically interesting 'bird'-like topologies, named as huanhuanbirds [20]. In order to obtain more diisoprenyl-cyclohexene/ane-type meroterpenoid dimers, a chemical investigation on the residual fractions of this fungus was carried out, which led to the isolation of two new diisoprenyl-cyclohexene-type meroterpenoid dimers (dimericbiscognienynes B and C, 1 and 2) (Fig. 1). Herein, we describe the isolation and structural elucidation of 1 and 2.

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Fig. 1. Chemical structures of 1, 2, and dimericbiscognialkyne A.

The culture was extracted thrice with EtOAc, and the organic solvent was evaporated to dryness under vacuum to afford a crude extract (43.2 g). Then the crude extract was subjected to silica gel CC (4 cm × 15 cm) using cyclohexane-MeOH (100:0 and 0:100, v/v) to afford a cyclohexane extract (C, 12.5 g) and a MeOH extract (w, 21.9 g). The MeOH extract (w, 21.9 g) was separated by ODS CC (4 cm × 30 cm) eluting with MeOH-H2O (20:80, 50:50, 70:30, and 100:0, v/v) to afford 4 fractions (w1-w4). Fraction w3 (3.1 g) was further subjected to ODS (4 cm × 45 cm) MPLC eluted with a gradient of MeOH-H2O (20:80 to 100:0, v/v) for 420 min at a flow rate of 20 mL/min to afford fractions w3-1-w3-9. Fraction w3-8 (673.2 mg) was subjected to silica gel CC using cyclohexane-ethyl acetate (100:0 to 0:100, v/v) to afford fractions w3-8-1-w3-8-8. Fraction w3-8-3 (47.3 mg) was subjected to preparative HPLC using MeCN-H2O (50:50, v/v) at a flow rate of 3 mL/min to yield 1 (tR: 24.3 min, 2.1 mg) and 2 (tR: 28.7 min, 5.3 mg).

Dimericbiscognienyne B (1): Colorless oil; [α]D27.1-123.6 (c 0.25, MeOH); UV (MeOH, nm) λmax (logε): 205 (4.86), 224 (4.59), 285 (4.30); IR (KBr, cm-1): νmax 3421, 2969, 2920, 2363, 1647, 1616, 1540, 1457, 1377, 1089; ECD λnmε) (c 9.7 × 10-4 mol/L, MeOH): 213 (+4.74); ESI-MS (positive): m/z 545 [M+Na]+; HR-ESI-MS (positive): m/z 545.2886 [M+Na]+ (calcd. for C32H42O6Na: 545.2879), 1H and 13C NMR data can be found in Table 1.

Table 1
NMR data of 1 and 2 in CDCl3.

Dimericbiscognienyne C (2): Colorless oil; [α]D27.1 +51.6 (c 0.5, MeOH); UV (MeOH, nm) λmax (logε): 224 (4.43), 315 (3.34); IR (KBr, cm-1): νmax 3442, 2965, 2924, 2360, 1718, 1668, 1437, 1376, 1276, 1089; ECD λnmε) (c 6.3 × 10-4 mol/L, MeOH): 213 (+6.70); ESI-MS (positive): m/z 543 [M+Na]+; HR-ESI-MS (positive): m/z 521.2899 [M+H]+ (calcd. for C32H41O6: 521.2903); 1H and 13C NMR data can be found in Table 1.

Dimericbiscognienyne B (1) was obtained as a colorless oil. The molecular formula of 1 was established as C32H42O6 (12 degrees of unsaturation) from its HR-ESI-MS (m/z 545.2886 [M+Na]+, calcd. for C32H42O6Na: 545.2879), which was 2 Da more than dimericbiscognienyne A. The 13C NMR data showed 32 carbon signals (Table 1). Combined with data from the DEPT 135 experiment, these carbons can be categorized as ten aromatic or olefinic carbons (δC 135.7, 134.6, 132.9, 132.7, 125.8, 125.6, 122.6, 119.0, 118.2, 116.8), two sp quaternary carbons (δC 88.4, 86.1), four sp3 quaternary carbons (δC 94.9, 73.5, 64.2, 48.1), six sp3 methine carbons (δC 75.2, 66.9, 65.8, 60.1, 41.7, 37.6), four sp3methylene carbons (δC 40.5, 38.1, 31.1, 26.3), and six methyl carbons (δC 26.0, 25.8, 23.9, 23.5, 18.1, 18.0). The 1H NMR data (Table 1) of 1 revealed the characteristic signals of six olefinic or aromatic protons [δH 5.62 (br s, 1 H), 5.47 (br s, 1 H), 5.25 (br t, 1H, J = 7.5 Hz), 5.19 (1 H), 5.17 (1 H), and 5.03 (br t, 1H, J = 7.5 Hz)] and six methyls [δH 1.78 (br s, 3 H), 1.74 (br s, 3 H), 1.73 (br s, 3 H), 1.71 (br s, 3 H), 1.64 (br s, 3 H), 1.64 (br s, 3 H)]. Except for the disappearence of one oxygenated methine signal [δC 59.4 (C-3′); δH 3.21 (H-3′)], one additional methylene signal [δC 40.5 (C-3′); δH 2.32 (Ha-3′), 1.32 (Hb-3′)], and the obvious downfield shift of C-2′ (δC 73.5), the NMR data of 1 are similar to those of dimericbiscognienyne A [20], which indicated that 1 should have the same skeleton as dimericbiscognienyne A, and 1 is a deoxidized derivative of dimericbiscognienyne A at C-3′. All the proton resonances were associated to the directly attached carbon atoms through the HSQC experiment (Table 1 and Supporting information). The key 1H-1H COSY correlations (Table S1 in Supporting information) of H-3′a with H-3′b/H-4′/H-5′, H-3′b with H-3′a/H-4′, and H-4′ with H-3′a/H-3′b/H-5′ and the HMBC cross-peaks (Table S1) from H-3′a/H-3′b to C-1′/C-2′/C-4′/C-5′, and from H-5′ to C-1′/C-3′/C-7′ confirmed the above deduction. The whole planar structure of 1 was established by the analysis of 2D NMR(Table S1), and the assignments of all proton and carbon resonances are shown in Table 1.

The relative configuration of 1 was deduced by comparison with dimericbiscognienyne A, which was previously established by X-ray crystallography analysis [20]. Their planar structures can be divided into two parts (A and B), and the planar structures of part A in 1 and dimericbiscognienyne A were identical. Because the 13C chemical shifts of part A in 1 were substantially identical with those of dimericbiscognienyne A [20], the relative configuration of part A in 1 was considered as the same as that of dimericbiscognienyne A, which was assigned as 1R*, 2S*, 3S*, 4R*, 5S*, 6R*, 7′R*. In part B of 1, the large value of 3JHb-3′, H-4′ (9.5 Hz) indicated that H-4′ was on the pseudoaxial orientation in the cyclohexene ring (C1′-C6′), and Hb-3′ was on the opposite axial orientation in the cyclohexene ring (C1′-C6′) of 1 (Fig. 2). In the NOESY experiment of 1, the observed correlations between H-1′ and Hb-3′/Ha-12′/Hb-12′ demonstrated that H-1′, Hb-3′, and C-12′ were on the same face in the cyclohexene ring (C1′-C6′) of 1 (Fig. 2 and Table S1). On the basis of the analyses of 3JHb-3′, H-4′ and the NOESY correlations, the relative configuration of the part B in 1 was assigned as 1′R*, 2′S*, 4′S* (Fig. 2). Combined with the above deductions and the key NOESY correlation between H-1′ and H-7′ (Fig. 2 and Table S1), the whole relative configuration of 1 was assigned as 1R*, 2S*, 3S*, 4R*, 5S*, 6R*, 1′R*, 2′S*, 4′S*, 7′R* (Fig. 2).

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Fig. 2. Key NOESY or ROESY correlations of 1 and 2.

Dimericbiscognienyne C (2) was obtained as a colorless oil. The molecular formula of 2 was established as C32H40O6 (13 degrees of unsaturation) from its HR-ESI-MS (m/z 521.2899 [M+H]+, calcd. for C32H41O6: 521.2903), which was 2 Da less than dimericbiscognienyne B (1). The 13C NMR data showed 32 carbon signals (Table 1). Combined with data from the DEPT 135 experiment, these carbons can be categorized as eleven sp2 carbons [including one carbonyl (δC197.0)], two sp quaternary carbons, four sp3 quaternary carbons [including three oxygenated ones (δC 94.9, 75.4, 64.2)], five sp3 methine carbons [including three oxygenated ones (δC 75.0, 66.7, 60.4)], four sp3 methylene carbons, and six methyl carbons. The 1H NMR data (Table 1) of 2 revealed that the characteristic signals of six olefinic or aromatic protons [δH 5.95 (br s, 1 H), 5.44 (br s, 1 H), 5.20 (1 H), 5.18 (2 H), 5.05 (br t, 1H, J = 6.9 Hz)], and six methyls [δH 1.76 (br s, 3 H), 1.76 (br s, 3 H), 1.73 (br s, 3 H), 1.73 (br s, 3 H), 1.65 (br s, 3 H), 1.64 (br s, 3 H)]. Except for the lacking of an oxygenated methine signal [δC 65.8 (C-4′); δH 4.56 (H-4′)] and the appearance of a carbonyl [δC 197.0 (C-4′)], the NMR data of 2 highly resemble those of 1 (Table 1), which indicated that 2 is an oxidation product of 1 at C-4′. The detailed analyses of 2D NMR (Table S2 in Supporting information) established the whole planar structure of 2, and the assignments of all proton and carbon resonances are shown in Table 1.

The planar structures of part A in 2 and 1 were also identical. Because the 13C chemical shifts of part A in 2 were substantially identical with those of 1 [20], the relative configuration of part A in 2 was considered as the same as that of 1, which was assigned as 1R*, 2S*, 3S*, 4R*, 5S*, 6R*, 7′R*. The ROESYcorrelations of between H-5 and 1-OH, and between H-4 and H-7′ in 2 were consistent with the above deduction (Fig. 2 and Table S2). In addition, the observed ROESY correlations between H-1′ and Hb-3′/Ha-12′/Hb-12′ demonstrated that H-1′, Hb-3′, and C-12′ were on the same face in the cyclohexene ring (C1′-C6′) of 2 (Fig. 2 and Table S2), and the relative configuration of part B in 2 was assigned as 1′R*, 2′S*. Combined with the above deductions and the key ROESY correlation between H-1′ and H-7′ (Fig. 2 and Table S2), the whole relative configuration of 2 was assigned as 1R*, 2S*, 3S*, 4R*, 5S*, 6R*, 1′R*, 2′S*, 7′R* (Fig. 2).

The ECD curves (Fig. 3) of 1 and 2 were similar to that of dimericbiscognienyne A, which suggested that 1 and 2 shared the same obsolute configurations with dimericbiscognienyne A. Therefore, the obsolute configurations of 1 and 2 were assigned as 1R, 2S, 3S, 4R, 5S, 6R, 1′R, 2′S, 4′S, 7′R, and 1R, 2S, 3S, 4R, 5S, 6R, 1′R, 2′S, 7′R, respectively.

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Fig. 3. ECD spectra of 1, 2 and dimericbiscognialkyne A.

Since the first report on the diisoprenyl-cyclohexene/ane-type meroterpenoidsfrom Beauveria felina SANK 13682 in 1983 [21], more than 50 diisoprenyl-cyclohexene/ane-type meroterpenoids have been reported, most of which are monomeric (C16). Only three dimers have been reported, including pestalofones B and C with unique 2-cyclohexylspiro[5.5] undecane ring system (Pestalotiopsis sp.) [12] and dimericbiscognialkyne A with unusual hexadecahydrobenzo[kl]xanthene ring system (Biscogniauxia sp.) [20]. In our chemical investigation, two new diisoprenyl-cyclohexene-type meroterpenoid dimers (dimericbiscognialkynes B and C) were obtained, which derive from two different monomeric diisoprenyl-cyclohexene-type meroterpenoids (biscognienynes A and B [20]) via a unique intermolecular redox coupling Diels-Alder adduct and a nucleophilic addition reaction (Scheme 1, Figs. S1 and S2). The result of this study added the members of the rare class of diisoprenyl-cyclohexene/ane-type meroterpenoid dimers.

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Scheme 1. Plausible biosynthetic pathways of 1 and 2.

Acknowledgments

This work was financially supported by grants from the National Natural Science Foundation of China (No. 3171101305), Chang Jiang Scholars Program (Hao Gao, 2017) from the Ministry of Education of China, the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2017A03036027), Guangdong Special Support Program (No. 2016TX03R28), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (Hao Gao, 2014), Pearl River Nova Program of Guangzhou (No. 201610010021), and K. C. Wong Education Foundation (Hao Gao, 2016).

Appendix A. Supplementary data

Supplementary material related to this article can befound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.05.019.

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