Secoheliosphanes A and B and ... - ACS Publications

Nov 30, 2017 - Medical Sciences and Peking Union Medical College, Beijing 100050, ... Institute of Medicinal Biotechnology, Chinese Academy of Medical...
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Article Cite This: J. Org. Chem. 2018, 83, 167−173

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Secoheliosphanes A and B and Secoheliospholane A, Three Diterpenoids with Unusual seco-Jatrophane and seco-Jatropholane Skeletons from Euphorbia helioscopia Zhen-Peng Mai,† Gang Ni,† Yan-Fei Liu,† Yu-Huan Li,‡ 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 ‡ Institute of Medicinal Biotechnology, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, P. R. China S Supporting Information *

ABSTRACT: Secoheliosphanes A (1) and B (2) and secoheliospholane A (3), possessing an unusual 7,8-secojatrophane skeleton and an unprecedented 9,10-seco-7,10epoxyjatropholane skeleton, respectively, were isolated from the whole plants of Euphorbia helioscopia, along with two biogenetically precursors, a new jatrophane diterpene, 2-epieuphornin I (4) and a known jatrophane diterpene, euphoscopin A (5). Structures of 1−4 including absolute configurations were elucidated on the basis of spectroscopic data, X-ray crystallography, and chemical conversion. Compounds 1 and 2 were prepared from 4 and 5, respectively, confirming their structural assignments. Notably, 1 and 2 presented the first examples of seco-jatrophane-type diterpenoids and 3 featured a novel 5/6/7/7-fused tetracyclic ring skeleton. Among them, compound 2 showed modest activity against HSV-1 with IC50 value of 6.41 μM.

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INTRODUCTION Diterpenoids in the widespread genus Euphorbia (Euphorbiaceae) are characteristic chemical constituents and exert a broad spectrum of biological activities, including antiviral, antiinflammatory, antimicrobial, cytotoxic, and modulation of multidrug resistance activities.1 In particular, lower diterpenoids display a variety of parent skeletons, such as jatrophanes, lathyranes, ingenanes, jatropholanes, and casbanes.2 Among them, jatrophane diterpenes are characterized by a highly oxygenated trans-bicyclo[10.3.0]pentadecane framework wherein there were five methyls separately located at C-2, C6, C-10 × 2, and C-13, as well as jatropholane diterpenes featured a 5/6/7/3 fused ring system, which was formed by transannular closure of lathyrane diterpenes at position C-5/C12.3 Of numerous known “Euphorbia diterpenoids”, only a very small number of seco-examples have been reported, i.e., secocasbanes, seco-lathyranes, seco-premyrsinanes, and seco-cembranes, with a clear absence of the seco-jatrophanes and secojatropholanes.4 Euphorbia helioscopia L. is an annual herb widely distributed in most parts of China and has been used to treat malaria, bacillary dysentery, osteomyelitis, and tumors as a Chinese folk medicine.5 Previous chemical investigations have proved that its metabolic pattern is heavily characterized by a series of complex macrocyclic diterpenes, especially jatrophanes. During our efforts to seek new biologically intriguing metabolites from traditional Chinese medicines, three novel diterpenes (1−3, © 2017 American Chemical Society

Figure 1), a new jatrophane diterpene (4, Figure 1), and a known jatrophane diterpene (5) were isolated from the whole plants of E. helioscopia.6 It was noteworthy that secoheliosphanes A (1) and B (2) and secoheliospholane A (3) separately

Figure 1. Structures of compounds 1−4.. Received: October 9, 2017 Published: November 30, 2017 167

DOI: 10.1021/acs.joc.7b02558 J. Org. Chem. 2018, 83, 167−173

Article

The Journal of Organic Chemistry

Table 1. 1H NMR (600 MHz) and 13C NMR (150 MHz) Data of Secoheliosphanes A (1) and B (2)

harbored an unusual 7,8-seco-jatrophane skeleton and an unprecedented 9,10-seco-7,10-epoxyjatropholane skeleton. Compounds 1 and 2 are hypothesized to arise from an enzymatic retro-aldol reaction in plants and were prepared from 4 and 5, respectively, revealing their biosynthetic events and confirming their structural assignments. Bioassay results indicated that compound 2 showed modest activity against HSV-1 with IC50 value of 6.41 μM.

secoheliosphanes A

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RESULTS AND DISCUSSION Secoheliosphane A (1) was obtained as a colorless gum. Its molecular formula, C 31H 40O 8 , was determined by the (+)-HRESIMS of a Na+ adduct of a molecule at m/z 563.2632 [M+Na]+ (calcd for C31H40O8Na, 563.2615), corresponding to 12 degrees of unsaturation. Its IR spectrum showed absorption bands assignable to carbonyl (1746 cm−1), α,β-unsaturated aldehyde (1717 and 1643 cm−1),7 and aromatic ring (1602 and 1453 cm−1) functionalities. The 1H NMR spectrum of 1 in CDCl3 displayed resonances attributed to an aldehyde group [δH 9.38 (1H, s, H-7)], a benzoyl group [δH 7.92 (2H, dd, J = 7.8, 1.2 Hz, H-2′ and H-6′), 7.56 (1H, t, J = 7.8 Hz, H-4′), and 7.42 (2H, t, J = 7.8 Hz, H-3′ and H-5′)], three olefinic methines [δH 6.76 (1H, dq, J = 10.2, 1.2 Hz, H5), 5.46 (1H, dd, J = 15.6, 8.4 Hz, H-12), and 5.41 (1H, d, J = 15.6 Hz, H-11)], two oxymethines [δH 5.67 (1H, d, J = 7.2 Hz, H-14) and 5.10 (1H, t, J = 7.2 Hz, H-3)]. In addition, six single methyl groups [δH 2.14 (H3-2″), 2.09 (H3-8), 2.06 (H3-2‴), 1.78 (H3-17), 1.19 (H3-19) and 1.18 (H3-18)], and two doublet methyl groups [δH 1.15 (d, J = 7.2 Hz, H-16) and 0.90 (d, J = 7.2 Hz, H-20)] were also displayed in the 1H NMR spectrum of 1. Besides carbon signals attributable to a benzoyloxy [δC 166.0, 133.5, 129.9, 129.5 × 2, and 128.7 × 2] and two acetyl groups [δC 170.0, 169.5, 22.5, and 21.1], the 13C NMR and DEPT spectra revealed 20 carbon resonances corresponding to six methyls, one methylenes, nine methines (three olefinic methine, two oxygenated methines, and an aldehyde), and four quaternary carbons (one oxygenated carbon, one olefinic carbon, and one carbonyl) (Table 1). Based on known jatrophane-skeleton diterpenoids with acyloxy groups from E. helioscopia,8 the 1D NMR data of 1 showed a set of typical signals for polyesterified jatrophane-type diterpene nature as well. But as the 11 degrees of unsaturation were accounted for by one benzoyl, two carbonyls, two olefinic bonds and three ester carbonyl groups, the remaining 1 degree of unsaturation required that 1 was monocyclic. That means one ring of bicyclic jatrophane-skeleton was opened. Aforementioned spectroscopic data suggests that 1 is a novel seco-jatrophane triester bearing two acetoxy and one benzoyloxy groups. The 1H−1H COSY correlations of H2-1/H-2(H3-16)/H-3/ H-4/H-5 and H-11/H-12/H-13(H3-20)/H-14 highlighted the following two key fragments a (C-1/C-2(C-16)/C-3/C-4/C-5) and b (C-11/C-12/C-13(C-20)/C-14) (Figure 2, blue lines). The HMBC correlations from 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; from H-4 to C-1, C-3, C-5, C-6, C-14, and C-15 constructed a five-membered carbon ring to which was attached two side chains via C-4 and C-15 units, and that was substituted with a methyl group (CH3-16) at C-2. Further HMBC correlations from H-14 to C-4, C-12, C-15, and C-20; from H3-20 to C-12, C-13, and C-14; and from H3-18 and H3-19 to C-9, C-10, and C-11, together with correlations from H3-8 to C-9 and C-10 arranged one entire carbon chain that connected to fivemembered ring at C-15. Also, the other side chain of five-

no.

δC

1a 1b 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 7′ 1′ 2′,6′ 3′,5′ 4′ 1″ 2″ 1‴ 2‴

40.9 37.7 82.4 47.0 148.2 141.2 194.7 25.6 211.4 50.2 134.6 132.6 38.3 75.5 91.5 18.1 9.8 24.2 24.0 17.5 166.0 129.9 129.5 128.7 133.5 170.0 21.1 169.5 22.5

secoheliosphane B

δH 2.94 1.57 2.45 5.10 3.79 6.76

dd (15.0, dd (15.0, m t (7.2) dd (10.2, dq (10.2,

δC 9.6) 8.4)

7.2) 1.2)

9.38 s 2.09 s

5.41 5.46 2.49 5.67

d (15.6) dd (15.6, 8.4) m d (7.2)

1.15 1.78 1.18 1.19 0.90

d (7.2) d (1.2) s s d (7.2)

7.92 dd (7.8, 1.2) 7.42 t (7.8) 7.56 t (7.8) 2.14 s 2.06 s

δH

40.8 38.2 82.5 47.4 147.5 141.5 194.6 25.6 211.1 50.2 135.4 131.5 39.9 75.3 92.4 18.2 10.0 24.3 24.1 18.7 165.9 129.9 129.5 128.7 133.4 169.9 21.2 169.6 22.5

3.01 1.62 2.46 5.18 3.73 6.78

dd dd m dd dd dq

(15.0, 7.8) (15.0, 9.0) (7.2, 5.4) (10.2, 7.2) (10.2, 1.2)

9.39 s 2.09 s

5.46 5.37 2.40 5.59

d (15.6) dd (15.6, 8.4) m d (7.2)

1.17 1.83 1.18 1.19 1.02

d (7.2) d (1.2) s s d (7.2)

7.93 dd (7.8, 1.2) 7.43 t (7.8) 7.57 t (7.8) 2.06 s 2.11 s

Figure 2. Key 2D NMR correlations of 1.

membered ring was deduced by the HMBC correlations from H3-17 to C-5, C-6 and C-7, along with the 1H−1H COSY correlations of H-4/H-5. After defining the 7,8-seco-jatrophane skeleton, diagnostic correlations from H-3 to C-7′; from H-14 to C-1″ indicated the presence of one benzoyloxy group at C-3 and one acetoxy group at C-14. At last, the remaining one acetoxy group could only be attached at C-15. Therefore, the planar structure of 1 was established as depicted in Figure 1. The relative configuration of 1, as shown in Figure 2, was elucidated by analysis of the NOESY spectrum. The geometry of Δ5 and Δ11 double bonds were both assigned as E based on the large coupling constant between H-11 and H-12 (J = 15.6 Hz) and the NOESY correlations of H-4/H3-17 and H-5/H-7 and H-11/H-13. As judged from its NOESY spectrum, the 168

DOI: 10.1021/acs.joc.7b02558 J. Org. Chem. 2018, 83, 167−173

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The Journal of Organic Chemistry cross-peaks observed between H3-16/H-3 and H3-16/H-4 indicated that H-3, H-4, and H3-16 were all α-oriented, while the NOESY correlations from H-2′ to H-2‴ demonstrated that OAc-15 and OBz-3 were both β-oriented. However, there were no useful correlations to support conclusions regarding the orientations of H-13 and H-14. To solve this problem, a retroaldol reaction was performed to afford 1 from the speculative precursor 4 (Figure 1), of which the absolute configuration was determined by X-ray crystallographic analysis. Treatment of 4 with Na2CO3 in absolute ethanol at 85 °C (reflux, 12 h) produced 4a in 72% yield.9 The 1H NMR spectrum and specific rotation of 4a (Figure S1, Supporting Information) were identical to those of natural secoheliosphane A (1). Accordingly, the absolute configuration of 1 was assigned to be 2R, 3S, 4S, 13R, 14R, and 15R. 2-epi-Euphornin I (4) formed colorless crystals and gave a molecular formula of C31H40O8 as established by the HRESIMS ([M+Na]+ m/z 563.2616, calcd 563.2615). Analyzing its 1D and 2D NMR data (Table 2) revealed that compound 4 was the 2-epi-isomer of euphornin I.8a The single-crystal X-ray diffraction analysis of 4 with Cu Kα radiation resulted in the Flack Parameter of 0.06 (10), allowing an explicit assignment of the absolute configuration of 4 as shown in Figure 3, consistent

with the sequential correlations of H3-16/H-3/H-4/H3-17/H11 and H3-20/H-14/H-12 in the NOESY spectrum (Figure S45, Supporting Information), respectively.

Figure 3. Single-crystal X-ray structure of 4.

Secoheliosphane B (2), a colorless gum, yield a molecular formula of C31H40O8 based on its 13C NMR and the (+)-HRESIMS peak at m/z 563.2638 [M+Na]+ (calcd 563.2615). On a detailed comparison of 1D and 2D NMR data (Table 1), compound 2 showed the same planar structure as that of 1. Specifically, C-11 and C-13 in 2 were deshielded by ΔδC + 0.8 and ΔδC + 1.6 ppm, respectively, whereas C-12 was shielded ΔδC −1.1 ppm. This suggested that compound 2 was the 13-epi-isomer of 1, which was verified by a retro-aldol reaction from the speculative precursor euphoscopin A (5).6 The similarity of NOESY correlations between 2 and 1 suggested that they had the same configuration, except for the configurations of C-13 and C-14 because there were no useful correlations to support them as well (Figure 4). Finally,

Table 2. 1H NMR (500 MHz) and 13C NMR (125 MHz) Data of Secoheliospholane A (3) and 2-epi-Euphornin I (4) secoheliospholane A no.

δC

1a 1b 2 3 4 5 6 7 8a 8b 9 10 11 12 13 14 15 16 17 18 19 20 7′ 1′ 2′, 6′ 3′, 5′ 4′ 1″ 2″ 1‴ 2‴ OH-6 OH-7

37.6 38.9 82.3 43.8 38.8 78.7 99.3 44.5 209.1 73.9 59.1 36.8 33.5 70.6 89.7 20.7 16.6 31.0 27.3 16.9 166.3 131.3 129.5 128.5 132.8 169.7 21.1 169.7 22.4

δH 2.80 1.16 2.24 5.25 2.16 2.22

dd (15.0, 9.0) dd (15.0, 7.0) m brd (5.0) m m

2.87 d (19.0) 2.44 d (19.0)

2.28 1.80 1.95 5.99

brs t (11.5) m d (3.0)

1.16 1.27 1.30 1.30 0.98

d (7.0) s s s d (7.0)

8.00 dd (8.0, 1.5) 7.45 t (8.0) 7.55 t (8.0) 2.15 s 2.07 s 2.30 s 2.67 s

2-epi-euphornin I δC 38.7 36.0 82.1 43.9 120.3 136.7 73.9 38.9 212.4 50.9 129.8 133.0 41.8 73.4 91.6 16.8 15.7 19.9 24.7 20.9 167.0 130.7 129.6 128.4 133.0 170.2 21.3 170.0 22.7

δH 2.61 1.31 2.36 4.69 3.62 5.51

dd (14.5, 6.0) dd (14.5, 12.0) overlap t (9.0) dd (11.0, 9.0) dt (11.0, 1.0)

3.96 d (6.5) 3.02 dd (14.5, 2.0) 2.35 overlap

5.13 5.23 2.37 5.98

d (15.5) dd (15.5, 9.0) overlap d (10.0)

1.03 1.58 1.09 1.19 0.95

d (6.5) d (1.0) s s d (6.5)

Figure 4. Key NOESY correlations of 2.

treatment of 5 with Na2CO3 in absolute ethanol at 85 °C (reflux, 12 h) produced 5a in 68% yield.9 The 1H NMR spectrum and specific rotation of 5a (Figure S2, Supporting Information) indicated excellent agreement with the natural secoheliosphane B (2). Accordingly, the absolute configuration of 2 was assigned to be 2R, 3S, 4S, 13S, 14R, and 15R. Secoheliospholane A (3) was obtained as colorless crystals with [α]20 D −25.9 (c 0.1, MeOH). Its molecular formula of C31H40O10, with 12 degrees of unsaturation, was deduced from the 13C NMR data and the HRESIMS ion peak at m/z 595.2530 [M+Na]+ (calcd for C31H40O10Na, 595.2514) in the HRESIMS. Its IR spectrum showed absorption bands assignable to hydroxyl (3444 cm−1), carbonyl (1748 and 1718 cm−1), and aromatic ring (1603 and 1453 cm−1) functionalities. The 1H NMR spectrum of 3 in CDCl3 implied a benzoyl group [δH 8.00 (2H, dd, J = 8.0, 1.5 Hz, H-2′ and H-

7.98 dd (8.0, 1.5) 7.42 t (8.0) 7.53 t (8.0) 2.18 s 2.15 s

169

DOI: 10.1021/acs.joc.7b02558 J. Org. Chem. 2018, 83, 167−173

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The Journal of Organic Chemistry 6′), 7.55 (1H, t, J = 8.0 Hz, H-4′), and 7.45 (2H, t, J = 8.0 Hz, H-3′ and H-5′)], two oxymethines [δH 5.99 (d, J = 3.0 Hz, H14) and δH 5.25 (brd, J = 5.0 Hz, H-3)], five tertiary methyl groups [δH 2.15 (H3-2″), 2.07 (H3-2‴), 1.30 × 2 (H3-18 and H3-19), and 1.27 (H3-17)], and two secondary methyl groups [δH 1.16 (d, J = 7.0 Hz, H3-16) and 0.98 (d, J = 7.0 Hz, H320)]. In addition, the spectrum showed resonances due to two exchangeable hydroxyl protons at δH 2.67 (OH-7) and 2.30 (OH-6). Analyses of the 13C NMR and DEPT spectra of 3 indicated the existence of 31 carbons. In combination with the aforementioned proton signals, the following typical carbon resonances [δC 169.7 and 22.4, 169.7 and 21.1, and 166.3, 132.8, 131.3, 129.5 × 2, and 128.5 × 2] revealed the presence of two acetoxy groups and one benzoyloxy group. Besides the above 11 carbon resonances, the remaining 20 carbon resnonances were attributed to five methyls, two methylenes, eight methines (two oxygenated methines), and five quaternary carbons (four oxygenated carbon and one carbonyl carbon). Moreover, deducting eight degrees of unsaturation accounted for one benzoyl, three ester carbonyls, and one carbonyl groups, and the remaining four degrees of unsaturation were indicative of the tetracyclic ring system of 3. The planar structure of 3 was determined by interpretation of the 2D NMR data, including HSQC, 1H−1H COSY, and HMBC spectra. In the 1H−1H COSY spectrum, two key fragments H-1/H-2(H3-16)/H-3/H-4/H-5/H-12(H-11)/H13(H3-20)/H-14 and H-2′/H-3′/H-4′/H-5′/H-6′ were first established by the observed correlations (Figure 5). In the

groups were placed at C-6 and C-7, respectively. HMBC correlations from H3-18 and H3-19 to C-10 and C-11; and from H-12 to C-10, together with the chemical shift of C-10 (δC 73.9), indicated that a propan-2-ol group was at C-11. Meanwhile, the presence of an ether bridge between C-7 and C-10 was deduced from the molecular composition and the chemical shift of C-7 (δC 99.3).10 At last, the remaining one acetoxy group could only be attached at C-15. The planar structure of 3 therefore was established to possess a 5/6/7/7 tetracyclic backbone as depicted in Figure 1. The relative configuration of 3 was elucidated by analyzing the correlations detected in the NOESY spectrum. The crosspeaks observed between H-2′/H-2‴, H-2′/H-5, H-5/H-13, and H-13/H-14 indicated that OBz-3, H-5, H-13, H-14, and 15OAc all possessed the same β-orientations. On the other hand, the NOESY correlations from H-3/H-4, H-3/H3-16, H-4/H317, H3-17/H-12, and H-12/H3-20 showed that these hydrogens were all α-oriented (Figure 5). The absolute configuration of compound 3 was confirmed by X-ray diffraction with Cu Kα radiation [Flack Parameter 0.03 (11)]. After repeated attempts, a high-quality single crystal of 3 was obtained from methanol-acetone solution. The X-ray crystallographic data (Figure 6) corroborated the planar

Figure 6. Single-crystal X-ray structure of 3.

structure and relative configuration of 3 elucidated via NMR data and further allowed the assignment of its absolute configuration as 2R, 3S, 4S, 5S, 6S, 7S, 11R, 12R, 13R, 14R, and 15R. Secoheliosphanes A (1) and B (2) have been identified as the first representative of diterpenoids featuring a novel secojatrophane-type skeleton, which were hypothesized to be formed by an enzyme-catalyzed retro-aldol reaction that is a very common step in plants and plays a pivotal role during the formation of natural products, especially the natural products with unusual skeletons.4e,11 To certify their speculative biosynthetic pathways and absolute configurations, 1 and 2 were prepared in basic condition by biological precursors 4 and 5, respectively (Scheme 1). On the other hand, secoheliospholane A (3) possessed an unprecedented 9,10-seco-7,10epoxyjatropholane skeleton, which was derived from jatrpholane diterpenoids. The hypothetical biogenetic pathway for 3 was also proposed in Scheme 2. Oxidation and esterification of jatropholane skeleton was envisioned to afford the intermediate 6, which followed by an oxidative cleavage between C-9 and C10 to give 7.4c And then, the intermediate was oxygenated at C6, C-7 double bond and C-9 to give the key intermediate 8 which possessed an epoxy group. A nucleophilic addition

Figure 5. Key 2D NMR correlations of 3.

HMBC spectrum of 3 (Figure 5), key correlations from the H21 to C-3, C-4, and C-15; from H-3 to C-1, C-7′, C-4, and C-15; from H-4 to C-1, C-2, C-3, and C-15; from H3-16 to C-1, C-2, and C-3, and from H-2′(H-6′) to C-7′ indicated that C-1−C-4 and C-15 constructed a five-membered carbon ring A in which a methyl group (CH3-16) and a benzoyloxy group substituted at C-2 and C-3, respectively. HMBC correlations from H-4 to C-5, C-12, and C-15; from H-5 to C-13 and C-15; from H-13 to C-5 and C-15; from H-14 to C-12 and C-1″; and from H3-20 to C-12, C-13, and C-14 demonstrated the presence of a sixmembered ring fused to C-4 and C-15 on the five-membered ring, which was substituted with a methyl group (CH3-20) at C-13 and an acetyoxy group at C-14. HMBC correlations from H-5 to C-7 and C-11; from H2-8 to C-6 and C-11; from H-12 to C-6 and C-9; from H3-17 and OH-6 both to C-5, C-6, and C-7; and from OH-7 to C-8 revealed the presence of a sevenmembered ring that was fused to the six-membered ring at C-5 and C-12 in which C-9 was a keto-carbonyl and two hydroxyl 170

DOI: 10.1021/acs.joc.7b02558 J. Org. Chem. 2018, 83, 167−173

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The Journal of Organic Chemistry Scheme 1. Hypothetical Biosynthetic Pathways for 1 and 2

Table 3. Antiviral Activity against HSV-1 and Cytotoxicity of Compounds in Vero Cellsa compound

TC50b (μM)

IC50 (μM)

1 2 3 4 5 acyclovird

57.74 ± 5.69 19.25 ± 2.87 >100 >100 48.07 ± 1.10 >100

>33.33 6.41 ± 1.56 >33.33 19.24 ± 4.99 >11.11 0.41 ± 0.15

SIc 3.0 >5.2 >243.9

a

Data represent mean values for three independent determinations. Cytotoxic concentration required to inhibit Vero cell growth by 50% c Selectivity index value equaled TC50/IC50. dPositive control. b

reaction of C6−C7-epoxy, with a concomitant addition of one molecule of water, generates an intermediate 9 in acidic condition, which underwent an oxidation reaction at C-7 to yield 10. At last, 7,10-epoxypholane 3, was formed by the intramolecular nucleophilic addition of 10-OH to the C-7 carbonyl in 10. Compounds 1−5 were all assessed the antiviral activity against the herpes simplex virus 1 (HSV-1) using Vero cells (Table 3).12 Acyclovir (ACV) was used as the positive control, with IC50 values of 0.41 μM. Compound 2 exhibited antiviral activity compared to ACV, with IC50 value of 6.41 μM. And for 2, the bioassay data indicated that the seco-jatrophane skeleton showed stronger antiviral activity against HSV-1 than its precursor (5), which possessed jatrophane skeleton. In summary, secoheliosphanes A (1) and B (2) represent a new seco-jatrophane skeleton formed through jatrophane diterpenoids with a retro-aldol reaction. Secoheliospholane A (3) bear a previously undescribed 5/6/7/7-fused tetracyclic ring skeleton which was speculated to be derived from jatropholane diterpenoids with the cyclopropane ring opened. Their structures were unambiguously confirmed by extensive spectroscopic analyses, especially by chemical conversion and X-ray crystallography. Remarkably, secoheliosphane B (2) showed antiviral activity against HSV-1 and provided a new structural template for the development of potential antiviral drugs.

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Agilent 6520 Accurate-Mass Q-TOF LC/MS spectrometer. The X-ray crystallography was collected on a Agilent Xcalibur Eos Gemini diffractometer with graphite monochromated Cu−Kα radiation (λ = 1.5418 Å). Analytical HPLC was conducted on an Agilent 1260 infinity system equipped with a DAD-UV detector. Preparative HPLC was performed on a CXTH system (Beijing Chuangxintongheng instrument Co. Ltd., P.R. China), equipped with a UV3000 detector using a YMC-Pack ODS-A column (250 × 20 mm, 5 μm, Kyoto, Japan). Polyamide (30−60 mesh, Changzhou Changfeng Chemical Factory, China), Silica gel (100−200, 200−300 mesh, Qingdao Marin Chemical Inc. Qingdao, P.R. China), CHP20P MCI gel (75−150 μm, Mitsubishi Chemical Corporation), Sephadex LH-20 (GE), and ODSA-HG (50 μm, YMC, Japan) were used for column chromatography (CC). Plant Material. The whole plants of Euphorbia helioscopia L. were purchased from Anguo Materia Medica Market in Hebei Province, China, in September 2015 and were identified by Professor Lin Ma, Institute of Materia Medica, Chinese Academy of Medical Scienes and Peking Union Medical College. A voucher specimen (ID-S-2624) has been deposited at the Herbarium of the Department of Medicinal Plants, the Institute of Materia Medica, Chinese Academy of Medical Sciences, Beijing. Extraction and Isolation. The dried and powdered whole plants of Euphorbia helioscopia (55 kg) were exhaustively extracted with 80% EtOH (3 × 140 L) under reflux (2 h, 3 times). The EtOH extract was evaporated in vacuo, and the crude extract (8.5 kg) was diluted with H2O and successively partitioned with petroleum ether, EtOAc, and nBuOH (three times with 20 L each). To remove most of the chlorophylls and flavones, the EtOAc extract fraction (1.2 kg) was treated with polyamide in water, 50% EtOH, and 95% EtOH, respectively. After removing the solvent, the 50% EtOH eluate (350 g) was chromatographed on a silica gel CC and eluted with a petroleum ether-acetone gradient system (30:1 to 1:1) to afford 18 fractions (A1−A18) on the basis of TLC analysis. A11 (14.1 g) was fractionated by an MCI gel CC, eluting with MeOH/H2O (70:30 to 90:10), produced four subfractions (A11-m1−A11-m4). A11-m2 (8.6 g) was subjected to MPLC on an ODS-A-HG column (MeOH/H2O, 70:30

EXPERIMENTAL SECTION

General Experimental Procedures. Melting points were measured on an X-6 analyzer. Optical rotations were measured on a JASCO P-2000 automatic digital polarimeter. UV spectra were recorded on a JASCO V-650 spectrometer. IR spectra were measured on a Nicolet 5700 FT-IR microscope instrument (FT-IR microscope transmission). NMR spectra were obtained at 500 or 600 MHz for 1H and 125 or 150 MHz for 13C, respectively, on Bruker AVANCE III 500 MHz or Bruker AVIIIHD 600 MHz spectrometers in CDCl3, with solvent peaks used as references. HRESIMS were measured using an

Scheme 2. Hypothetical Biosynthetic Pathways for 3

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The Journal of Organic Chemistry to 100:0) to afford 12 fractions (A11-m2−1−A11-m2−12). By using preparative HPLC, compound 4 (82.8 mg) was isolated from A11m2−2 (65% CH3CN), and compound 5 (452 mg) was isolated from A11-m2−10 (70% CH3CN). A11-m3 (2.8 g) was subjected to CC over silica gel eluting with a gradient of petroleum ether (15:1 to 2:1) in acetone, to give five subfractions, A11-m3−1−A11-m3−5. A11m3−3 (370 mg) was purified by Sephaex LH-20 (MeOH) and preparative HPLC (60% MeCN) to obtain 1 (4.0 mg) and 2 (25.2 mg). A17 (20.0 g) was separated over a column of silica gel, eluting with CHCl3-MeOH (100:1 to 15:1), to give nine fractions (A17−1− A17−9). A17−7 (7.5 g) was subjected to MPLC on an ODS-A-HG column (MeOH/H2O, 60:40 to 100:0) to yield 25 subfractions, A17− 7−1−A17−7−25. Among these subfractions, A17−7−8 (680 mg) was further purified by Sephadex LH-20 (MeOH) and preparative HPLC (65% MeCN) to obtain 3 (5.2 mg). Secoheliosphanes A (1). Colorless gum; [α]20 D + 24.6 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 228 (0.81) nm; IR νmax 2972, 1746, 1717, 1643, 1602, 1453, 714 cm−1; 1H and 13C NMR (CDCl3), see Table 1; (+)-HRESIMS m/z 563.2632 [M+Na]+ (calcd for C31H40O8Na, 563.2615). Secoheliosphanes B (2). Colorless gum; [α]20 D + 64.3 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 228 (0.39) nm; IR νmax 2969, 1746, 1717, 1642, 1602, 1453, 714 cm−1; 1H and 13C NMR (CDCl3), see Table 1; (+)-HRESIMS m/z 563.2638 [M+Na]+ (calcd for C31H40O8Na, 563.2615). Secoheliospholane A (3). Colorless crystals (methanol-acetone, 3:1); mp 258−261 °C; [α]20 D −25.9 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 228 (0.70) nm; IR νmax 3444, 2985, 1748, 1718, 1603, 1453, 714 cm−1; 1H and 13C NMR (CDCl3), see Table 2; (+)-HRESIMS m/ z 595.2530 [M+Na]+ (calcd for C31H40O10Na, 595.2514). 2-epi-euphornin I (4). Colorless crystals (MeOH); mp 102−105 °C; [α]20 D −83.2 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 228 (0.51) nm; IR νmax 3463, 2971, 1745, 1718, 1603, 1452, 714 cm−1; 1H and 13 C NMR (CDCl3), see Table 2; (+)-HRESIMS m/z 563.2616 [M +Na]+ (calcd for C31H40O8Na, 563.2615). Retro-Aldol Reaction of 4. To a suspension of Na2CO3 (5.0 mg, 0.047 mmol) in absolute ethanol (1.5 mL), compound 4 (10.0 mg, 0.019 mmol) was added. The resulting mixture was stirred strongly at 85 °C (reflux) for 12 h and followed by an extraction with EtOAc (5.0 mL) for three times. The organic phase was purified by preparative HPLC (60% MeCN in H2O, 10 mL/min, tR = 56.1 min) to yield compound 4a (7.2 mg, 72% yield). Retro-Aldol Reaction of 5. To a suspension of Na2CO3 (4.0 mg, 0.038 mmol) in absolute ethanol (1.0 mL), compound 5 (8.0 mg, 0.015 mmol) was added. The resulting mixture was stirred strongly at 85 °C (reflux) for 12 h and followed by an extraction with EtOAc (5.0 mL) for three times. The organic phase was purified by preparative HPLC (60% MeCN in H2O, 10 mL/min, tR = 52.2 min) to yield compound 5a (5.4 mg, 68% yield). Crystal Structure Analysis. X-ray crystal data of 3 and 4 were acquired on a Xcalibur Eos Gemini diffractometer with graphite monochromated Cu Kα radiation (λ = 1.5418 Å). Their crystal structures were elucidated by direct methods using SHELXS-97 program and refined by the full-matrix least-squares difference Fourier method. Crystallographic data of 3 and 4 have been deposited at The Cambridge Crystallographic Data Center with the deposition numbers of CCDC 1574439 for 3 and CCDC 1574440 for 4, and are available free of charge via the Internet at www.ccdc.cam.uk/products/csd/ request. Anti-HSV-1 Activity Assay. See ref 12.

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X-ray crystallographic data of 4 (CIF)

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 Sciences (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

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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.joc.7b02558. UV, CD, IR, HRMS, and 1D and 2D NMR data of 1−5 (PDF) X-ray crystallographic data of 3 (CIF) 172

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The Journal of Organic Chemistry (12) (a) Su, F. Y.; Zhao, Z.; Ma, S. G.; Wang, R. B.; Li, Y.; Liu, Y. B.; Li, Y. H.; Li, L.; Qu, J.; Yu, S. S. Org. Lett. 2017, 19, 4920−4923. (b) Lv, H. J.; Ma, S. G.; Qu, J.; Liu, Y. B.; Li, Y. H.; Zhang, D.; Li; Li, Y. S. S.; Yu, S.-S. J. Nat. Prod. 2016, 79, 2824−2837.

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