Manzamine Alkaloids from an Acanthostrongylophora sp. Sponge

Apr 28, 2017 - Manzamine Alkaloids from an Acanthostrongylophora sp. Sponge. Chang-Kwon Kim†∥ ... Fax: 82 2 762 8322. E-mail: [email protected]., *T...
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Manzamine Alkaloids from an Acanthostrongylophora sp. Sponge Chang-Kwon Kim,†,∥ Riswanto Riswanto,†,∥ Tae Hyung Won,† Heegyu Kim,‡ Berna Elya,§ Chung J. Sim,⊥ Dong-Chan Oh,† Ki-Bong Oh,*,‡ and Jongheon Shin*,† †

Natural Products Research Institute, College of Pharmacy, Seoul National University, San 56-1, Sillim, Gwanak, Seoul 151-742, Korea ‡ Department of Agricultural Biotechnology, College of Agriculture and Life Science, Seoul National University, San 56-1, Sillim, Gwanak, Seoul 151-921, Korea § Faculty of Pharmacy, Universitas Indonesia, Kampus UI depok, West Java 16424, Indonesia ⊥ Department of Biological Science, College of Life Science and Nano Technology, Hannam University, 461-6 Jeonmin, Yuseong, Daejeon 305-811, Korea S Supporting Information *

ABSTRACT: Five new manzamine alkaloids (1−5) and new salt forms of two known manzamines (6 and 7), along with seven known compounds (8−14) of the same structural class, were isolated from an Indonesian Acanthostrongylophora sp. sponge. On the basis of the results of combined spectroscopic analyses, the structure of kepulauamine A (1) was determined to possess an unprecedented pyrrolizine moiety, while others were functional group variants of known manzamines. These compounds exhibited weak cytotoxicity, moderate antibacterial activity, and mild inhibition against the enzyme isocitrate lyase.

M

modified at either their carboline or aliphatic moieties. The seven known compounds were structurally divided into four typical manzamines (8−11), a dimer (12), and two imidazolecontaining xestomanzamines (13 and 14). Bioactivity tests revealed that most of these compounds are weakly active against the K562 and A549 cancer cell lines. Several compounds also showed moderate antibacterial activity as well as weak inhibitory activity against isocitrate lyase (ICL).

anzamines are mixed biogenetic alkaloids consisting of a β-carboline heterocycle and polycyclic diamine moieties containing an isoquinoline ring.1 Since the first finding of manzamine A hydrochloride (= keramamine A) from the sponge Haliclona sp.,2 these compounds have been continuously found in diverse Indian and Pacific sponges.3 With significant structural variations on both their carboline and aliphatic portions, these compounds are widely recognized to possess diverse bioactivities such as cytotoxic, antibacterial, antiinflammatory, anti-infective, antiparasitic, and insecticidal activities.1b,4 Reinvigorated by the discovery of potent activities against diseases such as AIDS and tuberculosis,5 manzamines have attracted significant biomedical and synthetic chemistry interest.6 To date, almost 100 natural manzamines have been reported, which includes the recently reported acanthomanzamines and acantholactam, with remarkable structural variability on both carbon frameworks and functionalities.1c,3b,7,8 During our search for bioactive metabolites from tropical sponges, we encountered a red encrusting sponge, Acanthostrongylophora sp., from Indonesia. Guided by 1H NMR data and cytotoxicity results, the organic extract was separated by diverse chromatographic methods to yield 14 compounds of the manzamine class. Determined based upon the combined spectroscopic analyses, the structures of five new manzamines (1−5) and two new hydrogen chloride salts (6 and 7) of known manzamines are reported herein. Among the new compounds, kepulauamine A (1) possesses a pyrrolizine-type moiety, which is unprecedented among manzamines, while the others are functional group derivatives of known compounds, © 2017 American Chemical Society and American Society of Pharmacognosy

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RESULTS AND DISCUSSION The molecular formula of kepulauamine A (1) was deduced to be C36H43N4O by positive HRESIMS analysis. The counterion anticipated from the molecular ion cluster was designated to be a chlorine from the observation of a cluster of C36H42ClN4O ([M + Cl − H]−) from a negative-mode mass analysis. The 13C NMR spectrum of this compound showed signals for 17 carbons in the deshielded region (δC 144.3−113.6, Table 1), which were thought to be attributed to both aromatic and olefinic moieties based on the occurrence of corresponding proton signals in two distinct regions (δH 8.50−7.44, 6 H; 6.69−5.32, 5 H) in the 1H NMR spectrum (Table 2). Also observed in the 13C NMR data were two unprotonated, four methine, and 13 methylene carbons in the shielded region, suggesting the presence of a highly aliphatic portion of this compound. Received: February 11, 2017 Published: April 28, 2017 1575

DOI: 10.1021/acs.jnatprod.7b00121 J. Nat. Prod. 2017, 80, 1575−1583

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Article

chemical shift, an oxygen, possibly as a hydroxy group, inherent in the molecular formula was placed at C-12. Further extension of the cyclohexene moiety was initiated by deducing a linear proton spin system consisting of two methylene protons (H2-22 and H2-23) directly correlated with the H-24 methine proton from the COSY and TOCSY data. Then, the linkage of C-22 with an isolated methylene carbon (C-36) via a nitrogen (N-21) was revealed by the crucial HMBC correlation at H2-22/C-36 and the characteristic chemical shifts of the following methylene carbons: C-22, δC 48.8, δH 3.18 and 2.81; C-36, δC 54.5, δH 3.83 and 3.62 (Tables 1 and 2). An additional three-bond HMBC correlation at H26/C-36 connected the latter methylene carbon at C-25, thus constructing another six-membered nitrogenous ring (N-21− C-25 and C-36) to indicate an isoquinoline. In the meanwhile, a combination of the COSY and TOCSY data revealed a long linear proton system (H2-13−H2-20) bearing a double bond (H-15 and H-16) in the middle. The linkage of this portion at C-12 and N-21 by C-13 and C-20, respectively, of the predescribed isoquinoline moiety was secured by a number of key HMBC correlations: H-13/C-12, H-20/C-22, H-22/C-20, H-26/C-13, and H-36/C-20. Overall, an isoquinoline-bearing tricyclic system, commonly found in manzamines, was confidently defined. The remaining portion of 1 was also determined from 2D NMR analyses. A long linear assembly of protons (H2-28−H235) was deduced from combined COSY and TOCSY data. Aided by the HSQC and HMBC data, a double bond was placed at C-32 (δC 128.0, δH 5.89) and C-33 (δC 129.5, δH 6.08). The small coupling constant (J32,33 = 6.6 Hz) between the olefinic protons suggested the presence of a small-sized ring system possessing this double bond. This interpretation was accommodated by a nitrogenous linkage (N-27) connecting C28, C-31, and C-34, thus indicating a 2,3,5,7a-tetrahydro-1Hpyrrolizine moiety, which was confirmed by the mutual HMBC correlations among these positions; H-28/C-31 and C-34, H31/C-32, and H-34/C-31. Finally, the linkages of N-27 and C36 with C-26 and C-25, respectively, of the tricyclic system were also established from several HMBC correlations: H-26/ C-28, C-31, and C-34 and H-35/C-25. The presence of N-27 as a quaternary ammonium salt was also supported by the remarkable deshielding (δΔ 10−15) of the neighboring C-28, C-31, and C-34 carbons compared to those of an ordinary cyclic amine system.9 This pyrrolizine-type moiety of 1 is unprecedented among the manzamine alkaloids. Kepulauamine A (1) possessed two series of stereogenic centers at C-12, C-24, C-25, and C-26 of its cyclohexene moiety and at N-27, C-31, and C-34 of its pyrrolizine moiety, whose configurations were assigned using the ROESY data (Figure 2). First, the orientation of the C-24/C-25 ring juncture of the bicyclic portion was assigned as cis based on the cross-peaks at H-24/H-28 (δH 4.04), H-24/H-35 (δH 3.13), and H-26/H-36 (δH 3.62), as is commonly found in manzamines. The 12-OH group was also α-oriented to the cyclohexene portion based on the cross-peaks at H-11/H-13 (δH 2.37) and H-13 (δH 2.17)/H-26. Combining these data, the relative configurations of the bicyclic portion were assigned as 12S*, 24R*, 25R*, and 26R*. Similarly, the relative configurations of the pyrrolizine moiety were assigned as 27S*, 31R*, and 34R* based on the cross-peaks at H-11/H-28 (δH 4.50), H-24/H-34, H-26/H-31, H-28 (δH 4.04)/H-34, H29 (δH 2.11)/H-34, and H-30 (δH 1.93)/H-32. All of the ROESY correlations were in good accordance with DFT

The planar structure of 1 was determined by a combination of COSY, TOCSY, HSQC, and HMBC experiments. First, the six highly deshielded protons (δH 8.50−7.44) were found to form two separate proton spin systems based on the COSY data (Figure 1). The HSQC-based direct matching of these protons with their attached carbons, followed by the HMBCbased long-range correlations with neighboring carbons, readily constructed a β-carboline system (C-1−C-9a), which was supported by a comparison of the NMR data with those of manzamines including the known congeners manzamines A (9) and B (11). The attachment of a trisubstituted double bond at C-1 of the carboline moiety was accomplished by the HMBC correlations of an olefinic proton at δH 6.69 (H-11) with the carbons at δC 138.2 (C-1) and 144.9 (C-10). Extension of this partial structure to construct a cyclohexene moiety (C-10−C-12 and C-24−C-26) was made from the HMBC correlations of the H11 olefinic proton and two isolated methine protons at δH 4.40 (H-26) and 3.90 (H-24) with neighboring carbons, including the unprotonated carbons at δC 71.4 (C-12) and 45.9 (C-25): H-11/C-1, C-10, and C-12, H-24/C-10, C-11, and C-25, and H-26/C-11, C-12, C-24, and C-25. On the basis of its carbon 1576

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Table 1. 13C NMR (ppm, Type) Assignments for Compounds 1−7 position 1 3 4 4a 4b 5 6 7 8 8a 9a 10 11 12 13 14 15 16 17 18 19 20 22 23 24 25 26 28 29 30 31 32 33 34 35 36

1a 138.2, 134.3, 117.3, 135.5, 121.8, 123.7, 122.8, 132.5, 113.8, 139.2, 134.1, 144.9, 143.8, 71.4, 45.4, 21.8, 129.8, 132.8, 26.3, 27.7, 22.0, 57.4, 48.8, 23.4, 34.7, 45.9, 77.2, 65.0, 25.5, 29.2, 91.3, 128.0, 129.5, 86.9, 42.3, 54.5,

C CH CH C C CH CH CH CH C C C CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH CH CH CH CH2 CH2

2b 138.3, 131.0, 114.4, 121.4, 121.1, 120.7, 120.0, 127.4, 111.4, 141.7, 136.5, 31.9, 59.0, 60.7, 39.5, 23.6, 131.4, 132.7, 24.4, 27.7, 24.4, 50.5, 44.9, 23.9, 33.0, 39.8, 59.4, 47.6, 26.8, 27.4, 22.8, 128.9, 131.9, 18.0, 32.1, 56.5,

C CH CH C C CH CH CH CH C C CH CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2

3b 141.7, 61.1, 20.0, 108.7, 122.9, 118.7, 119.8, 124.8, 111.7, 137.8, 128.3, 33.3, 57.3, 60.8, 39.6, 23.6, 131.2, 132.8, 24.5, 27.9, 24.9, 50.8, 44.8, 24.2, 32.8, 39.5, 59.4, 47.6, 26.7, 27.2, 22.6, 129.0, 131.7, 18.1, 32.3, 56.7,

C CH2 CH2 C C CH CH CH CH C C CH CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2

4a 143.2, 136.0, 115.7, 131.5, 122.4, 122.9, 121.6, 130.4, 113.2, 143.1, 135.6, 41.5, 66.8, 73.8, 43.7, 20.6, 129.2, 132.6, 28.5, 23.8, 26.7, 60.8, 58.2, 25.6, 33.6, 41.7, 62.3, 47.3, 26.6, 25.9, 27.5, 132.5, 130.3, 20.4, 30.7, 64.0,

C CH CH C C CH CH CH CH C C CH CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2

5b 143.1, 138.7, 113.4, 129.3, 121.8, 121.6, 120.1, 128.3, 111.6, 140.0, 133.5, 139.0, 137.3, 70.1, 40.8, 21.7, 128.4, 132.4, 25.9, 25.7, 26.7, 53.4, 49.5, 32.6, 40.2, 47.3, 75.0, 50.6, 27.7, 36.0, 69.8, 139.9, 128.0, 55.3, 44.3, 68.5,

C CH CH C C CH CH CH CH C C C CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH CH CH CH CH2 CH2

6a 134.3, 132.4, 117.5, 124.7, 122.3, 122.5, 122.5, 129.9, 113.3, 143.9, 136.4, 134.2, 138.7, 70.0, 44.9, 22.2, 130.3, 130.7, 24.7, 30.9, 22.5, 51.7, 47.8, 22.8, 35.0, 43.3, 68.4, 52.2, 25.3, 24.9, 28.1, 132.6, 131.5, 20.7, 31.7, 57.9,

7a

C CH CH C C CH CH CH CH C C C CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2

140.4, 62.0, 20.7, 111.9, 126.6, 120.4, 122.0, 125.4, 113.2, 139.8, 128.2, 134.6, 138.1, 70.0, 44.6, 22.5, 130.3, 131.6, 25.4, 28.1, 23.2, 51.5, 48.0, 23.0, 34.6, 43.3, 69.0, 52.5, 25.1, 24.6, 25.2, 132.5, 130.6, 22.1, 31.3, 58.1,

C CH2 CH2 C C CH CH CH CH C C C CH C CH2 CH2 CH CH CH2 CH2 CH2 CH2 CH2 CH2 CH C CH CH2 CH2 CH2 CH2 CH CH CH2 CH2 CH2

a,b

Data were obtained in MeOH-d4 and CDCl3, respectively.

found to be identical to those in 11 based on ROESY analysis and a comparison of NMR data. Thus, the structure of compound 2, designated as manzamine B N-oxide, was defined as a new derivative of manzamine B with differences in the carboline moiety. Compound 3, a yellow, amorphous powder, was determined to be C36H48N4O2 based on HRFABMS analysis. Although the 13 C and 1H NMR data for this compound were reminiscent of those of 2, suggesting the same manzamine-type structural nature, significant differences were found in terms of the carboline moiety. That is, the HSQC data revealed the replacement of two of the aromatic methine groups of 2 with methylene groups in 3: δC 61.1 and 20.0, δH 4.38, 4.29, 3.20, and 3.11 (Tables 1 and 2). The new methylene protons were found to be directly spin-coupled based on the COSY data. These differences were accommodated by the hydrogenation at C-3 and C-4, thus the loss of aromaticity at the pyridine moiety, based on combined 2D NMR analyses, including the following crucial HMBC correlations: H-3/C-1 and C-4a and H-4/C-4a and C-9a. Thus, the structure of 3, designated to be 3,4dihydromanzamine B N-oxide, was defined as a new manzamine alkaloid.

calculations (Supporting Information). The overall absolute configurations were assigned as 12S, 24R, 25R, 26R, 27S, 31R, and 34R based on biosynthetic considerations involving the other manzamines, including congener 5, which is discussed later. Thus, the structure of kepulauamine A (1) was determined to be a new manzamine alkaloid possessing a 2,3,5,7a-tetrahydro-1H-pyrrolizine moiety. The molecular formula of compound 2 was deduced to be C36H46N4O2 by HRFABMS analysis. The 13C and 1H NMR data of this compound were similar to those of manzamines, in particular, the congener manzamine B (11). Aided by the results of combined 2D NMR analyses (Figure 2), a detailed examination of the NMR data showed noticeable shielding of the carbons and protons in the alkylpyridine portion (C-1, C3−C-4a, and C-9a) of the β-carboline system (Tables 1 and 2). These differences, in conjunction with the presence of an additional oxygen inherent in the molecular formula, were accommodated by the presence of an N-oxide group at the N-2 position. Similar trends in NMR shielding effects were also observed for other manzamines bearing a pyrido-N-oxide group.10 The configurations of the stereogenic centers, including the α orientation of the C-11 epoxide, were also 1577

DOI: 10.1021/acs.jnatprod.7b00121 J. Nat. Prod. 2017, 80, 1575−1583

8.49, d (5.3)

d (7.9) dd (7.9, 7.0) dd (7.9, 7.0) d (7.9)

s m m m m m m m m m m m m m m m br dd (11.6, 10.9) m m br s s ddd (13.8, 10.6, 5.0) dd (10.6, 5.0) m m m m br d (8.8)

8.36, 7.44, 7.75, 7.77,

6.69, 2.37, 2.17, 2.53, 2.27, 5.67, 5.66, 2.44, 2.05, 2.09, 1.90, 1.89, 1.79, 3.32, 3.28, 3.18, 2.81, 2.29, 1.89, 3.90, 4.40, 4.50, 4.04, 2.22, 2.11, 2.36, 1.93, 5.37,

4

5 6 7 8 10 11 13

1578

5.89, br d (6.6) 6.08, br d (6.6) 5.32, br dd (8.8, 7.8)

3.13, br dd (10.7, 7.8)

32 33 34

35

31

30

29

24 26 28

23

22

20

19

18

15 16 17

14

8.50, d (5.3)

1a

3

position

7.94, d (8.1) 7.19, dd (8.1, 7.8) 7.46, dd (8.1, 7.8) 7.34, d (8.1) 5.35, dd (9.8, 4.3) 3.53, d (4.3) 2.48, m 1.31, dd (13.1, 13.1) 2.37, m 2.25, m 5.35, m 5.35, m 2.26, m 2.06, m 1.61, m 1.44, m 1.77, m 1.56, m 2.46, m 2.38, m 2.89, m 2.62, m 1.85, m 1.10, m 2.01, br dd (9.8, 2.4) 3.00, s 3.46, m 2.55, m 1.99, m 1.66, m 1.78, m 1.65, m 2.42 m 2.05, m 5.47, ddd (10.6, 5.0, 5.0) 5.58, ddd (10.6, 5.0, 5.0) 2.26, m 1.97, m 2.85, m

7.70, d (6.8)

8.14, d (6.8)

2b 4.38, ddd (14.4, 14.2, 7.3) 4.29, ddd (14.4, 7.3, 4.5) 3.20, ddd (16.3, 14.2, 7.1) 3.11, ddd (16.3, 7.1, 4.3) 7.46, d (8.0) 7.07, ddd (8.0, 7.3, 0.9) 7.16, ddd (8.0, 7.3, 0.9) 7.25, d (8.0) 4.91, dd (9.6, 4.3) 3.40, d (4.3) 2.48, m 1.33, dd (13.5, 13.5) 2.40, m 2.22, m 5.34, m 5.35, m 2.39, m 2.03, m 1.59, m 1.42, m 1.75, m 1.57, m 2.42, m 2.31, m 2.80, m 2.49, m 1.78, m 1.05, m 1.71, br dd (9.6, 2.5) 2.91, s 3.44, m 2.53, m 1.94, m 1.62, m 1.77, m 1.61, m 2.40, m 2.00, m 5.46, ddd (10.8, 5.0, 5.0) 5.58, ddd (10.8, 5.0, 5.0) 2.22, m 1.84, m 2.71, m

3b

Table 2. 1H NMR [δ, mult, (J in Hz)] Assignments for Compounds 1−7

8.21, 7.32, 7.62, 7.70, 4.47, 4.96, 2.41, 1.94, 2.38, 2.07, 5.44, 5.69, 2.47, 2.25, 2.01, 1.29, 2.03, 1.94, 3.89, 3.23, 4.50, 3.52, 2.42, 2.30, 2.86, 3.78, 3.45, 3.20, 2.02, 1.94, 1.87, 1.53, 2.43, 2.31, 5.70, 5.64, 2.47, 2.14, 2.36,

d (7.8) t (7.8) t (7.8) d (7.8) m m m m m m br t (10.9) m m m m m m m br dd (13.6, 12.5) m m br dd (10.8, 9.2) m m br s s m m m m m m m m m m m m m

8.13, d (4.7)

8.32, d (4.7)

4a

s m m m m ddd (12.7, 10.7, 8.6) ddd (12.7, 10.7, 4.8) m m m m m m ddd (13.2, 11.8, 3.6) ddd (13.2, 3.6, 1.7) m m m m m s m m m m m m br dd (7.2, 6.4)

d (7.7) dd (7.7, 2.2) m m

2.28, dd (13.2, 7.5)

5.85, dd (10.2, 6.4) 5.27, dd (10.2, 8.5) 3.99, br dd (8.5, 7.5)

6.38, 2.05, 1.76, 2.34, 2.13, 5.65, 5.54, 2.54, 1.73, 1.75, 1.35, 1.47, 1.35, 2.62, 2.44, 2.77, 1.96, 2.00, 1.57, 3.03, 3.52, 3.11, 3.01, 2.11, 1.60, 1.90, 1.60, 4.56.

8.11, 7.28, 7.55, 7.54,

7.83, d (5.0)

8.44, d (5.0)

5b

6.30, 2.30, 1.90, 2.68, 2.22, 5.64, 5.65, 2.56, 2.03, 1.92, 1.28, 1.99, 1.41, 3.74, 3.51, 4.21, 2.99, 2.22, 1.41, 3.49, 3.85, 3.87, 3.36, 2.02, 1.97, 1.95, 1.71, 2.40, 2.13, 5.70, 5.66, 2.06, 2.01, 2.58,

8.16, 7.35, 7.55, 7.61, s m m m m m m m m m m m m m m dd (12.3, 10.5) br d (10.5) m m m s m m m m m m m m m m m m m

d (7.9) dd (7.7, 7.1) dd (7.9, 7.1) d (7.9)

8.17, d (6.4)

8.25, d (6.4)

6a

6.32, 2.17, 1.80, 2.60, 2.22, 5.64, 5.67, 2.33, 2.08, 1.73, 1.46, 2.20, 1.96, 3.46, 3.35, 3.95, 3.06, 2.25, 1.78, 3.16, 3.81, 3.87, 3.36, 2.07, 1.94, 1.95, 1.74, 2.56, 2.02, 5.71, 5.65, 2.07, 2.02, 2.34,

7.55, 7.11, 7.21, 7.38,

s m m m m br br m m m m m m m m m m m m br s m m m m m m m m br br m m m

s s

s

s s

d (7.9) dd (7.9, 7.2) dd (7.9, 7.2) d (7.9)

4.48, ddd (14.4, 14.2, 7.3) 4.30, ddd (14.0, 8.7, 5.5) 3.35, m 3.27, m

7a

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DOI: 10.1021/acs.jnatprod.7b00121 J. Nat. Prod. 2017, 80, 1575−1583

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Figure 1. COSY and TOCSY (bold lines) and selected HMBC (arrows) correlations of 1, 2, and 5.

1.86, m 3.71, d (12.3) 3.60, d (12.3)

The molecular formula of compound 4 was established as C36H48N4O2 based on HRFABMS analysis. The spectroscopic data for this compound were similar to those of other manzamines. First, based on the results of combined 1D and 2D NMR analyses, this compound was found to possess the same β-carboline moiety as 1. Then, the remaining aliphatic portion was also found to have the identical carbon−proton and proton−proton correlations as 2. Therefore, the significant differences in the NMR data at C-11 and its vicinity must be attributed to the ring-opening of the epoxide in 2 to a diol group in 4: C-11, δC 66.8, δH 4.96; C-12, δC 73.8 (Tables 1 and 2). The 11-OH and 12-OH were both found to have the α orientation based on the ROESY cross-peaks at H-11/H-13 (δH 2.41) and H-13 (δH 1.94)/H-26 (Figure 2). Because the ROESY data, aided by the DFT model (Supporting Information), revealed the same cross-peaks at the other stereogenic centers as for common manzamines, the absolute configurations at the diol-bearing carbons were assigned as 11S and 12R. Thus, the structure of 4, designated as 11hydroxymanzamine J based on the structural resemblance, was defined as a new manzamine alkaloid bearing an 11,12dihydroxy group. The molecular formula of compound 5 was deduced to be C36H44N4O2 based on HRFABMS analysis. The NMR data of this compound showed characteristic carbon and proton signals for a β-carboline moiety. For the remaining aliphatic portion, detailed examination of the 13C NMR data, in conjunction with the eight degrees of unsaturation from the mass data, indicated the presence of three double bonds and five rings, corresponding to one additional ring than in 2−4. This additional ring was thought to bear a C−N bond based on the presence of a conspicuous methine group (δC 55.3, δH 3.99) in the HSQC data (Tables 1 and 2). Given this, combined 2D NMR analyses secured the presence of the isoquinoline and macrocyclic moiety bearing double bonds at C-11 and C-15, as in other manzamines. A

a,b

Data were obtained in MeOH-d4 and CDCl3, respectively.

0.81, dd (14.6, 7.7) 2.29, m 2.21, m 2.22, m 3.83, d (12.5) 3.62, d (12.5) 36

position

Table 2. continued

1a

0.91, m 3.10, m 2.49, m

2b

3b

4a

1.76, m 2.78, m 2.34, m

5b

1.66, m 3.42, m 3.35, m

6a

1.86, m 3.34, m 3.32, m

7a

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group was placed at H-31 based on the direct coupling with the H-32 olefinic proton (J31,32 = 6.4 Hz) and the HMBC correlations at H-31/C-32 and H-33/C-31 (Figure 1). Then, the configurations at the heterobearing C-12 and C-34 positions were both assigned as S based on the crucial ROESY cross-peaks at H-13 (δH 2.05)/H-26, H-24/H-35 (δH 2.28), H-26/H-36 (δH 2.78), H-31/H-34, H-34/H-35 (δH 2.28), and H-35 (δH 1.76)/H-36 (δH 2.78) (Figure 2). Finally, the configuration of the isolated C-31 stereogenic center was assigned as R based on the MTPA analysis (Figure 3). Thus, the structure of compound 5, designated as 31-hydroxymanzamine A, was determined as a new manzamine alkaloid bearing an eight-membered ring.

Figure 3. ΔδH(5S − 5R) values (ppm) obtained for the (S)- and (R)MTPA esters of 5.

Chromatographic separation of the extract yielded two additional compounds, 6 and 7. Despite the noticeable differences in the 13C and 1H NMR data of these compounds compared to those of the known compounds (Tables 1 and 2), comprehensive analyses of 1D and 2D NMR and mass data defined these compounds to be the hydrogen chloride salts of manzamine J N-oxide and 3,4-dihydromanzamine J N-oxide, respectively.10,11 In addition to the spectroscopic interpretation, the salt nature of these compounds was supported by their significant polarity, similar to 1, another manzamine salt, in chromatographic separation. The salt formation at N-27 of these compounds was further supported by a DFT calculation study and a ROESY cross-peak at H-3/H-11 (Supporting Information).12 The extract also contained seven known compounds, which were identified to be the four typical manzamines 32,33-dihydro-31-hydroxymanzamine A (8),13 manzamine A (9),2a 6-deoxymanzamine X (10),10 and manzamine B (11),14 a manzamine dimer, neo-kauluamine (12),15 and two imidazole-containing carbolines, xestomanzamines A (13) and B (14),16 based on combined spectroscopic data, which were in good agreement with the data in the literature. These results demonstrated that the Indonesian specimen of Acanthostrongylophora sp. is a rich source of manzamine alkaloids. Manzamine alkaloids are widely recognized to possess diverse bioactivities.1b In our measurements, most of these compounds exhibited weak inhibition against K562 and A549 cancer cell lines (Table 3). Several compounds were also moderately active against diverse strains of pathogenic bacteria. In enzyme inhibition tests, most of the compounds were inactive against sortase A (SrtA) and Na+/K+-ATPase, while a

Figure 2. Key ROESY (black arrows) correlations of 1, 4, and 5.

hydroxy group was also placed at C-12 based on the carbon chemical shift (δC 70.1) and HMBC correlations at H-13/C-12 and H-26/C-12. For the macrocyclic N-27−C-35 portion, the COSY and TOCSY data indicated a long linear spin system consisting of all of the remaining protons. The direct coupling (J33,34 = 8.5 Hz) with the H-33 olefinic proton placed the Nbearing methine at C-34. The connection of this methine at the C-25 ring juncture of the isoquinoline via the C-35 methylene carbon was also confirmed by the HMBC correlations at H-34/ C-25 and H-35/C-25, C-33, and C-34. Similarly, a hydroxy 1580

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Table 3. Results of Bioactivity Testsa A549

K562

MIC (ng/mL)

LC50 μM) compound 1 2 3 4 5 6 7 8 9 10 11 12 13 14 doxorubicin ampicillin pHMBc 3-NPd ouabain

4.6 12 5.8 6.2 5.8 4.7 5.7 8.2 8.3 6.7 6.5 13 12 9.9 0.92

7.2 9.8 5.2 8.2 7.2 8.1 9.5 8.4 11 9.1 9.6 12 13 9.2 1.1

Gram(+) Bacteria

SrtA Gram(−) Bacteria

ICL Na+/K+-ATPase IC50 (HM

A

B

C

D

E

F

8.0 >100 13 2.0 25 32 32 50 >100 100 >100 >100 >100 50

32 100 6.3 4.0 6.3 >100 64 25 50 100 100 1.6 50 25

16 100 3.1 4.0 6.3 >100 64 13 50 50 100 1.6 25 25

16 50 3.1 8.0 1.6 32 >100 1.6 3.1 3.1 50 0.78 25 6.3

8.0 >100 25 2.0 13 >100 32 100 >100 >100 >100 >100 >100 50

64 >100 >100 16 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100

0.39

0.78

0.39

0.39

1.6

6.3

NDb >150 >150 ND >150 81 >150 >150 >150 >150 >150 >150 >150 >150

>150 >150 70 60 140 26 >150 68 140 140 >150 27 >150 75

>150 >150 >150 120 >150 >150 110 >150 >150 >150 >150 >150 >150 >150

25 19 6.6

a

A: Staphylococcus aureus (ATCC 6538p), B: Bacillus subtilis (ATCC 6633), C: Kocuria rhizophila (NBRC 12708), D: Salmonella enterica (ATCC 14028), E: Proteus hauseri (NBRC 3851), F: Escherichia coli (ATCC 35270). bNot detected. cpara-Hydroxymercuribenzoic acid. d3-Nitropropionic acid. at a depth of 10 m on February 19, 2012. The sponge was massive in shape and dark red in life, measuring 70 × 60 mm and 15 mm thick. The surface was uneven and shaggy, and the texture was breadcrumblike. The skeleton was a tangential irregular network of strongyles with free oxeas at the ectosome, forming a network of multispicular, longitudinal tracks at the choanosome (lengths of 120−140 μm and diameters of 5−6 μm). The specimens were similar to those of Acanthostrongylophora ashmoria Hooper, 1984, in terms of the general morphological features and spicules, except that A. ashmoria had a hard and stony texture. A voucher specimen (registry no. spo. 67) was deposited in the MBIK (Marine Biodiversity Institute of Korea), Seocheon, Korea, under the curatorship of C.J.S. Extraction and Isolation. Freshly collected specimens were immediately frozen and stored at −25 °C until use. Lyophilized specimens were macerated and repeatedly extracted with MeOH (3 × 3 L) and CH2Cl2 (2 × 3 L). The combined extracts (172.22 g) were successively partitioned between H2O (127.84 g) and n-BuOH (40.17 g); the latter fraction was repartitioned between H2O−MeOH (15:85, 15.49 g) and n-hexane (23.61 g). An aliquot (9.35 g) of the aqueous MeOH layer was separated by C18 reversed-phase flash chromatography using sequential mixtures of MeOH and H2O as the eluents (a column size of 150 × 100 mm, six fractions in H2O−MeOH gradient, from 50:50 to 0:100, v/v, fraction size, 1 L), followed by acetone and then EtOAc. Based on the 1H NMR and cytotoxicity results, the fractions eluted with H2O−MeOH (50:50; 3.12 g), H2O−MeOH (30:70; 0.52 g), H2O−MeOH (0:100; 0.82 g), and acetone (0.81 g) were chosen for separation. The H2O−MeOH (50:50) fraction was separated by Sephadex LH-20 gel permeation chromatography, eluting with 1:1 v/v CHCl3−MeOH (a column size of 500 × 20 mm, fraction size, 10 mL) to yield 25 fractions. Guided by the results of the 1H NMR analyses, the 11th fraction was separated by reversed-phase HPLC (YMC-ODS column, 10 mm × 250 mm; 1.8 mL/min, H2O−MeOH gradient (75:25 with 0.01% TFA to 45:55 with 0.01% TFA)), yielding four peaks rich in secondary metabolites. Purification of each compound was accomplished by analytical HPLC (YMC-ODS column, 4.6 × 250 mm; 0.7 mL/min, H2O−MeCN gradient (90:10 with 0.01% TFA to 60:40 with 0.01% trifluoroacetic acid, TFA)) to yield, in order of

number of compounds, such as 6 and 12, showed mild inhibition against Candida albicans-derived isocitrate lyase, a key enzyme in microbial virulence. In summary, 14 manzamine alkaloids, including five new compounds (1−5) and two new manzamine salts (6 and 7), were isolated and structurally defined from an Indonesian Acanthostrongylophora sp. sponge. Among the new compounds, kepulauamine A (1) possesses a pyrrolizine-type moiety, which is unprecedented among the manzamines. These compounds exhibited diverse bioactivities such as cytotoxicity, antibacterial activity, and ICL inhibition.

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EXPERIMENTAL SECTION

General Experimental Procedures. Optical rotations were measured using a JASCO P-1020 polarimeter using a 1 cm cell. UV spectra were acquired using a Hitachi U-3010 spectrophotometer. IR spectra were recorded on a JASCO 4200 FT-IR spectrometer using a ZnSe cell. NMR spectra were recorded in CDCl3 and MeOH-d4 with solvent peaks as the δH 7.26/δC 77.0 and δH 3.30/δC 49.0 internal standards, respectively, on Bruker Avance 400, 500, 600, and 800 MHz spectrometers. Proton and carbon NMR spectra were measured at 400 and 100 MHz (2, 5, and 7), 500 and 125 MHz (3 and 6), 600 and 150 MHz (4), and 800 and 200 MHz (1), respectively. High-resolution ESI mass spectrometry data were obtained at the National Instrumentation Center for Environmental Management (Seoul, Korea) on a Thermo-Finnigan LTQ-Orbitrap instrument equipped with a Dionex U-3000 HPLC system. High-resolution FAB mass spectrometric data were obtained at the Korea Basic Science Institute and acquired using a JEOL JMS 700 mass spectrometer with metanitrobenzyl alcohol as the FABMS matrix. HPLC was performed on a SpectraSYSTEM p2000 equipped with a refractive index detector (SpectraSYSTEM RI-150), a Gilson 321 pump, and a UV−vis detector (Gilson UV-Vis-151). All solvents used were of spectroscopic grade or were distilled prior to use. Animal Material. Specimens of an Acanthostrongylophora sp. (voucher number 12KL-15) sponge were collected by hand via scuba diving from Kepulauan Seribu Marine National Park, north of Jakarta, 1581

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elution, compounds 6, 1, 7, and 4, with retention times (tR) of 25.1, 28.4, 32.7, and 42.5 min, respectively. The H2O−MeOH (30:70) fraction was separated by reversed-phase HPLC (2.0 mL/min, H2O−MeOH gradient (70:30 with 0.05% TFA to 30:70 with 0.05% TFA)), yielding five peaks rich in secondary metabolites. Each compound was further purified by normal-phase HPLC (YMC-silica column, 4.6 × 250 mm; 1.0 mL/min, MeOH− EtOAc, 3:97, with 0.1% diethylamine) to afford, in order of elution, compounds 13, 14, 11, 2, and 3, with retention times (tR) of 17.1, 21.4, 28.7, 32.3, and 35.5 min, respectively. The H2O−MeOH (0:100) fraction was separated by reversed-phase HPLC (2.0 mL/min, H2O− MeOH gradient (60:40 with 0.05% TFA to 25:75 with 0.05% TFA)), yielding three peaks rich in secondary metabolites. Further purification of each compound was separated by analytical HPLC (0.8 mL/min, H2O−MeOH, 70:30, with 0.05% TFA) to yield, in order of elution, compounds 8, 5, and 12, with retention times (tR) of 23.1, 26.4, and 32.5 min, respectively. The acetone fraction was separated by reversedphase HPLC (2.0 mL/min, H2O−MeOH, 5:95) to yield compounds 10 and 9, with retention times (tR) of 22.1 and 28.5 min, respectively. The purified metabolites were isolated in the following amounts: 2.6, 11.6, 6.4, 4.8, 5.4, 5.3, 7.8, 13.0, 51.7, 39.0, 13.7, 9.1, 5.1, and 29.7 mg of 1−14, respectively. Kepulauamine A (1): pale yellow, amorphous powder; [α]25 D +13 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 214 (4.16), 291 (3.72), 346 (3.47), 356 (3.46) nm; IR (ZnSe) νmax 3375, 2928, 1677, 1441, 1200, 1131 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRESIMS m/z 547.3434 [M]+ (calcd for C36H43N4O, 547.3431), 581.3066 [M + Cl − H]− (calcd for C36H42ClN4O, 581.3047) Manzamine B N-oxide (2): yellow, amorphous powder; [α]25 D +45 (c 0.5, CHCl3); UV (MeOH) λmax (log ε) 215 (2.99), 253 (3.24), 289 (0.73), 320 (1.90), and 350 (0.71) nm; IR (ZnSe) νmax 3340, 2930, 1457, 1186 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRFABMS m/z 567.3701 [M + H]+ (calcd for C36H47N4O2, 567.3699). 3,4-Dihydromanzamine B N-oxide (3): yellow, amorphous powder; [α]25 D +48 (c 0.5, CHCl3); UV (MeOH) λmax (log ε) 215 (2.67), 261 (1.46), 289 (0.64), 321 (1.03), 346 (1.29) nm; IR (ZnSe) νmax 3341, 2930, 1447, 1154 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRFABMS m/z 569.3857 [M + H]+ (calcd for C36H49N4O2, 569.3856). 11-Hydroxymanzamine J (4): pale yellow, amorphous powder; [α]25 D +44 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 205 (4.11), 223 (3.39), 274 (3.37), 322 (3.24) nm; IR (ZnSe) νmax 3420, 2920, 1640, 1540, 1090 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRFABMS m/z 551.3753 [M − H2O + H]+ (calcd for C36H47N4O, 551.3750). 31-Hydroxymanzamine A (5): pale yellow, amorphous powder; [α]25 D +37 (c 0.5, CHCl3); UV (MeOH) λmax (log ε) 218 (3.10), 237 (2.43), 289 (1.24), 343 (0.63), 358 (0.68) nm; IR (ZnSe) νmax 3316, 2928, 1455, 1072 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRFABMS m/z 565.3546 [M + H]+ (calcd for C36H45N4O2, 565.3543). Manzamine J N-oxide-HCl (6): dark brown, amorphous powder; [α]25 D +78 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 261 (3.40), 330 (2.28) nm; IR (ZnSe) νmax 3182, 2910, 1677, 1451, 1200, 1131 cm−1; 1 H and 13C NMR data, Tables 1 and 2; HRESIMS m/z 567.3698 [M]+ (calcd for C36H47N4O2, 569.3699), 601.3329 [M + Cl − H]− (calcd for C36H46ClN4O2, 601.3309). 3,4-Dihydromanzamine J N-oxide-HCl (7): dark brown, amorphous powder; [α]25 D +58 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 217 (4.40), 352 (4.28) nm; IR (ZnSe) νmax 3257, 2928, 1668, 1441, 1200, 1141 cm−1; 1H and 13C NMR data, Tables 1 and 2, respectively; HRESIMS m/z 569.3845 [M]+ (calcd for C36H49N4O2, 569.3856), 603.3431 [M + Cl − H]− (calcd for C36H48ClN4O2, 603.3466). Preparation of the (S)- and (R)-MTPA Esters of Compound 5. To a solution of 5 (1.2 mg) in dry pyridine (500 μL) were added (R)(−)-MTPA chloride (20 μL) and DMAP (0.3 mg). The mixture was allowed to stand under N2 at room temperature for 2 h. After consumption of the starting material was confirmed by HPLC-DAD analysis, MeOH (1 mL) was added. The solvent was removed under

vacuum, and the residue was separated by reversed-phase HPLC (YMC ODS column, 4.6 mm × 250 mm; H2O−MeOH, 40:60, with 0.05% TFA) to give 0.6 mg of the (S)-MTPA ester 5S. The corresponding (R)-MTPA ester 5R was also obtained from the same esterification reaction of 5 with (S)-(+)-MTPA chloride. (S)-MTPA ester of 5 (5S): 1H NMR (CDCl3, 600 MHz) δH 8.902 (1H, br s, H-3), 8.377 (1H, d, J = 6.1 Hz, H-4), 8.254 (1H, d, J = 8.0 Hz, H-5), 7.792 (1H, m, H-7), 7.772 (1H, m, H-8), 7.529−7.511 (2H, m, MTPA-Ar), 7.472 (1H, m, H-6), 7.453−7.438 (3H, m, MTPA-Ar), 6.398 (1H, s, H-11), 6.279 (1H, br s, H-32), 5.945 (1H, dd, J = 9.1, 5.3 Hz, H-31), 5.679 (1H, dd, J = 9.1, 9.1 Hz, H-33), 5.525 (1H, dd, J = 9.5, 9.5 Hz, H-15), 5.340 (1H, dd, J = 9.5, 9.5 Hz, H-16), 4.909 (1H, br s, H-34), 4.399 (1H, br s, H-28a), 3.658 (1H, m, H-36a), 3.549 (1H,m, H-20a), 3.490 (3H, s, H-MTPA-OMe), 3.410 (1H, br s, H26), 3.217−3.159 (3H, m, H-20b, H-24, and H-35a), 3.044−2.847 (4H, m, H-14a, H-22b, H-28b, and H-36b), 2.577 (1H, br s, H-14b), 2.385−2.291 (2H, m, H-13a and H-17a), 2.273 (1H, m, H-29a), 2.251 (1H, m, H-30a), 2.219−2.003 (3H, m, H-19a, H-19b, and H-23a), 1.931−1.910 (3H, m, H-13b, H-17b, and H-18a), 1.882 (1H, m, H29b), 1.840 (1H, m, H-30b), 1.633−1.595 (1H, m, H-18b), 1.277 (1H, m, H-23b); LRESIMS m/z 781.4 [M + H]+ (calcd for C46H52F3N4O4, 781.4). (R)-MTPA ester of 5 (5R): 1H NMR (CDCl3, 600 MHz) δH 8.914 (1H, br s, H-3), 8.338 (1H, br s, H-4), 8.241 (1H, d, J = 8.3 Hz, H-5), 7.775 (1H, m, H-7), 7.755 (1H, m, H-8), 7.525−7.512 (2 H, m, HMTPA-Ar), 7.463 (1H, m, H-6), 7.449−7.431 (3H, m, H-MTPA-Ar), 6.417 (1H, m, H-32), 6.364 (1H, s, H-11), 5.965 (1H, m, H-31), 5.744 (1H, dd, J = 9.6, 9.6 Hz, H-33), 5.516 (1H, dd, J = 9.7, 9.7 Hz, H-15), 5.342 (1H, dd, J = 9.7, 9.7 Hz, H-16), 4.901 (1H, brs, H-34), 4.408 (1H, br s, H-28a), 3.651 (1H, d, J = 12.3 Hz, H-36a), 3.560 (3H, s, HMTPA-OMe), 3.440 (1H, br s, H-26), 3.375 (1H, m, H-20a), 3.227− 3.222 (2H, m, H-20b and H-35a), 3.104 (1H, m, H-24), 3.029−2.907 (4H, m, H-22a, H-22b, H-28b, and H-36b), 2.827 (1H, m, H-14a), 2.559 (1H, m, br s, H-17a), 2.365 (1H, m, H-14b), 2.286 (1H, m, H35b), 2.248 (1H, m, H-29a), 2.183 (1H, m, H-30a), 2.151−2.009 (4H, m, H-13a, H-19a, H-19b, and H-23a), 1.890−1.802 (2H, m, H-13b and H-17b), 1.825 (1H, m, H-29b), 1.739 (1H, m, H-30b), 1.713− 1.667 (2H, m, H-18a and H-18b), 1.259 (1H, m, H-23b); LRESIMS m/z 781.4 [M + H]+ (calcd for C46H52F3N4O4, 781.4). Biological Assays. Antimicrobial assays were performed according to the method described previously.17 Cytotoxicity assays were performed in accordance with literature protocols.18 Isocitrate lyase, Na+/K+-ATPase, and sortase A inhibition assays were performed according to previously described methods.19a−c

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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.jnatprod.7b00121. 1 H, 13C, and 2D NMR spectra of compounds 1−7 and DFT calculations of 1 and 4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel (J. Shin): 82 2 880 2484. Fax: 82 2 762 8322. E-mail: [email protected]. *Tel (K.-B. Oh): 82 2 880 4646. Fax: 82 2 873 2039. E-mail: [email protected]. ORCID

Dong-Chan Oh: 0000-0001-6405-5535 Jongheon Shin: 0000-0002-7536-8462 Author Contributions ∥

C.-K. Kim and R. Riswanto contributed equally to this study.

Notes

The authors declare no competing financial interest. 1582

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(14) Sakai, R.; Kohmoto, S.; Higa, T. Tetrahedron Lett. 1987, 28, 5493−5496. (15) El Sayed, K. A.; Kelly, M.; Kara, U. A. K.; Ang, K. K. H.; Katsuyama, I.; Dunbar, D. C.; Khan, A. A.; Hamann, M. T. J. Am. Chem. Soc. 2001, 123, 1804−1808. (16) Kobayashi, M.; Chen, Y.-J.; Aoki, S.; In, Y.; Ishida, T.; Kitagawa, I. Tetrahedron 1995, 51, 3727−3736. (17) Oh, K.-B.; Lee, J. H.; Chung, S.-C.; Shin, J.; Shin, H. J.; Kim, H.K.; Lee, H.-S. Bioorg. Med. Chem. Lett. 2008, 18, 104−108. (18) (a) Mosmann, T. J. Immunol. Methods 1983, 65, 55−63. (b) Ulukaya, E.; Ozdikicioglu, F.; Oral, A. Y.; Demirci, M. Toxicol. In Vitro 2008, 22, 232−239. (19) (a) Johansson, M.; Karlsson, L.; Wennergren, M.; Jansson, T.; Powell, T. L. J. Clin. Endocrinol. Metab. 2003, 88, 2831−2837. (b) Oh, K.-B.; Kim, S.-H.; Lee, J.; Cho, W.-J.; Lee, T.; Kim, S. J. Med. Chem. 2004, 47, 2418−2421. (c) Lee, H.-S.; Lee, T.-H.; Yang, S. H.; Shin, H. J.; Shin, J.; Oh, K.-B. Bioorg. Med. Chem. Lett. 2007, 17, 2483−2486.

ACKNOWLEDGMENTS We thank the Basic Science Research Institute in Daegu, Korea, for providing mass spectrometric data. This study was supported by the Medical Research Center (grant no. 20090083533) of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning, Korea.

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REFERENCES

(1) (a) Peng, J.; Rao, K. V.; Choo, Y. -M.; Hamann, M. T. Modern Alkaloids: Structure, Isolation, Synthesis and Biology; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008; p 189. (b) Ashok, P.; Ganguly, S.; Murugesan, S. Drug Discovery Today 2014, 19, 1781−1791. (c) Ashok, P.; Lathiya, H.; Murugesan, S. Eur. J. Med. Chem. 2015, 97, 928−936. (2) (a) Sakai, R.; Higa, T. J. Am. Chem. Soc. 1986, 108, 6404−6405. (b) Nakamura, H.; Deng, S.; Kobayashi, J.; Ohizumi, Y. Tetrahedron Lett. 1987, 28, 621−624. (3) (a) Rao, K. V.; Donia, M. S.; Peng, J.; Garcia-Palomero, E.; Alonso, D.; Martinez, A.; Medina, M.; Franzblau, S. G.; Tekwani, B. L.; Khan, S. I.; Wahyuono, S.; Willett, K. L.; Hamann, M. T. J. Nat. Prod. 2006, 69, 1034−1040. (b) Blunt, J. W.; Copp, B. R.; Keyzers, R. A.; Munro, M. H. G.; Prinsep, M. R. Nat. Prod. Rep. 2016, 33, 382−431 and earlier reports in the series. (4) (a) Yousaf, M.; Hammond, N. L.; Peng, J.; Wahyuono, S.; McIntosh, K. A.; Charman, W. N.; Mayer, A. M. S.; Hamann, M. T. J. Med. Chem. 2004, 47, 3512−3517. (b) Shilabin, A. G.; Kasanah, N.; Tekwani, B. L.; Hamann, M. T. J. Nat. Prod. 2008, 71, 1218−1221. (c) Guzmán, E. A.; Johnson, J. D.; Linley, P. A.; Gunasekera, S. E.; Wright, A. E. Invest. New Drugs 2011, 29, 777−785. (d) Radwan, M.; Hanora, A.; Khalifa, S.; Abou-El-Ela, S. H. Cell Cycle 2012, 11, 1765− 1772. (5) Peng, J.; Hu, J.-F.; Kazi, A. B.; Li, Z.; Avery, M.; Peraud, O.; Hill, R. T.; Franzblau, S. G.; Zhang, F.; Schinazi, R. F.; Wirtz, S. S.; Tharnish, P.; Kelly, M.; Wahyuono, S.; Hamann, M. T. J. Am. Chem. Soc. 2003, 125, 13382−13386. (6) (a) Winkler, J. D.; Axten, J. M. J. Am. Chem. Soc. 1998, 120, 6425−6426. (b) Winkler, J. D.; Londregan, A. T.; Hamann, M. T. Org. Lett. 2006, 8, 2591−2594. (c) Peng, J.; Kudrimoti, S.; Prasanna, S.; Odde, S.; Doerksen, R. J.; Pennaka, H. K.; Choo, Y.-M.; Rao, K. V.; Tekwani, B. L.; Madgula, V.; Khan, S. I.; Wang, B.; Mayer, A. M. S.; Jacob, M. R.; Tu, L. C.; Gertsch, J.; Hamann, M. T. J. Med. Chem. 2010, 53, 61−76. (d) Jakubec, P.; Hawkins, A.; Felzmann, W.; Dixon, D. J. J. Am. Chem. Soc. 2012, 134, 17482−17485. (7) El-Desoky, A. H.; Kato, H.; Eguchi, K.; Kawabata, T.; Fugiwara, Y.; Losung, F.; Mangindaan, R. E. P.; de Voogd, N. J.; Takeya, M.; Yokosawa, H.; Tsukamoto, S. J. Nat. Prod. 2014, 77, 1536−1540. (8) Furusato, A.; Kato, H.; Nehira, T.; Eguchi, K.; Kawabata, T.; Fugiwara, Y.; Losung, F.; Mangindaan, R. E. P.; de Voogd, N. J.; Takeya, M.; Yokosawa, H.; Tsukamoto, S. Org. Lett. 2014, 16, 3888− 3891. (9) (a) Li, S.; Chesnut, D. B. Magn. Reson. Chem. 1985, 23, 625−638. (b) Pérez-Castorena, A.-L.; Arciniegas, A.; Pérez, R.; Gutierrez, H.; Toscano, R. A.; Villasenor, J. L.; de Vivar, A. R. J. Nat. Prod. 1996, 62, 1039−1043. (c) Pretsch, E.; Bühlmann, P.; Badertscher, M. Structure Determination of Organic Compounds; Springer: Berlin Heidelberg, 2009; p 117. (10) Edrada, R. A.; Proksch, P.; Wray, V.; Witte, L.; Muller, W. E. G.; Van Soest, R. W. M. J. Nat. Prod. 1996, 59, 1056−1060. (11) Yamada, M.; Takahashi, Y.; Kubota, T.; Fromont, J.; Ishiyama, A.; Otoguro, K.; Yamada, H.; O̅ mura, S.; Kobayashi, J. Tetrahedron 2009, 65, 2313−2317. (12) (a) Watanabe, D.; Tsuda, M.; Kobayashi, J. J. Nat. Prod. 1998, 61, 689−692. (b) Samoylenko, V.; Khan, S. I.; Jacob, M. R.; Tekwani, B. L.; Walker, L. A.; Hufford, C. D.; Muhammad, I. Nat. Prod. Commun. 2009, 4, 185−192. (13) Rao, K. V.; Santarsiero, B. D.; Mesecar, A. D.; Schinazi, R. F.; Tekwani, B. L.; Hamann, M. T. J. Nat. Prod. 2003, 66, 823−828. 1583

DOI: 10.1021/acs.jnatprod.7b00121 J. Nat. Prod. 2017, 80, 1575−1583