Introduction

Marine sponges of the genus Plakortis are known to contain stable cycloperoxides, and most of their compounds possess a 1,2-dioxane ring, substituted with an acetate moiety at C-3 and three functionalities at C-4, C-6, and C-6. Since the first study to isolate plakortin [1] from the Caribbean sponge Plakortis halichondrioides, a number of compounds containing characteristic cycloperoxides have been identified, including plakortolide [2], plakortic acid [3], plakortenone [4], haterumadioxin [5], manadoperoxide [6], and plakortide [7]. These compounds were found to exhibit various biological activities, i.e., antibiotic [1], cytotoxic [2–5], and antimalarial [6, 8] activities, and enhanced the Ca2+-pumping activity of the cardiac sarcoplasmic reticulum [7]. In our screening for the antimicrobial activities of the marine organisms, we found that the sponge P. bergquistae, which was collected in Indonesia, exhibited antibacterial activity. We herein reported the isolation and structure elucidation of five new polyketide endoperoxides, manadodioxans A−E (1−5), from this sponge (Fig. 1).

Fig. 1
figure 1

Structures of 1−5

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Results and discussion

The marine sponge P. bergquistae (150 g, wet weight) was collected in Indonesia and immediately extracted with EtOH after collection. The extract was partitioned between EtOAc and water, and bioassay-guided purification from the EtOAc-soluble fraction afforded 1 (0.2 mg), 2 (0.4 mg), 3 (0.7 mg), 4 (1.3 mg), and 5 (1.8 mg).

Manadodioxan A (1) had the molecular formula C19H30O5, which was determined by high-resolution electrospray ionization mass spectrometry (HRESIMS) and 13C NMR (Table 1) spectrometries. The 1H NMR spectrum (Table 1) showed three triplet methyl signals at δ 0.87 (t, J = 7.7 Hz, H3-16), 0.90 (t, J = 7.4 Hz, H3-14), and 1.04 (t, J = 7.6 Hz, H3-18), two singlet methyl signals at δ 2.29 (H3-12) and 3.70 (1-OMe), an oxygen-bearing signal at δ 4.49 (m), three olefin signals at δ 5.93 (s, H-7), 6.16 (d, J = 16.2 Hz, H-10), and 7.05 (d, J = 16.2 Hz, H-9), and methylene/methine signals at δ 1−3. An analysis of 2D spectra, including COSY, HSQC, and HMBC spectra, revealed the presence of a 1,2-dioxane ring, substituted with four alkyl functionalities at C-3, C-4, C-6, and C-6 (Fig. 2). HMBC correlations from H2-2 and 1-OMe to C-1 showed that a methyl ester was attached to C-2. The connections of two ethyl groups at C-4 and C-6 were identified by HMBC correlations from H3-14 and H2-13 to C-4 and from H3-16 and H2-15 to C-6. Another functional group at C-6 contained an α,β,γ,δ-unsaturated ketone and a 9E configuration was indicated by the coupling constant, J 9,10 = 16.2 Hz. NOE correlations, H-7/H-9 and H-10/H3-18, indicated the geometry of double bond C-7/C-8. The relative configurations of C-3, C-4, and C-6 were established by NOE correlations, H2-2/H-5ax (δ 1.40), H-7/H-5 eq (δ 1.79), and H-7/H-4 (Fig. 3). Thus, the structure of 1 was determined.

Table 1 1H and 13C NMR data for compound 1
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Fig. 2
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COSY (bold lines) and key HMBC (arrows) correlations for 1 and 4

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Fig. 3
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Key NOE correlations for 1 and 4

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The molecular formula of manadodioxan B (2) was determined to be C20H36O6 based on HRESIMS. 1H and 13C NMR spectra (Table 2) showed that 2 was a congener of 1. An analysis of 2D spectra clearly revealed that the α,β,γ,δ-unsaturated ketone moiety in 1 was absent in 2. The terminal singlet methyl group (H3-12) in 1 was replaced with the triplet methyl group C-12 (δH 0.98 (t, J = 7.4 Hz); δC 10.4) in 2, which was successively connected to the methylene C-11 (δH 1.36 and 1.72; δC 25.5), hydroxyl-bearing carbon C-10 (δH 3.50; δC 73.2), methoxy-bearing carbon C-9 (δH 3.46 (d, J = 6.5 Hz); δC 89.8), and trisubstituted double bond C-8 (δC 140.9)/C-7 (δH 5.42 (s); δC 131.0), as confirmed by 2D spectra. HMBC correlations from H3-18 and H2-17 to C-8 showed that an ethyl group was attached to the olefin carbon C-8, which also exists in 1. NOE correlations determined relative configurations at C-3, C-4, and C-6, the same as 1. Relative configurations at C-9 and C-10 have not yet been determined. Manadodioxan C (3) had the molecular formula C21H38O6, as established by HRESIMS, indicating one more methylene unit in 3 than in 2. The 1H and 13C NMR spectra of 2 and 3 (Table 2) were very similar, and an analysis of 2D spectra indicated the presence of an ethoxy group (δH 4.15 and δC 60.7 (CH2); δH 1.24 and δC 14.2 (CH3)) placed at C-1 of 3 instead of the methoxy group in 2. NOE correlations, H-7/H-9, H-7/9-OMe, and H-10/H3-18, indicated the geometries of double bonds C-7/C-8 of 2 and 3.

Table 2 NMR data for 2 and 3 in CDCl3
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ESIMS of manadodioxan D (4) showed an ion peak at m/z 377.2310 [M + Na]+, and the molecular formula was determined to be C20H34O5 based on HRESIMS. An analysis of 2D spectra established the entire carbon framework possessing a 1,2-dioxane ring with four alkyl functionalities containing an α,β-unsaturated ketone (Table 3). Relative configurations at C-3, C-4, and C-6 were indicated by NOE correlations, H3-18/H-5eq (δ 1.50), H-17 (δ 1.99)/H-4ax, and H-2 (δ 3.00)/H-5ax (δ 1.23) (Fig. 3). The long alkyl chain at C-6 was oriented in the equatorial position and the functionalities at C-3 and C-4 were oriented in the axial and equatorial positions, respectively, in 4. Although the planar structure of 4 was identical to that of “compound 1”, which was isolated from a Jamaican Plakortis sponge [9], the carbon chemical shifts at C-7 and C-17 in CDCl3 were different; δC 36.3 and 24.8 in 4 and δC 32.2 and 29.8 in “compound 1”, respectively. Although the planar structure has only been reported for “compound 1” in the literature, the difference observed in carbon chemical shifts at C-7 and C-17 indicated that the long alkyl chain at C-6 may be oriented in the axial position in “compound 1”, while those at C-3 and C-4 were the same as those in 4. Manadodioxan E (5) had the molecular formula C20H36O4, and its 1H and 13C NMR chemical shifts (Table 3) were superimposable on those of plakortide G [7], except for the absence of the methoxy group that existed in plakortide G. Relative configurations at C-3, C-4, and C-6 of 5 were confirmed to be the same as those of 4, based on NOE correlations. Thus, the structure of 5 was determined to be a free acid of plakortide G.

Table 3 NMR data for 4 and 5 in CDCl3
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The antimicrobial activities of manadodioxans D (4) and E (5) were tested against bacteria, Escherichia coli and Bacillus cereus, a fungus, Candida albicans, and a yeast, Saccharomyces cerevisiae (Table 4) (the antimicrobial activities of 1−3 were not tested due to their limited amounts) using the paper disk method. Compound 5 more potently inhibited the growth of the Gram-negative bacterium, E. coli, than that of the Gram-negative bacterium, B. cereus, but was inactive for C. albicans and S. cerevisiae at 10 μg/disk. On the other hand, 4 was inactive for the four microorganisms tested in this study. The presence of the carbonyl group at C-13 in 4 may have sequestered antimicrobial activity.

Table 4 Growth inhibitory activities of 4 and 5 against Escherichia coli, Bacillus cereus, Candida albicans, and Saccharomyces cerevisiae
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Experimental

General

Optical rotations were recorded on a JASCO DIP-1000 polarimeter. IR spectra were recorded on a PerkinElmer Frontier FT-IR spectrometer. UV spectra were measured on a JASCO V-550 spectrophotometer. NMR spectra were recorded on a Bruker Avance 600, Bruker Avance 500, or JEOL JNM-ECX-400 NMR spectrometer in CDCl3. Chemical shifts were referenced to residual solvent peaks (δH 7.24 and δC 77.0). Mass spectra were measured on a Bruker BioTOF mass spectrometer.

Biological material

The marine sponge was collected at a depth of 10 m in North Sulawesi, Indonesia, in December 2006 and immediately soaked in EtOH. The sponge was identified as Plakortis bergquistae. A voucher specimen (RMNH POR 8528) has been deposited at the Naturalis Biodiversity Center, the Netherlands.

Extraction and isolation

The marine sponge (150 g, wet weight) was extracted with MeOH. The concentrated aqueous residue was successively extracted with EtOAc. The EtOAc fraction (1.1 g) was subjected to silica gel column chromatography with n-hexane/EtOAc to afford fraction A (130 mg) eluted with n-hexane/EtOAc (1:1) and fraction B (405 mg) eluted with n-hexane/EtOAc (9:1). Fraction A was purified by ODS column chromatography with MeOH/H2O to afford 4 (1.3 mg, 75 % MeOH-H2O) and 5 (1.8 mg, 90 % MeOH-H2O). Fraction B was purified by ODS and C30 HPLC to afford 1 (0.2 mg), 2 (0.4 mg), and 3 (0.7 mg).

Manadodioxan A (1): an amorphous solid. [α] 21D –100° (c = 0.22, MeOH). IR ν max (film) cm−1: 3453, 2926, 2855, 1738, 1667, 1592, 1460, 1363, 1257, 1164 cm−1. UV λ max (MeOH) nm (log ε): 204 (4.65), 278 (4.78). 1H (500 MHz, CDCl3) and 13C NMR (125 MHz, CDCl3) spectroscopic data, see Table 1. HRESIMS [M+Na]+ m/z 361.1997 (calcd for C19H30O5Na, 361.1989).

Manadodioxan B (2): an amorphous solid. [α] 21D –97° (c = 0.33, MeOH). IR ν max (film) cm−1: 3453, 2925, 1739 cm−1. UV λ max (MeOH) nm (log ε): 206 (4.60). 1H (500 MHz, CDCl3) and 13C NMR (125 MHz, CDCl3) spectroscopic data, see Table 2. Key NOE correlations: H2-2/H-5ax (δ 1.32), H-7/H-5eq (δ 1.75), and H-7/H-4. HRESITOFMS [M+Na]+ m/z 395.2399 (calcd for C20H36O6Na, 395.2408).

Manadodioxan C (3): an amorphous solid. [α] 21D –110° (c = 0.77, MeOH). IR ν max (film) cm−1: 3453, 2928, 2856, 1729, 1460, 1379, 1274, 1113 cm−1. UV λ max (MeOH) nm (log ε): 218 (5.34). 1H (600 MHz, CDCl3) and 13C NMR (150 MHz, CDCl3) spectroscopic data, see Table 2. Key NOE correlations: H2-2/H-5ax (δ 1.31), H-7/H-5eq (δ 1.75), and H-7/H-4. HRESIMS [M+Na]+ m/z 409.2557 (calcd for C21H38O6Na, 409.2564).

Manadodioxan D (4): an amorphous solid. [α] 21D +43° (c = 0.68, MeOH). IR ν max (film) cm−1: 3450, 2928, 1714, 1460 cm−1. UV λ max (MeOH) nm (log ε): 204 (4.53, sh), 224 (4.68). 1H (400 MHz, CDCl3) and 13C NMR (100 MHz, CDCl3) spectroscopic data, see Table 3. HRESITOFMS m/z 377.2310 [M+Na]+ (calcd for C20H34O5Na, 377.2302).

Manadodioxan E (5): an amorphous solid. [α] 21D +54° (c = 2.2, MeOH). IR ν max (film) cm−1: 3352, 2926, 2841, 1653, 1450, 1114, 1016, 662. UV λ max (MeOH) nm (log ε): 204 (4.27). 1H (600 MHz, CDCl3) and 13C NMR (150 MHz, CDCl3) spectroscopic data, see Table 3. NOE correlations: H3-18/H-5eq (δ 1.50), H-17 (δ 2.01)/H-4ax, and H-2 (δ 3.03)/H-5ax (δ 1.22). HRESITOFMS m/z: 363.2460 [M+Na]+ (calcd for C20H36O4Na, 363.2509).

Antifungal assay

Growth inhibitory activity was determined by the paper disk method. Paper disks (ϕ 6 mm), impregnated with 10 μg of 4 or 5, were incubated on agar plates containing E. coli or B. cereus at 37 °C and C. albicans or S. cerevisiae at 27 °C overnight.