Bromopyrrole Alkaloids from a Marine Sponge Agelas sp.

Abstract

Five new bromopyrrole alkaloids, 2-bromokeramadine (1), 2-bromo-9,10-dihydrokeramadine (2), tauroacidins C (3) and D (4), and mukanadin G (5), were isolated from an Okinawan marine sponge Agelas sp. The structures of 1–5 were elucidated on the basis of spectroscopic data and conformational analysis. Mukanadin G (5) has a tricyclic skeleton consisting of a fused tetrahydrobenzaminoimidazole and 2,5-dioxopyrrolidine moieties. Antimicrobial activities of 1–3, and 5 as well as three related known bromopyrrole alkaloids, keramadine (6), tauroacidin A (7), and taurodispacamide A (8) were evaluated.

Marine sponges have been recognized as a rich source of bioactive metabolites with unique chemical structures.¹⁾ Among them, bromopyrrole alkaloids are known to be one of the most common metabolites contained in marine sponges. These alkaloids have attracted widespread interest due to their fascinating chemical structures with high N to C ratio (ca. 1 : 2).^2–4) Bromopyrrole alkaloids have been reported to show various bioactivities such as immunosuppressive, anticancer, and antimicrobial activities.^2–4) In our continuing search for metabolites from Okinawan marine sponges, we have previously reported the isolation of a series of bromopyrrole alkaloids from Agelas spp.^5–11) Recently, we have also reported monomeric bromopyrrole alkaloids, nagelamides U–W,¹²⁾ and dimeric bromopyrrole alkaloids, nagelamides X–Z,¹³⁾ from the extracts of a sponge Agelas sp. (SS-162). Nagelamides U–Z exhibited antifungal activity against Candida albicans.^12,13) Further investigation of the extracts resulted in the isolation of five new bromopyrrole alkaloids, 2-bromokeramadine (1), 2-bromo-9,10-dihydrokeramadine (2), tauroacidins C (3) and D (4), and mukanadin G (5) together with three known bromopyrrole alkaloids, keramadine (6), tauroacidin A (7), and taurodispacamide A (8). In this paper, we describe the isolation and structure elucidation of 1–5 as well as antimicrobial activities of 1–3, and 5–8.

The sponge Agelas sp. (SS-162, 3.9 kg, wet weight) collected at Kerama islands, Okinawa, was extracted with MeOH. Chromatographic separations of the extract resulted in the isolation of 2-bromokeramadine (1, 0.000051%, wet weight), 2-bromo-9,10-dihydrokeramadine (2, 0.00016%), tauroacidin C (3, 0.000033%), tauroacidin D (4, 0.000024%), and mukanadin G (5, 0.00052%). In the purification process, three known bromopyrrole alkaloids, keramadine (6),¹⁴⁾ tauroacidin A (7),¹⁵⁾ and taurodispacamide A (8),¹⁶⁾ were isolated and identified by comparison of their physicochemical data with the reported data.

2-Bromokeramadine (1) was isolated as a pale yellow amorphous solid, and the electrospray ionization mass spectroscopy (ESI-MS) showed the pseudomolecular ion peaks at m/z 402, 404, and 406 (1 : 2 : 1), indicating the existence of two bromine atoms in the molecule. The high-resolution (HR)-ESI-MS revealed the molecular formula of 1 to be C₁₂H₁₄N₅OBr₂ (m/z 401.95608 [M]⁺, Δ+0.12 mmu). ¹H- and ¹³C-NMR data for 1 (Table 1) were reminiscent of those for keramadine (6)¹⁴⁾ (Chart 1), a bromopyrrole alkaloid with a monobromopyrrole and an N-methylated aminoimidazole moieties. However, the signal of one aromatic quaternary carbon (C-2) in 1 was discerned in place of that of the sp ² methine (CH-2) in 6. Given the molecular formula, 1 was presumed to be a 2,3-dibromo form of 6. This was confirmed by analysis of the 2 dimension (2D) NMR spectra including the ¹H–¹H correlation spectroscopy (¹H–¹H COSY), hetero nuclear multiple bond coherence (HMBC), and rotating Overhauser enhancement and exchange spectroscopy (ROESY) spectra (Fig. 1). Thus, the structure of 1 was elucidated as 2-bromokeramadine (Chart 1).

Table 1. ¹H- and ¹³C-NMR Data for 2-Bromokeramadine (1), 2-Bromo-9,10-dihydrokeramadine (2), and Tauroacidins C (3) and D (4) in DMSO-d₆

Position	1		2		3		4
	δC	δH (J in Hz)	δC	δH (J in Hz)	δC	δH (J in Hz)	δC	δH (J in Hz)
1	–	12.72 (1H, br s)	–	12.70 (1H, br s)b)	–	12.70 (1H, br s)	–	12.71 (1H, br s)
2	104.8	–	104.5	–	104.8	–	105.1	–
3	97.9	–	97.9	–	97.9	–	98.1	–
4	112.7	6.91 (1H, br s)	112.7	6.92 (1H, br s)	113.1	6.96 (1H, d, 2.5)	113.3	6.96 (1H, br s)
5	127.9	–	128.3	–	127.9	–	127.6	–
6	158.9	–	159.1	–	159.1	–	159.2	–
7	–	8.49 (1H, t, 5.6)	–	8.28 (1H, t, 6.0)	–	8.23 (1H, t, 5.8)	–	8.47 (1H, t, 5.8)
8	37.9	4.00 (2H, t, 5.6)	37.8	3.25 (2H, q, 6.0)	41.2	3.41 (2H, m)a)	48.4	4.09, 4.04 (1H each, dd, 18.3, 5.8)
9	133.1	5.83 (1H, dt, 11.5, 5.6)	26.6	1.73 (2H, m)	76.1	4.27 (1H, dt, 7.7, 5.6)	203.5	–
10	114.0	6.25 (1H, d, 11.5)	20.5	2.46 (2H, t, 7.3)	113.3	6.04 (1H, d, 7.7)	41.0	3.18 (1H, dd, 18.1, 2.8)
								3.00 (1H, dd, 18.1, 7.8)
11	123.7	–	127.8	–	133.4	–	57.2	4.76 (1H, dd, 7.8, 2.8)
12	–	–	–	–	–	11.10 (1H, br s)	–	8.91 (1H, br s)
13	146.6	–	146.8	–	166.5	–	169.0	–
13-NH	–	7.75 (2H, br s)	–	7.68 (2H, br s)	–	8.49 (1H, br s)	–	7.96 (1H, br s)
14	–	12.51 (1H, br s)	–	12.43 (1H, br s)b)	–	9.38 (1H, br s)	–	8.73 (1H, br s)
15	112.0	7.09 (1H, s)	108.4	6.71 (1H, s)	167.5	–	180.2	–
1'					–	9.73 (1H, t, 5.6)	–	9.22 (1H, t, 5.8)
2'					40.0	3.66 (2H, m)	39.8c)	3.58 (2H, m)
3'					49.2	2.76 (2H, t, 7.2)	49.3	2.72 (2H, m)
12-NMe	29.3	3.38 (3H, s)a)	29.0	3.33 (3H, s)
9-OMe					69.8	3.26 (3H, s)

a) Overlapped with signal of HOD. b) Interchangeable. c) Overlapped with signal of DMSO-d₆.

Chart 1. Structures of Five New (1–5) and Three Known (6–8) Bromopyrrole Alkaloids Isolated from a Marine Sponge Agelas sp. (SS-162)

Fig. 1. Selected 2D NMR Correlations for 2-Bromokeramadine (1)

The molecular formula of 2-bromo-9,10-dihydrokeramadine (2), C₁₂H₁₆N₅OBr₂, was established by the HR-ESI-MS (m/z 403.97185 [M]⁺, Δ+0.24 mmu). Inspection of the ¹H- and ¹³C-NMR spectra (Table 1) suggested that 2 was structurally related to 1, whereas the signals of two sp³ methylenes (CH₂-9 and CH₂-10) were observed in 2 in place of the resonances of the Z-olefin in 1. The connectivities of N-7 to C-11 were revealed by ¹H–¹H COSY cross-peaks of 7-NH/H₂-8, H₂-8/H₂-9, and H₂-9/H₂-10 and HMBC correlations for H₂-9/C-11 and H₂-10/C-15 (Fig. 2). Therefore, the structure of 2 was assigned as 2-bromo-9,10-dihydrokeramadine (Chart 1).

Fig. 2. Selected 2D NMR Correlations for 2-Bromo-9,10-dihydrokeramadine (2)

Tauroacidin C (3) was obtained as a pale yellow amorphous solid. The HR-ESI-MS suggested the molecular formula of 3 to be C₁₄H₁₈N₆O₅Br₂S (m/z 562.93298 [M+Na]⁺, Δ+1.15 mmu). The ¹H- and ¹³C-NMR spectra of 3 (Table 1) resembled to tauroacidin A (7)¹⁵⁾ (Chart 1) except for the presence of the signal due to a methoxy group in 3. An HMBC correlation for the methoxy protons to C-9 suggested that the methoxy group was attached to C-9 (Fig. 3). Tauroacidin C (3) was optically inactive, although 3 has a chiral center (C-9). The optical resolution of 3 on chiral HPLC resulted in the separation of enantiomers, the ratio of which was approximately 1 : 1. Therefore, 3 was concluded to be a racemate. Related bromopyrrole alkaloids, tauroacidins A (7) and B, also reported as racemates.¹⁵⁾ Thus, the structure of 3 was assigned as shown in Chart 1.

Fig. 3. Selected 2D NMR Correlations for Tauroacidin C (3)

The molecular formula of tauroacidin D (4) was elucidated to be C₁₃H₁₆N₆O₅Br₂S by the HR-ESI-MS (m/z 548.91647 [M+Na]⁺, Δ+0.29 mmu). The 1D NMR spectra of 4 (Table 1) implied that 4 is also an analogue of 7. The presence of a ketone carbonyl group in 4 was revealed by a carbon signal at δ_C 203.5 and an IR absorption at 1710 cm⁻¹. The carbonyl carbon was assigned as C-9 on the basis of 2D NMR analysis (Fig. 4). The 2D NMR analysis also indicated that C-10 and C-11 in 4 were saturated, whereas those in 7 were unsaturated. The optical resolution using chiral HPLC suggested 4 to be a racemate. Therefore, the structure of tauroacidin D (4) was elucidated as shown in Chart 1.

Fig. 4. Selected 2D NMR Correlations for Tauroacidin D (4)

Mukanadin G (5) was obtained as a pale yellow amorphous solid. The molecular formula of 5, C₁₅H₁₅N₆O₃Br₂, was established by the HR-ESI-MS (m/z 484.95637 [M]⁺, Δ−0.32 mmu). The presence of carbonyl functionalities was indicated by IR absorptions at 1773, 1718, and 1687 cm⁻¹. The 1D NMR spectra of 5 (Table 2) implied that 5 is a bromopyrrole alkaloid possessing a dibromopyrrole amide (N-1–N-7) and aminoimidazole (C-11–C-15) moieties. Interpretation of the ¹H–¹H COSY spectrum of 5 suggested the connectivities of the dibromopyrrole amide moiety (N-7) to C-10, C-9 to C-20, and C-16 to C-20. HMBC correlations for H-16 with C-11 and C-15, and for H₂-10 with C-11 and C-15 revealed that C-11 and C-15 were attached to C-10 and C-16, respectively, forming a tetrahydrobenzaminoimidazole moiety. In addition, the existence of a 2,5-dioxopyrrolidine ring (C-16–C-20) fused to the tetrahydrobenzaminoimidazole moiety was deduced by HMBC correlations for H-9 to C-19 and 18-NH to C-16, C-17, C-19, and C-20, taking the chemical shifts of C-17 (δ_C 176.1) and C-19 (δ_C 178.5) into consideration. Therefore, the gross structure of mukanadin G (5) was assigned as shown in Fig. 5.

Table 2. ¹H- and ¹³C-NMR Data for Mukanadin G (5) in DMSO-d₆

Position	δC	δH (J in Hz)
1	–	12.70 (1H, br d, 2.3)
2	104.6	–
3	97.8	–
4	112.8	6.94 (1H, d, 2.3)
5	128.1	–
6	159.1	–
7	–	8.23 (1H, t, 6.0)
8a	40.6	3.85 (1H, m)
8b		3.55 (1H, dt, 13.7, 6.0)
9	34.6	2.18 (1H, m)
10a	21.2	2.53 (1H, d, 12.6)
10b		2.12 (1H, ddd, 13.8, 12.6, 2.3)
11	121.5	–
12	–	12.33 (1H, br s)a)
13	147.6	–
13-NH2	–	7.42 (2H, br s)
14	–	12.00 (1H, br s)a)
15	115.5	–
16	40.3	3.92 (1H, br d, 8.0)
17	176.1	–
18	–	11.31 (1H, br s)
19	178.5	–
20	42.6	3.62 (1H, dd, 8.0, 4.0)

a) Interchangeable.

Fig. 5. Selected 2D NMR Correlations for Mukanadin G (5)

The relative stereochemistry of mukanadin G (5) was assigned as follows. Since the syn relationship for H-16/H-20 was implied by a steric restriction of the cyclohexene ring, stereochemical analysis was carried out on two diastereomers (H-9/H-20-syn and H-9/H-20-anti). Conformational searches for each diastereomer on the MacroModel program (MMFFs force field) gave the most stable conformers 5a (syn) and 5b (anti) (Fig. 6). Comparison of an experimental value of ³J_H-9/H-10b (13.8 Hz) in 5 with calculated values (11.8 Hz in 5a and 5.6 Hz in 5b) implied the syn relationship for H-9/H-20 as well as the pseudo-axial orientation for H-9. This was confirmed by a correlation for H-9/H-16 observed in the ROESY spectrum of 5. Thus, the relative stereochemistry of mukanadin G (5) was assigned as shown. Similarly to 3 and 4, mukanadin G (5) was concluded to be a racemate on the basis of chiral HPLC analysis. This is the first report of the isolation of 5 from natural source, whereas Lindel and colleagues reported the synthesis of 5 as a Diels–Alder adduct of oroidin and maleimide in the presence of Y(OTf)₃.¹⁷⁾

Fig. 6. The Most Stable Conformers of Possible Diastereomers 5a (H-9/H-20-syn) and 5b (H-9/H-20-anti) of Mukanadin G (5) (N-1–N-7 Were Not Shown)

In the course of our search for antimicrobial natural products from marine organisms,^{12,13,18–21)} antimicrobial activities for four new (1–3 and 5) and three known (6–8) bromopyrrole alkaloids against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Micrococcus luteus, Aspergillus niger, Trichophyton mentagrophytes, Candida albicans, and Cryptococcus neoformans were evaluated (Table 3). As a result, mukanadine G (5) showed moderate antifungal activity against C. albicans and C. neoformans (IC₅₀ 16 and 8.0 µg/mL, respectively), while keramadine (6) and tauroacidin A (7) exhibited antibacterial activity against M. luteus (MIC 4.0 and 8.0 µg/mL, respectively).

Table 3. Antimicrobial Activities of 1–3 and 5–8

Strain	1	2	3	5	6	7	8
Escherichia colia)	>32	>32	>32	>32	>32	>32	>32
Staphylococcus aureusa)	32	>32	>32	>32	>32	>32	>32
Bacillus subtilisa)	>32	>32	>32	>32	>32	>32	>32
Micrococcus luteusa)	>32	>32	>32	>32	4.0	8.0	>32
Aspergillus nigerb)	>32	>32	>32	32	>32	>32	>32
Trichophyton mentagrophytesb)	>32	32	>32	>32	>32	>32	>32
Candida albicansb)	>32	32	>32	16	>32	>32	>32
Cryptococcus neoformansb)	>32	>32	>32	8.0	>32	>32	>32

a) MIC value (µg/mL). b) IC₅₀ value (µg/mL).

Experimental

General

Optical rotations and IR spectra were recorded on a JASCO P-1030 digital polarimeter and a JASCO FT/IR-230 spectrophotometer, respectively. UV spectra were recorded using a Shimazu UV-1600PC spectrophotometer. NMR spectra were measured by a Bruker AMX-600 spectrometer and a JEOL ECA 500 spectrometer. The resonances of CHD₂SOCD₃ (δ_H 2.49) and DMSO-d₆ (δ_C 39.5) were used as internal references for ¹H- and ¹³C-NMR chemical shifts, respectively. HR-ESI-MS spectra were recorded on a Thermo Scientific Exactive spectrometer.

Sponge Description

The sponge Agelas sp. (Order Agelasida, Family Agelasidae) (SS-162) collected at Kerama Islands, Okinawa, was kept frozen until used. The sponge has a smooth surface. Sponges are firm, springy and compressible. Skeleton is dense reticulate fibre skeleton with grainy texture. Primary fibres cored and echinated by verticillate spined acanthostyles. Spicules are verticillate, regularly spined acanthostyles, 210×12 µm, some oxeote modifications and thin forms occur. The voucher specimen is deposited at the Graduate School of Pharmaceutical Sciences, Hokkaido University.

Extraction and Isolation

The sponge Agelas sp. (SS-162, 3.9 kg, wet weight) was extracted with MeOH (5 L×3) to give the extract (256 g). A part (135 g) of the extract was partitioned successively with EtOAc (750 mL×3), n-BuOH (750 mL×3), and water (750 mL). The EtOAc-soluble materials were partitioned between n-hexane (500 mL×3) and 10% MeOH aq. (500 mL) to yield 10% MeOH aq.-soluble materials (34.5 g). The 10% MeOH aq.-soluble materials were subjected to a silica gel column (CHCl₃/MeOH/AcOH, 80 : 20 : 2→0 : 100 : 2) to give seven fractions (frs. 1–7). Fractionation of fr. 4 on an ODS column (MeOH/H₂O/TFA, 30 : 70 : 0.1→100 : 0 : 0.1) gave nine fractions (frs. 4.1–4.9). Fraction 4.4 was loaded on ODS HPLC (YMC ODS-AQ, 20×250 mm, flow rate 7.0 mL/min, UV detection at 254 nm, eluent MeCN/H₂O/TFA, 30 : 70 : 0.1), and then purified using ODS HPLC (YMC Hydrosphere C18, 10×250 mm, 2.5 mL/min, 254 nm, MeCN/H₂O/TFA, 22 : 78 : 0.1) to afford mukanadin G (5, 20.1 mg, 0.00052%), 2-bromokeramadine (1, 2.0 mg, 0.000051%), and 2-bromo-9,10-dihydrokeramadine (2, 6.3 mg, 0.00016%). The n-BuOH-soluble materials (7.14 g) were separated by a Toyopearl HW-40 column (MeOH/H₂O/TFA, 30 : 70 : 0.1→100 : 0 : 0.1) and an ODS column (MeOH/H₂O/TFA, 10 : 90 : 0.1→100 : 0 : 0.1) to afford a fraction containing bromopyrrole alkaloids. The fraction was purified by ODS HPLC (YMC ODS-AQ, 20×250 mm, 5.0 mL/min, 254 nm, MeOH/H₂O/TFA, 40 : 60 : 0.1) and HILIC HPLC (Cosmosil HILIC, 10×250 mm, 3.5 mL/min, 254 nm, MeCN/H₂O, 89 : 11) to give tauroacidin C (3, 1.3 mg, 0.000036%). During the purification process, keramadine (6, 20.0 mg, 0.00051%), tauroacidin A (7, 8.8 mg, 0.00023%), and taurodispacamide A (8, 12.5 mg, 0.00032%) were isolated. Residual part (121 g) of the MeOH ext. was partitioned with n-hexane and 90% MeOH aq. The 90% MeOH aq.-soluble materials were partitioned between n-BuOH and H₂O. The organic layer was applied to a silica gel column (CHCl₃/MeOH/AcOH, 80 : 20 : 2→0 : 100 : 2) to give seven fractions (frs. 1′–7′). Fraction 5′ was separated by a Toyoperal HW-40 column (MeOH/H₂O/TFA, 30 : 70 : 0.1→100 : 0 : 0.1) to afford nine fractions (frs. 5.′1–5.′9). Fraction 5.′4 was subjected to an ODS column (MeOH/H₂O/TFA, 30 : 70 : 0.1→100 : 0 : 0.1), and then purified using ODS HPLC (YMC ODS-AQ, 20×250 mm, 6.0 mL/min, 254 nm, MeCN/H₂O/TFA, 25 : 75 : 0.1) and HILIC HPLC (Cosmosil HILIC, 10×250 mm, 3.5 mL/min, 254 nm, MeCN/H₂O, 85 : 15) to isolate tauroacidin D (4, 1.0 mg, 0.000024%).

2-Bromokeramadine (1): Pale yellow amorphous solid, UV (MeOH) λ_max 278 (ε 10700) nm, IR (KBr) ν_max 3120 and 1672 cm⁻¹, ¹H- and ¹³C-NMR data (Table 1), ESI-MS: m/z 402, 404, and 406 (1 : 2 : 1) [M]⁺, HR-ESI-MS: m/z 401.95608 [M]⁺ (Calcd for C₁₂H₁₄N₅O⁷⁹Br₂, 401.95596).

2-Bromo-9,10-dihydrokeramadine (2): Pale yellow amorphous solid, UV (MeOH) λ_max 209 (ε 12200, sh) and 275 (11500) nm, IR (KBr) ν_max 3142 and 1686 cm⁻¹, ¹H- and ¹³C-NMR data (Table 1), ESI-MS: m/z 404, 406, and 408 (1 : 2 : 1) [M]⁺, HR-ESI-MS: m/z 403.97185 [M]⁺ (Calcd for C₁₂H₁₆N₅O⁷⁹Br₂, 403.97161).

Tauroacidin C (3): Pale yellow amorphous solid, [α]_D²¹ ≈0 (c 0.15, MeOH), UV (MeOH) λ_max 267 (ε 9200) and 311 (3000, sh) nm, IR (KBr) ν_max 3176 and 1686 cm⁻¹, ¹H- and ¹³C-NMR data (Table 1), ESI-MS: m/z 563, 565, and 567 (1 : 2 : 1) [M+Na]⁺, HR-ESI-MS: m/z 562.93298 [M+Na]⁺ (Calcd for C₁₄H₁₈N₆O₅⁷⁹Br₂SNa, 562.93183).

Tauroacidin D (4): Colorless amorphous solid, [α]_D²⁴ ≈0 (c 0.025, MeOH), UV (MeOH) λ_max 211 (ε 12700, sh), 246 (11300), and 277 (8700) nm, IR (KBr) ν_max 3273, 1710, and 1634 cm⁻¹, ¹H- and ¹³C-NMR data (Table 1), ESI-MS: m/z 549, 551, and 553 (1 : 2 : 1) [M+Na]⁺, HR-ESI-MS: m/z 548.91647 [M+Na]⁺ (Calcd for C₁₃H₁₆N₆O₅⁷⁹Br₂SNa, 548.91618).

Mukanadin G (5): Pale yellow amorphous solid, [α]_D²² ≈0 (c 0.25, MeOH), UV (MeOH) λ_max 275 (ε 9700) nm, (KBr) ν_max 3305, 1773, 1718, and 1687 cm⁻¹, ¹H- and ¹³C-NMR data (Table 2), ESI-MS: m/z 485, 487, and 489 (1 : 2 : 1) [M]⁺, HR-ESI-MS: m/z 484.95637 [M]⁺ (Calcd for C₁₅H₁₅N₆O₃⁷⁹Br₂, 484.95669).

Optical Resolution of Tauroacidins C (3) and D (4) and Mukanadin G (5)

Racemic forms of 3–5 were analyzed using chiral HPLC (CHIRALPAK ZWIX (+), DAICEL Corp., 4.0×250 mm, flow rate 0.3 mL/min UV detection 254 nm) with eluent MeOH/H₂O (98 : 2 with 50 mM formic acid and 25 mM diethylamine) to separate each enantiomer (t_R 31.2 and 32.4 min for 3, 39.0 and 47.5 min for 4, 28.0 and 29.7 min for 5).

Antimicrobial Assay

Antimicrobial assay against Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Micrococcus luteus, Aspergillus niger, Candida albicans, Cryptococcus neoformans, and Trichophyton mentagrophytes was carried out as previously described.²²⁾

Acknowledgment

We thank S. Oka and A. Tokumitsu, Equipment Management Center, Hokkaido University, for measurements of MS spectra, and Z. Nagahama for help with sponge collection. This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

References

• 1) Blunt J. W., Copp B. R., Keyzers R. A., Munro M. H. G., Prinsep M. R., Nat. Prod. Rep., 30, 237–323 (2013).
• 2) Aiello A., Fattorusso E., Menna M., Taglialatela-Scafati O., “Modern Alkaloids: Structure, Isolation, Synthesis and Biology,” ed. by Fattorusso E., Taglialatela-Scafati O., Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim, 2008, pp. 271–304.
• 3) Forte B., Malgesini B., Piutti C., Quartieri F., Scolaro A., Papeo G., Mar. Drugs, 7, 705–753 (2009).
• 4) Al-Mourabit A., Zancanella M. A., Tilvi S., Romo D., Nat. Prod. Rep., 28, 1229–1260 (2011).
• 5) Araki A., Kubota T., Aoyama K., Mikami Y., Fromont J., Kobayashi J., Org. Lett., 11, 1785–1788 (2009).
• 6) Yasuda T., Araki A., Kubota T., Ito J., Mikami Y., Fromont J., Kobayashi J., J. Nat. Prod., 72, 488–491 (2009).
• 7) Kubota T., Araki A., Ito J., Mikami Y., Fromont J., Kobayashi J., Tetrahedron, 64, 10810–10813 (2008).
• 8) Araki A., Kubota T., Tsuda M., Mikami Y., Fromont J., Kobayashi J., Org. Lett., 10, 2099–2102 (2008).
• 9) Araki A., Tsuda M., Kubota T., Mikami Y., Fromont J., Kobayashi J., Org. Lett., 9, 2369–2371 (2007).
• 10) Tsuda M., Yasuda T., Fukushi E., Kawabata J., Sekiguchi M., Fromont J., Kobayashi J., Org. Lett., 8, 4235–4238 (2006).
• 11) Endo T., Tsuda M., Okada T., Mitsuhashi S., Shima H., Kikuchi K., Mikami Y., Fromont J., Kobayashi J., J. Nat. Prod., 67, 1262–1267 (2004).
• 12) Tanaka N., Kusama T., Takahashi-Nakaguchi A., Gonoi T., Fromont J., Kobayashi J., Tetrahedron Lett., 54, 3794–3796 (2013).
• 13) Tanaka N., Kusama T., Takahashi-Nakaguchi A., Gonoi T., Fromont J., Kobayashi J., Org. Lett., 15, 3262–3265 (2013).
• 14) Nakamura H., Ohizumi Y., Kobayashi J., Hirata Y., Tetrahedron Lett., 25, 2475–2478 (1984).
• 15) Kobayashi J., Inaba K., Tsuda M., Tetrahedron, 53, 16679–16682 (1997).
• 16) Fattorusso E., Taglialatela-Scafati O., Tetrahedron Lett., 41, 9917–9922 (2000).
• 17) Pöverlein C., Breckle G., Lindel T., Org. Lett., 8, 819–821 (2006).
• 18) Takahashi Y., Tanaka N., Kubota T., Ishiyama H., Shibazaki A., Gonoi T., Fromont J., Kobayashi J., Tetrahedron, 68, 8545–8550 (2012).
• 19) Tanaka N., Asai M., Takahashi-Nakaguchi A., Gonoi T., Fromont J., Kobayashi J., Org. Lett., 15, 2518–2521 (2013).
• 20) Tanaka N., Momose R., Takahashi Y., Kubota T., Takahashi-Nakaguchi A., Gonoi T., Fromont J., Kobayashi J., Tetrahedron Lett., 54, 4038–4040 (2013).
• 21) Tanaka N., Momose R., Takahashi-Nakaguchi A., Gonoi T., Fromont J., Kobayashi J., Tetrahedron, 70, 832–837 (2014).
• 22) Nagai H., Mikami Y., Yazawa K., Gonoi T., Yasumoto T., J. Antibiot., 46, 520–522 (1993).