Krempfielins Q and R, Two New Eunicellin-Based Diterpenoids from the Soft Coral Cladiella krempfi

Abstract

Two new eunicellin-based diterpenoids, krempfielins Q and R (1 and 2), and one known compound cladieunicellin K (3) have been isolated from a Formosan soft coral Cladiella krempfi. The structures of these two new metabolites were elucidated by extensive spectroscopic analysis. Anti-inflammatory activity of new metabolites to inhibit the superoxide anion generation and elastase release in N-formyl-methionyl-leucyl phenylalanine/cytochalasin B (FMLP/CB)-induced human neutrophil cells and cytotoxicity of both new compounds toward five cancer cell lines were reported.

1. Introduction

Soft corals of the genus Cladiella have been known to be rich sources of eunicellin-type metabolites and several bioactivities of these compounds have been studied [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Our previous studies on the soft coral Cladiella krempfi have resulted in the isolation of a series of new eunicellin-based diterpenoids, krempfielins A–P [15,16,17,18]. Our contineous investigation on the chemical constituents of soft coral C. krempfi has afforded two new compounds, krempfielins Q and R (1 and 2 in Chart 1), and one minor known compound cladieunicellin K (3) [11]. The molecular structures of 1 and 2, including the relative configurations, were established by the detailed spectroscopic analysis and by comparison with related physical and spectral data of known compound, krempfielin E (4) [16]. For the anti-inflammatory activity of these two new compounds to inhibit the superoxide anion generation and elastase release in N-formyl-methionyl-leucyl phenylalanine/cytochalasin B (FMLP/CB)-induced human neutrophils and the cytotoxicity of them against five human cell lines, human T cell lymphoblast-like cell line (CCRF-CEM), human erythromyeloblastoid leukemia (K562), human acute lymphoblastic leukemia cell line (Molt 4), human ductal breast epithelial tumor cell line (T47D) and human colorectal adenocarcinoma cell line (DLD-1) were also evaluated.

Chart 1. Structures of metabolites 1–4.

Chart 1. Structures of metabolites 1–4.

2. Results and Discussion

Krempfielin Q (1) showed the molecular ion peak [M + Na]⁺ at m/z 505.2779 in the HRESIMS (Figure S1) and established a molecular formula of C₂₆H₄₂O₈, implying six degrees of unsaturation. The IR absorption bands at ν_max 3445 and 1733 cm⁻¹ revealed the presence of hydroxy and ester carbonyl functionalities, respectively. Its ¹³C NMR spectrum (Figure S2) showed signals of 26 carbons (Table 1) which were assigned by the assistance of the distortionless enhancement by polarization transfer (DEPT) spectrum to five methyls (including one acetate methyl δ_C 21.1), seven sp³ methylenes, one sp² methylene, eight sp³ methines (including four oxymethines), two sp³ and three sp² quaternary carbons (including two ester carbonyls). The NMR spectroscopic data of 1 (Figures S2,S3 and Table 1) displayed signals for 1,1-disubstituted double bond (δ_C 148.4 C, 110.7 CH₂; δ_H 4.91 and 4.81 s). Two ester carbonyls (δ_C 172.2 and 171.2) were assigned from the ¹³C NMR spectrum and their signals were correlated with the methylene protons (δ_H 2.33, 2H, m) of an n-butyrate and protons of an acetate methyl (δ_H 2.08 s, 3H), respectively. Therefore, the remaining three degrees of unsaturation identified 1 as a tricyclic molecule. The ¹H-¹H correlation spectroscopy (COSY) and heteronuclear multiple bond correlation (HMBC) correlations (Figure 1) were further used for establishing the molecular skeleton of 1. It was found that the COSY experiment showed the presence of three isolated consecutive proton spin systems. These evidences and the analysis of HMBC spectrum (Figure 1) suggested that 1 is an eunicellin-based diterpenoid. Furthermore, the acetoxy group attaching at C-19 was confirmed by the HMBC correlations from oxymethylene (δ_H 3.96 (H₂-19)) and acetate methyl protons (δ_H 2.08) to the ester carbonyl carbon appearing at δ 171.2 (C). Thus, the remaining n-butyryloxy group showed to be positioned at C-3, which was confirmed by an oxygen-bearing quaternary carbon resonating at δ 86.0 ppm. On the basis of above analysis, the planar structure of 1 was established.

Figure 1. Selected COSY (▬) and HMBC (→) correlations of 1 and 2.

Figure 1. Selected COSY (▬) and HMBC (→) correlations of 1 and 2.

The relative configuration of 1 was deduced by the analysis of nuclear Overhauser effect (NOE) correlations, as shown in Figure 2. The observation of the NOE correlations of H-1 with H-10 and H₃-20 suggested that these protons had the same orientation and were assumed to be β-oriented. The NOE interactions found between the oxymethine proton H-8 with H-10 and H₃-16 assigned the α-orientation of the two hydroxy groups positioned at C-7 and C-8. The NOE correlations of H-2 with both H-14 and H₃-15, but not with H-1 and H-10; H-14 with both H-9 and H₂-19; and H-5α (δ_H 1.62) with both H-6 and H₃-15, permitted that H-2, H-6, H-9, H-14, and H₃-15 were assigned to be α-oriented. Furthermore, the configuration of C-18 was suggested to be R * on the basis of NOE correlations of H-1 with H₃-20, H-14 with H₂-19, and H-2 with H-18. The relative configuration of 1 was thus established. Comparison of the ¹H and ¹³C NMR spectroscopic data of 1 with those of its 7,16-dehydration derivative, krempfielin E (4) [16], further confirmed the structure of 1.

Table 1. ¹³C and ¹H NMR data for compounds 1–2.

C	1 a		2 b
	δC	δH	δC	δH
1	44.9 (CH) c	2.27 m d	43.5 (CH)	2.35 m
2	92.3 (CH)	3.54 br s	90.8 (CH) e	3.63 d (1.5)
3	86.0 (qC)		86.0 (qC)
4	35.6 (CH2)	1.84 dd (14.4, 10.4) f	34.6 (CH2) e	1.88 m
		2.61 dd (14.4, 9.2)		2.49 dd (14.0, 9.0)
5	29.5 (CH2)	1.42 m	29.9 (CH2)	1.43 m
		1.62 m		1.70 m
6	77.6 (CH)	4.64 d (6.0)	77.0 (CH)	4.63 d (7.5)
7	79.6 (qC)		79.4 (qC)
8	80.0 (CH)	3.60 br d (8.4)	79.1 (CH)	3.50 br t (8.0)
9	81.3 (CH)	3.85 dd (9.2, 6.4)	82.2 (CH)	4.05 dd (9.5, 5.5)
10	53.3 (CH)	3.34 t (6.8)	50.6 (CH)	3.35 t (6.5)
11	148.4 (qC)		143.0 (qC)
12	31.4 (CH2)	2.08 m	72.8 (CH)	5.44 d (4.5)
		2.31 m
13	25.4 (CH2)	1.17 dd (12.8, 2.8)	29.4 (CH2)	1.50 m
		1.68 m		1.88 m
14	38.9 (CH)	1.51 m	32.7 (CH)	1.90 m
15	23.1 (CH3)	1.41 s	23.2 (CH3)	1.46 s
16	17.7 (CH3)	1.26 s	17.7 (CH3)	1.26 s
17	110.7 (CH2)	4.81 s	116.3 (CH2) e	5.23 s
		4.91 s
18	34.1 (CH)	1.92 m	34.0 (CH)	1.99 m
19	67.7 (CH2)	3.96 d (7.2)	67.8 (CH2)	3.90 dd (11.0, 7.0)
				4.02 dd (11.0, 7.0)
20	11.0 (CH3)	0.85 d (6.4)	11.4 (CH3)	0.90 d (6.5)
3-OCOPr	172.2 (qC)		172.2 (qC)
	37.3 (CH2)	2.33 m	37.3 (CH2)	2.30 m
	18.4 (CH2)	1.68 m	18.3 (CH2)	1.63 m
	13.7 (CH3)	0.99 t (7.2)	13.6 (CH3)	0.98 t (7.0)
12-OAc			170.4 (qC)
			21.5 (CH3)	2.08 s
19-OAc	171.2 (qC)		171.0 (qC)
	21.1 (CH3)	2.08 s	20.9 (CH3)	2.07 s

^{a 13}C and ¹H spectra recorded at 100 and 400 MHz in CDCl₃; ^{b 13}C and ¹H spectra recorded at 125 and 500 MHz in CDCl₃; ^c Deduced from DEPT; ^d Mutiplicity m deduced from HSQC; ^e Broad signal; and ^fJ values (Hz) in parentheses.

Table 1. ¹³C and ¹H NMR data for compounds 1–2.

C	1 a		2 b
	δC	δH	δC	δH
1	44.9 (CH) c	2.27 m d	43.5 (CH)	2.35 m
2	92.3 (CH)	3.54 br s	90.8 (CH) e	3.63 d (1.5)
3	86.0 (qC)		86.0 (qC)
4	35.6 (CH2)	1.84 dd (14.4, 10.4) f	34.6 (CH2) e	1.88 m
		2.61 dd (14.4, 9.2)		2.49 dd (14.0, 9.0)
5	29.5 (CH2)	1.42 m	29.9 (CH2)	1.43 m
		1.62 m		1.70 m
6	77.6 (CH)	4.64 d (6.0)	77.0 (CH)	4.63 d (7.5)
7	79.6 (qC)		79.4 (qC)
8	80.0 (CH)	3.60 br d (8.4)	79.1 (CH)	3.50 br t (8.0)
9	81.3 (CH)	3.85 dd (9.2, 6.4)	82.2 (CH)	4.05 dd (9.5, 5.5)
10	53.3 (CH)	3.34 t (6.8)	50.6 (CH)	3.35 t (6.5)
11	148.4 (qC)		143.0 (qC)
12	31.4 (CH2)	2.08 m	72.8 (CH)	5.44 d (4.5)
		2.31 m
13	25.4 (CH2)	1.17 dd (12.8, 2.8)	29.4 (CH2)	1.50 m
		1.68 m		1.88 m
14	38.9 (CH)	1.51 m	32.7 (CH)	1.90 m
15	23.1 (CH3)	1.41 s	23.2 (CH3)	1.46 s
16	17.7 (CH3)	1.26 s	17.7 (CH3)	1.26 s
17	110.7 (CH2)	4.81 s	116.3 (CH2) e	5.23 s
		4.91 s
18	34.1 (CH)	1.92 m	34.0 (CH)	1.99 m
19	67.7 (CH2)	3.96 d (7.2)	67.8 (CH2)	3.90 dd (11.0, 7.0)
				4.02 dd (11.0, 7.0)
20	11.0 (CH3)	0.85 d (6.4)	11.4 (CH3)	0.90 d (6.5)
3-OCOPr	172.2 (qC)		172.2 (qC)
	37.3 (CH2)	2.33 m	37.3 (CH2)	2.30 m
	18.4 (CH2)	1.68 m	18.3 (CH2)	1.63 m
	13.7 (CH3)	0.99 t (7.2)	13.6 (CH3)	0.98 t (7.0)
12-OAc			170.4 (qC)
			21.5 (CH3)	2.08 s
19-OAc	171.2 (qC)		171.0 (qC)
	21.1 (CH3)	2.08 s	20.9 (CH3)	2.07 s

^{a 13}C and ¹H spectra recorded at 100 and 400 MHz in CDCl₃; ^{b 13}C and ¹H spectra recorded at 125 and 500 MHz in CDCl₃; ^c Deduced from DEPT; ^d Mutiplicity m deduced from HSQC; ^e Broad signal; and ^fJ values (Hz) in parentheses.

Krempfielin Q (2) showed the molecular ion peak [M + Na]⁺ at m/z 563.2835 in the HRESIMS (Figure S4) which established a molecular formFfigure sula of C₂₈H₄₄O₁₀, implying seven degrees of unsaturation for this compound. The IR absorptions at ν_max 3444 and 1732 cm⁻¹ were consistent with the presence of hydroxy and ester carbonyl functionalities. The ¹³C NMR spectrum of 2 showed signals of 28 carbons (Figure S5 and Table 1), which were differentiated by the DEPT spectrum as six methyls (including two acetate methyls δ_C 21.5 and 20.9), six sp³ methylenes, one sp² methylene, nine sp³ methines (including five oxymethines), two sp³ and four sp² quaternary carbons (including three ester carbonyls). The NMR spectroscopic data of 2 (Figures S5–S7, and Table 1) showed the presence of 1,1-disubstituted double bond (δ_C 143.0 C, 116.3 CH₂; δ_H 5.23 s, 2H). Three ester carbonyls (δ_C 172.2, 171.0 and 170.4) were assigned from the ¹³C NMR spectrum and their signals were correlated with the methylene protons (δ_H 2.30, 2H, m) of an n-butyrate and protons of two acetate methyl (δ_H 2.08 s and 2.07 s, each 3H), respectively, indicated the presence of one n-butyrate and two acetoxy groups. The remaining three degrees of unsaturation identified 2 also a tricyclic diterpenoid. The molecular framework of this compound was also established by COSY and HMBC correlations (Figure 1). Comparison of the NMR data of 2 with those of the known compound krempfielin E (4) [16] revealed that 2 is the C-12 acetylated derivative of krempfielin E. The stereochemistry of compound 2 was determined by the NOESY correlations as shown in Figure 2.

Figure 2. Key NOESY (↔) correlations for 1 and 2.

Figure 2. Key NOESY (↔) correlations for 1 and 2.

Many cytotoxic and anti-inflammatory eunicellin-based compounds have been discovered from soft corals [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Recently, we isolated several eunicellins with anti-inflammatory activity by significantly inhibiting superoxide anion generation and elastase release in human neutrophiles induced by FMLP/CB [17,18]. The same in vitro anti-inflammatory effects of the diterpenoids 1 and 2 also were tested. At a concentration of 10 μM, compound 2 exhibited some anti-inflammatory activity in reducing the generation of superoxide anion (13.17% ± 2.09% inhibition) and in inhibiting the elastase release (11.09% ± 5.55% inhibition), relative to the control cells stimulated with FMLP/CB only (Table 2). The cytotoxicity of 1 and 2 against five human carcinoma cell lines, CCRF-CEM, K562, Molt 4, T47D and DLD-1 were also evaluated by the MTT assay, and both compounds did not show activity against the proliferation of these cancer cell lines. The impurity of compound 2 might affect the bioactivity and the biological activities of 3 were not measured due to the paucity of this compound.

Table 2. Effects of compounds 1 and 2 on superoxide anion generation and elastase release in FMLP/CB-induced human neutrophils.

Compound	Superoxide Anion		Elastase
	Inh %	IC50 (μM) a	Inh %	IC50 (μM)
1	5.46 ± 5.19	>10	2.99 ± 2.82	>10
2	13.17 ± 2.09 **	>10	11.09 ± 5.55	>10

Percentage of inhibition (Inh %) at 10 μM concentration. Results are presented as mean ± S.E.M. (the standard error of mean) (n = 3 or 4). ** p < 0.01 compared with the control value; and ^a Concentration necessary for 50% inhibition (IC₅₀).

Table 2. Effects of compounds 1 and 2 on superoxide anion generation and elastase release in FMLP/CB-induced human neutrophils.

Compound	Superoxide Anion		Elastase
	Inh %	IC50 (μM) a	Inh %	IC50 (μM)
1	5.46 ± 5.19	>10	2.99 ± 2.82	>10
2	13.17 ± 2.09 **	>10	11.09 ± 5.55	>10

Percentage of inhibition (Inh %) at 10 μM concentration. Results are presented as mean ± S.E.M. (the standard error of mean) (n = 3 or 4). ** p < 0.01 compared with the control value; and ^a Concentration necessary for 50% inhibition (IC₅₀).

3. Experimental Section

3.1. General Experimental Procedures

Optical rotations were measured on a JASCO P-1020 polarimeter (Jasco, Tokyo, Japan). IR spectra were recorded on a JASCO FT/IR-4100 spectrophotometer (Jasco). ESIMS were obtained with a Bruker APEX II mass spectrometer (Bruker Daltonics, Billerica, MA, USA). The NMR spectra were recorded in CDCl₃, either on a Varian UNITY INOVA-500 FT-NMR (Varian Inc., Palo Alto, CA, USA) or a Varian 400MR FT-NMR (Varian Inc.). Silica gel (Merck, Darmstadt, Germany, 230–400 mesh) was used for column chromatography. Precoated silica gel plates (Kieselgel 60 F-254, 0.2 mm, Merck) were used for TLC analysis (Merck). High-performance liquid chromatography (HPLC) was performed on a Hitachi L-2130 HPLC apparatus (Hitachi, Tokyo, Japan) equipped with Hitachi L-2455 diode array detector (Hitachi) and a Supelco C18 column (250 mm × 21.2 mm, 5 μm, Supelco, Bellefonte, PA, USA).

3.2. Animal Material

C. krempfi was collected by hand using scuba off the coast of Penghu islands of Taiwan in June 2008, at a depth of 5–10 m, and stored in a freezer until extraction. A voucher sample (specimen No. 200806CK) was deposited at the Department of Marine Biotechnology and Resources, National Sun Yat-sen University, Kaohsiung, Taiwan.

3.3. Extraction and Separation

The octocoral (1.1 kg fresh wet weight) was collected and freeze-dried. The freeze-dried material was minced and extracted exhaustively with EtOH (3 × 10 L). The EtOH extract of the frozen organism was partitioned between CH₂Cl₂ and H₂O. The CH₂Cl₂-soluble portion (14.4 g) was subjected to column chromatography on silica gel and eluted with EtOAc in n-hexane (0%–100% of EtOAc, stepwise) and then further with MeOH in EtOAc with increasing polarity to yield 41 fractions. Fraction 30, eluted with MeOH–EtOAc (1:10), was rechromatographed over a silica gel open column using acetone in n-hexane (0%–100% of acetone, stepwise) as the mobile phase to afford six subfractions (A1–A6). Subfraction A4 (eluted with n-hexane–acetone 1:1) was further separated by reverse phase HPLC (CH₃CN–H₂O, 1.5:1) to afford compound 3 (1.0 mg). Fraction 32, eluted with MeOH–EtOAc (1:10), was rechromatoraphed over a silica gel open column using acetone in n-hexane (0%–100% of acetone, stepwise) as the mobile phase to afford eight subfractions (B1–B8). Subfraction B2 (eluted with n-hexane–acetone 2:1) was separated by reverse phase HPLC (CH₃CN–H₂O, 1:1) to afford compound 1 (2.2 mg). Subfraction B3 (eluted with n-hexane–acetone 1.8:1) was subjected to reverse phase HPLC (CH₃CN–H₂O, 1:1.6) and yielded compound 2 (2.0 mg).

3.3.1. Krempfielin Q (1)

Colorless oil; = +84.7 (c 0.77, CHCl₃); IR (neat) v_max 3445, 3076, 2963, 2932, 1733, 1646, 1455, 1373, 1237, 1183, and 1067 cm⁻¹; ¹³C and ¹H NMR data, see Table 1; ESIMS m/z 505 [M + Na]⁺; HRESIMS m/z 505.2779 [M + Na]⁺ (calcd. for C₂₆H₄₂O₈Na, 505.2777).

3.3.2. Krempfielin R (2)

White powder; = +82.0 (c 0.57, CHCl₃); IR (neat) v_max 3444, 3038, 2963, 2930, 1732, 1650, 1456, 1373, 1239, 1182 and 1074 cm⁻¹; ¹³C and ¹H NMR data, see Table 1; ESIMS m/z 563 [M + Na]⁺; HRESIMS m/z 563.2835 [M + Na]⁺ (calcd. for C₂₈H₄₄O₁₀Na, 563.2832).

3.4. Cytotoxicity Testing

Cell lines were purchased from the American Type Culture Collection (ATCC). Cytotoxicity assays of compounds 1 and 2 were performed using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetra-zolium bromide) colorimetric method [28,29].

3.5. In Vitro Anti–Inflammatory Assay–Superoxide Anion Generation and Elastase Release by Human Neutrophils

Human neutrophils were obtained by means of dextran sedimentation and Ficoll centrifugation. Measurements of superoxide anion generation and elastase release were carried out according to previously described procedures [30,31]. LY294002, a phosphatidylinositol-3-kinase inhibitior, was used as a positive control for inhibition of superoxide anion generation and elastase release with IC₅₀ values of 1.88 ± 0.45 and 4.12 ± 0.92 μM, respectively. Briefly, superoxide anion production was assayed by monitoring the superoxide dismutase-inhibitable reduction of ferricytochrome c. Elastase release experiments were performed using MeO-Suc-Ala-Ala-Pro-Val-p-nitroanilide as the elastase substrate [32].

4. Conclusions

Two new eunicellin-based diterpenoids 1 and 2 were isolated together with a known one from the continuing investigation of a soft coral Cladiella krempfi. Although both compounds were not cytotoxic towards a limited panel of cancer cell lines, 2 could inhibit the generation of superoxide anion and the release of elastase in FMLP/CB-induced human neutrophils.