Three New Clerodane Diterpenes from Polyalthia longifolia var. pendula

Abstract

Three new clerodane diterpenes, (4→2)-abeo-cleroda-2,13E-dien-2,14-dioic acid (1), (4→2)-abeo-2,13-diformyl-cleroda-2,13E-dien-14-oic acid (2), and 16(R&S)-methoxycleroda-4(18),13-dien-15,16-olide (3), were isolated from the unripe fruit of Polyalthia longifolia var. pendula (Annonaceae) together with five known compounds (4–8). The structures of all isolates were determined by spectroscopic analysis. The anti-inflammatory activity of the isolates was evaluated by testing their inhibitory effect on NO production in LPS-stimulated RAW 264.7 macrophages. Among the isolated compounds, 16-hydroxycleroda-3,13-dien-15,16-olide (6) and 16-oxocleroda-3,13-dien-15-oic acid (7) showed promising NO inhibitory activity at 10 µg/mL, with 81.1% and 86.3%, inhibition, respectively.

1. Introduction

Polyalthia longifolia var. pendula, known as “Indian Mast Tree”, is a lofty evergreen tree which is distributed in tropical and subtropical regions. It is commonly cultivated in Asia and especially in Taiwan, as ornamental street tree due to its effectiveness in combating noise pollution [1]. In India, the aqueous plant decoction and its alcoholic extract have been used for the treatment of skin diseases, helminthiasis, pyrexia, diabetes, and, hypertension [2]. A series of diterpenoids showing cytotoxicity [3,4], antibacterial [5,6], antifungal [5], and anti-inflammatory activities [7] were reported from this plant. In line with our continuing efforts to identify interesting natural products with unique structures and biological activities from Taiwanese flora, we have investigated the secondary metabolite content of Polyalthia longifolia var. pendula unripe fruit methanolic extract. As a result we have separated two new rearranged (4→2)-abeo-clerodane diterpenes: (4→2)-abeo-cleroda-2,13E-dien-2,14-dioic acid (1), (4→2)-abeo-2,13-diformyl-cleroda-2,13E-dien-14-oic acid (2) as well as one new clerodane diterpene: 16(R&S)-methoxycleroda-4(18),13-dien-15,16-olide (3). Moreover, the isolation of five known compounds including solidagonal acid (4) [8], 16-hydroxycleroda-4(18),13-dien-15,16-olide (5) [9,10], 16-hydroxycleroda-3,13-dien-15,16-olide (6) [11], 16-oxocleroda-3,13-dien-15-oic acid (7) [11] and 3β,5β,16-trihydroxyhalima-13-en-15,16-olide (8) [4] (Figure 1) are also reported in the current study. The anti-inflammatory activity of the isolates was examined by evaluating their inhibitory activity on nitric oxide (NO) production in LPS-stimulated macrophage (RAW264.7 cells).

Figure 1. Compounds 1–8 isolated from the Indian Mast Tree Polyalthia longifolia var. pendula.

Figure 1. Compounds 1–8 isolated from the Indian Mast Tree Polyalthia longifolia var. pendula.

2. Results and Discussion

Compounds 1 and 2 were isolated as oils. The UV absorption band at ca. 220 nm and the IR absorption bands at 1,680–1,690 cm⁻¹ indicated the presence of a conjugated carbonyl moiety. The ¹³C-NMR data of 1 and 2 revealed the presence of 20 carbons. The DEPT and HMBC experiments showed two sets of conjugated systems for C2-C3-C4 and C16-C13-C14-C15 in 1 and 2 (Table 1). Their ¹H-NMR spectra indicated the presence of a single methyl at ca. δ_H 2.00–2.08, two methyls at ca. δ_H 0.85–0.96, and a secondary methyl group at δ_H 0.87–0.92 (d, J = 6.5 Hz) (Table 1). No olefinic protons for the C2-C3-C4 conjugated system were detected in the ¹H-NMR spectra. Accordingly, the data suggested that compounds 1 and 2 belong to the clerodane-type diterpene type with a (4→2) rearranged ring A moiety, similar to that of solidagonal acid (4) [8].

The molecular formula of compound 1 was predicted as C₂₀H₃₀O₄ by ESI-MS, indicating six indices of hydrogen deficiency (IHD). The UV and the IR spectra indicated the presence of a carboxylic acid group (IR: 3,361 cm⁻¹) and a conjugated carboxylic acid group (UV: 220 nm; IR: 1,684, 1,654 cm⁻¹), which was also supported by the ¹³C-NMR signals at δ_C 169.6 and 169.4 (Table 1). The 1D and 2D NMR spectroscopic data of compounds 1 and 4 were similar, except for the downfield-shifted proton signal at δ_H 2.00 and the carbon signal at δ_C 169.5 suggesting that 1 possesses a rearranged clerodane skeleton with a carboxylic moiety connected to C-2 (Table 1), which is different from the aldehyde group in 4. The HMBC correlations between δ_H 5.70 (H-14)/δ_C 169.4 (C-15), 34.5 (C-12), and 17.7 (C-16), suggested the presence of an α,β-unsaturated carboxylic acid in 1. The HMBC correlations [δ_H 1.47 (H₂-11)/δ_C 160.2 (C-13), 54.1 (C-10), 37.2 (C-8), 34.5 (C-12), 17.1 (C-20); δ_H 2.12, 2.07 (H₂-12)/δ_C 160.2 (C-13), 115.8 (C-14), 38.1 (C-11), 17.7 (C-16)] indicated that the α,β-unsaturated carboxylic acid is connected to C-9 by a short methylene chain [-C(11)H₂-C(12)H₂-] (Figure 2). Furthermore, a trans- ring junction between rings A/B was suggested based on the NOESY cross peaks [δ_H 1.55 (H-8)/δ_H 1.69 (H-10); δ_H 1.60 (H_ax-7)/δ_H 0.95 (Me-19); δ_H 0.87 (Me-17)/δ_H 1.60 (H_ax-7), δ_H 0.95 (Me-19), δ_H 0.92 (Me-20); δ_H 0.95 (H-19)/δ_H 2.30 (H_eq-1) and δ_H 1.67 (H_eq-6)] (Figure 3) [12,13]. Additionally, the configuration of the double bond Δ¹³ was suggested as E by comparing the obtained chemical shifts of hydrogens and carbon (C-13 to C-16) with previous NMR data. We found close similarity of proton signals: δ_H 5.70 (H-14) and 2.15 (H-16); as well as carbon signals: δ_C 160.2 (C-13) and 115.8 (C-14) 169.4 (C-15) and 17.7 (C-16) with known solidagonal acid (4) and other similar structures [8,14,15]. Thus, compound 1 was assigned as (4→2)-abeo-cleroda-2,13E-dien-2,14-dioic acid (1).

Table 1. ¹H-NMR (500 MHz) and ¹³C-NMR (125 MHz) data of 1–3 (δ in ppm and J in Hz).

No.	1 a		2 b		3 c
	δC	δH	δC	δH	δC	δH
1	29.3	2.30 dd (14.6, 6.0)	25.5	2.64 m	21.1	1.40–1.55 m
		2.35 dd (14.6, 2.1)		2.15 m
2	126.7	-	137.3	-	32.4	2.05 m
						2.24 m
3	169.6 *	-	189.8	9.95 s	28.01/28.03	1.20 m
						1.84 m
4	164.9	-	174.9	-	159.0	-
5	50.1	-	51.1	-	40.0	-
6	34.4	1.37 td (11.5, 5.0)	38.5	1.65 m	36.8	1.45–1.58 m
		1.67 m		1.45 m
7	28.1	1.57 m	28.3	1.60 m	26.9	1.40–1.55 m
		1.60 m
8	37.2	1.55 m	37.1	1.64 m	36.1	1.38–1.43 m
9	37.7	-	38.5	-	38.72/38.68	-
10	54.1	1.69 dd (6.0, 2.1)	53.8	1.71 m	48.1	1.03 dt (12.0, 3.0 )
11	38.1	1.47 td (8.5, 4.5)	34.0	1.36 m	34.00/34.02	1.40–1.60 m
12	34.5	2.07 m	19.3	2.52 m	20.7	2.02 m
		2.12 m		2.66 m		2.23 m
13	160.2	-	154.7	-	168.9	-
14	115.8	5.70 s	135.3	6.49 s	117.11/117.20	6.09 br s
15	169.4 *	-	167.2	-	170.4	-
16	17.7	2.15 s	194.4	9.55 s	104.0	5.91 s
17	14.0	0.87 d (6.0)	14.9	0.92 d (6.5)	15.8	0.77 d (6.5)
						0.79 d (6.5)
18	10.2	2.00 s	9.9	2.08 s	103.0	4.48 d (3.5)
19	15.9	0.95 s	17.1	0.96 s	20.5	1.02 s
20	17.1	0.92 s	18.0	0.85 s	17.7	0.74 s
16-OCH3					56.1	3.43 s/3.43 s

^a Measured in CD₃OD; ^b Measured in CDCl₃; ^c Measured in DMSO-d₆; * Signals are interchangeable.

Table 1. ¹H-NMR (500 MHz) and ¹³C-NMR (125 MHz) data of 1–3 (δ in ppm and J in Hz).

No.	1 a		2 b		3 c
	δC	δH	δC	δH	δC	δH
1	29.3	2.30 dd (14.6, 6.0)	25.5	2.64 m	21.1	1.40–1.55 m
		2.35 dd (14.6, 2.1)		2.15 m
2	126.7	-	137.3	-	32.4	2.05 m
						2.24 m
3	169.6 *	-	189.8	9.95 s	28.01/28.03	1.20 m
						1.84 m
4	164.9	-	174.9	-	159.0	-
5	50.1	-	51.1	-	40.0	-
6	34.4	1.37 td (11.5, 5.0)	38.5	1.65 m	36.8	1.45–1.58 m
		1.67 m		1.45 m
7	28.1	1.57 m	28.3	1.60 m	26.9	1.40–1.55 m
		1.60 m
8	37.2	1.55 m	37.1	1.64 m	36.1	1.38–1.43 m
9	37.7	-	38.5	-	38.72/38.68	-
10	54.1	1.69 dd (6.0, 2.1)	53.8	1.71 m	48.1	1.03 dt (12.0, 3.0 )
11	38.1	1.47 td (8.5, 4.5)	34.0	1.36 m	34.00/34.02	1.40–1.60 m
12	34.5	2.07 m	19.3	2.52 m	20.7	2.02 m
		2.12 m		2.66 m		2.23 m
13	160.2	-	154.7	-	168.9	-
14	115.8	5.70 s	135.3	6.49 s	117.11/117.20	6.09 br s
15	169.4 *	-	167.2	-	170.4	-
16	17.7	2.15 s	194.4	9.55 s	104.0	5.91 s
17	14.0	0.87 d (6.0)	14.9	0.92 d (6.5)	15.8	0.77 d (6.5)
						0.79 d (6.5)
18	10.2	2.00 s	9.9	2.08 s	103.0	4.48 d (3.5)
19	15.9	0.95 s	17.1	0.96 s	20.5	1.02 s
20	17.1	0.92 s	18.0	0.85 s	17.7	0.74 s
16-OCH3					56.1	3.43 s/3.43 s

^a Measured in CD₃OD; ^b Measured in CDCl₃; ^c Measured in DMSO-d₆; * Signals are interchangeable.

Figure 2. Selected HMBC ( ) and COSY ( ) correlations of compounds 1–2.

Figure 2. Selected HMBC ( ) and COSY ( ) correlations of compounds 1–2.

The main fragment of abeo-clerodanes 1–2.

Figure 3. Selected NOESY correlations of compounds 1–2.

Figure 3. Selected NOESY correlations of compounds 1–2.

The molecular formula of 2 was predicted as C₂₀H₂₈O₄ by ESI-MS and ¹³C-NMR data, indicating seven IHD. Two proton signals at δ_H 9.95 and 9.55, as well as tertiary resonances at δ_C 194.4 and 189.8, indicated that 2 possesses two aldehyde groups. Through comparison of the ¹H-NMR data of 1 and 2, it was noticed that the olefinic proton (H-14) signal in 2 was shifted downfield (δ_H 6.49) instead of δ_H 5.70 in 1. Also the disappearance of the methyl signal at δ_H 2.15 implied that the methyl group (H₃-16) in 2 should be replaced by an aldehyde group (Table 1), which was confirmed by the HMBC correlations [δ_H 9.55 (H-16)/δ_C 154.7 (C-13); δ_H 6.49 (H-14)/δ_C 194.4 (C-16), 154.7 (C-13)]. The C-2 position of the remaining aldehyde group was suggested by comparing the ¹H-NMR spectra of 2 and 4 and was also supported by the HMBC correlations [δ_H 9.95 (H-3)/δ_C 137.3 (C-2), 25.5 (C-1); δ_H 2.08 (H₃-18)/δ_C 174.9 (C-4), 137.3 (C-2)] (Figure 2). An octahydroindene system with a trans-junction of rings A and B was deduced based on the NOESY correlations (Figure 3), which was also supported by the results reported by Manabe et al. [16]. The geometry of the C-13~C-14 fragment in 2 was deduced by comparing the NMR data with similar compounds possessing the same fragment. It was found that the chemical shifts of the protons [δ_H 6.49 (H-14), 9.55 (H-16] and carbons [δ_C 154.7 (C-13),135.3 (C-14), 167.2 (C-15), 194.4 (C-16)] are similar to compounds possessing similar fragments with trans-configuration such as 16-oxocleroda-3,13-dien-15-oic acid (7) [11,17] and 16-oxocleroda-4(18),13E-dien-15-oic acid and 16-oxo-ent-halima-5(10),13E-dien-15-oic acid [9]. Therefore, the configuration of C13-C14 was determined as trans. The structure of 2 was thus assigned as (4→2)-abeo-2,13-diformyl-cleroda-2,13E-dien-14-oic acid (2).

The molecular formula of compound 3 was predicted as C₂₁H₃₂O₃ by the ESI-MS data, and it was thus deduced to have six IHD. In the ¹H- and ¹³C-NMR spectra of 3, the downfield shifted proton signals [δ_H 6.09 (1H, brs), 5.91 (1H, s)] as well as two methine carbons [δ_C 117.1, 104.0] and two quaternary carbons [δ_C 170.4, 168.9] indicated the presence of an α,β-unsaturated lactone moiety, which was also supported by the UV absorption band at 207 nm and the IR absorption band at 1,758 cm⁻¹. Moreover, a terminal double bond, one methoxy and three methyl groups were suggested based on certain characteristic proton signals [δ_H 4.48 (2H, d); 3.43 (3H, s), 1.02 (3H, s), 0.77 (3H, d, J = 6.5 Hz), 0.74 (3H, s)] as well as the ¹³C-NMR signals of a methylene carbon [δ_C 103.0], a quaternary carbon [δ_C 159.0], a methoxy carbon [δ_C 56.1], and three methyl carbons [δ_C 20.5, 17.7, 15.8]. The detailed analysis of ¹H and ¹³C-NMR data of 3 indicated that it possesses a skeleton similar to that of 16-hydroxycleroda-4(18),13-dien-15,16-olide (5) [8,9] (Table 1). The NOESY cross peaks [δ_H 1.02 (H₃-19)/δ_H 0.74 (H₃-20); δ_H 1.03 (H-10)/δ_H 1.43 (H-8)] confirmed that the junction of A/B ring in 3 is trans. The HMBC correlation [δ_H 3.43/δ_C 104.0] indicated that the additional methoxy is connected to C-16. Thus, the structure of 3 was assigned as 16(R&S)-methoxycleroda-4(18),13-dien-15,16-olide.

Moreover five known compounds were also identified from the extract, including solidagonal acid (4) [8], 16-hydroxycleroda-4(18),13-dien-15,16-olide (5) [9,10], 16-hydroxycleroda-3,13-dien-15,16-olide (6) [11], 16-oxocleroda-3,13-dien-15-oic acid (7) [11], and 3β,5β,16-trihydroxyhalima-13-en-15,16-olide (8) [4], by comparing their UV, IR, ¹H-NMR, ¹³C-NMR and MS data with those reported in literature.

Nitric oxide (NO), an important intracellular and intercellular signaling molecule, acts as a mediator in the cardiovascular, nervous, and immune systems [18]. It is also involved in various biological reactions, such as vasorelaxation [19], inhibition of platelet aggregation [20], neurotransmission [21], inflammation [22], immunoregulation [23], and angiogenesis regulation [24]. This mediator plays a critical role in inflammatory response and controlling its levels remains an important therapeutic target [25].

Several studies have evaluated the anti-inflammatory activity of methanolic extract of P. longifolia leaves and roots [7,26]. Taking these previous reports on the anti-inflammatory activity of P. longifolia into consideration, we thought that the isolated compounds might possess anti-inflammatory activity. Thus, we investigated the inhibitory effect of the isolated compounds on the NO production in LPS-stimulated macrophage (RAW 264.7 cells) by the Griess reaction. The results indicated that 16-hydroxycleroda-3,13-dien-15,16-olide (6) and 16-oxocleroda-3,13(14)E-diene-15-oic acid (7) reduced NO production in LPS-induced cells in a dose-dependent manner (Figure 4). At 10 µM, compounds 6 and 7 exhibited promising NO inhibitory activity, with 81.1% and 86.3%, inhibition respectively, without affecting cell viability (Figure 4 and Figure 5), and their IC₅₀ values were approaching 1 μΜ. These results supported a previous report, which demonstrated that 6 could inhibit microglia-mediated inflammation and inflammation-related neuronal cell death [27].

Figure 4. Effect of 6 and 7 isolated from P. longifolia var. pedula on the expression of RAW 264.7 NO. RAW 264.7 macrophages (5 × 10⁵/mL) were pre-treated with compounds 6 and 7, and DMSO (control) for 30 min, followed by stimulation with LPS (1 µg/mL) for 24 h. NO concentration in the culture medium was assayed by the Griess reaction. The data were expressed as the means ± S.E. from three separate experiments.

Figure 4. Effect of 6 and 7 isolated from P. longifolia var. pedula on the expression of RAW 264.7 NO. RAW 264.7 macrophages (5 × 10⁵/mL) were pre-treated with compounds 6 and 7, and DMSO (control) for 30 min, followed by stimulation with LPS (1 µg/mL) for 24 h. NO concentration in the culture medium was assayed by the Griess reaction. The data were expressed as the means ± S.E. from three separate experiments.

Figure 5. Effect of 6 and 7 isolated from P. longifolia var. pedula on cell viability. RAW 264.7 macrophages (5 × 10³/well) were treated with compounds 6 and 7, DMSO (control) in the presence or absence of LPS (1 µg/mL) for 24 h, followed by incubating with MTT reagent. After 30 min of incubation, the absorbance (A₅₅₀ − A₆₉₀) was measured by spectrophotometry [26]. The data were expressed as the means ± S.E. from three separate experiments.

Figure 5. Effect of 6 and 7 isolated from P. longifolia var. pedula on cell viability. RAW 264.7 macrophages (5 × 10³/well) were treated with compounds 6 and 7, DMSO (control) in the presence or absence of LPS (1 µg/mL) for 24 h, followed by incubating with MTT reagent. After 30 min of incubation, the absorbance (A₅₅₀ − A₆₉₀) was measured by spectrophotometry [26]. The data were expressed as the means ± S.E. from three separate experiments.

3. Experimental

3.1. General

Melting points were measured on Yanaco MP-500D melting point apparatus (Yanaco, Kyoto, Japan) and were used uncorrected. Optical rotations were measured on a JASCO P-1020 polarimeter (JASCO, Tokyo, Japan). UV spectra were recorded on a Hitachi U-2800 UV-Vis spectrophotometer (Hitachi, Tokyo, Japan). IR spectra were taken on a Shimadzu IR Prestige-21 FT-IR spectrometer (Shimadzu, Nakagyo-ku, Japan). 1D and 2D NMR spectra were recorded on Bruker 500 AVII NMR spectrometers (Bruker BioSpin GmbH, Karlsruhe, Germany). HR-FAB-MS were measured with a Finnigan/Thermo Quest MAT 95XL spectrometer, and ESI-MS/MS were obtained on a Bruker HCT ultra PTM Discovery system (Bruker Daltonics, Bremen, Germany). Sephadex LH-20 (Amersham Biosciences, Uppsala, Sweden) and Silica gel 60 (230–400 mesh or 70–230 mesh, Merck, Darmstadt, Germany) were used for column chromatography; precoated Si gel plates (silica gel 60 F₂₅₄, Merck) were used for analytical TLC. The spots were detected by spraying with 50% H₂SO₄ aqueous solution and then heating on a hot plate. HPLC was performed on a Hitachi L-2130 pump equipped with a Hitachi L-2420 UV-Vis detector (Hitachi). Discovery^® HS C₁₈ (5 μm, 250 × 4.6 mm i.d., Supelco, Bellefonte, PA, USA) and semi-preparative Discovery^® HS C₁₈ (5 μm, 250 × 10 mm i.d., Supelco) columns were used for analytical and preparative purposes, respectively.

3.2. Plant Material

The unripe fruits of P. longifolia var. pendula (500 g) were collected in Kaohsiung City, Taiwan, in September, 2005. A voucher specimen (PLP) was deposited in the Department of Marine Biotechnology and Resources, National Sun Yat-sen University, Kaohsiung, Taiwan.

3.3. Extraction and Isolation

The unripe fruits of P. longifolia var. pendula (500 g) were extracted with methanol (4L × four times). After removing the solvent, the MeOH extract (27.9 g) was partitioned with n-hexane and water to yield n-hexane (8.0 g) and aqueous layers. The n-hexane layer was further separated into four fractions (Ha-Hd) by column chromatography (CC) on silica gel with n-hexane–CHCl₃ and CHCl₃–MeOH as eluents. Fraction Hb (2.5 g) was applied to a silica gel column eluted with gradient of n-hexane–CHCl₃ and CHCl₃–MeOH to yield 30 fractions (Hb-1~Hb-30). Fraction Hb-12 (25.8 mg) was separated by silica gel column chromatography eluted with n-hexane–chloroform (5:1 to 1:9) to afford compound 7 (10.0 mg). Fraction Hb-13 (37.9 mg) was separated by reversed-phase HPLC (MeOH–H₂O (0.05% TFA), 90:10) to obtain 5 (1.0 mg, t_R 11.1 min) and 6 (7.0 mg, t_R 11.8 min). Fraction Hb-17 (254.0 mg) was purified by reversed-phase HPLC (MeOH–H₂O (0.05% TFA), 75:25) to obtain 8 (3.0 mg, t_R 11.1 min). Fraction Hb-18 (210.0 mg) was separated by reversed-phase HPLC (MeOH–H₂O (0.05% TFA), 75:25) to obtain 1 (2.1 mg, t_R 37.7 min). Compound 4 (2.0 mg, t_R 20.0 min) were isolated from the subfraction Hb-19 (37.6 mg) by reversed-phase HPLC (MeOH–H₂O (0.05% TFA), 70:30). Compound 3 (2.5 mg, t_R 7.2 min) was isolated from the subfraction Hb-20 (139.0 mg) by reversed-phase HPLC (MeOH–H₂O (0.05% TFA), 95:5). Fraction Hc-4 (180.0 mg) was separated by reversed-phase HPLC (MeCN–H₂O (0.05% TFA), 75:25) yielding 2 (3.8 mg, t_R 17.7 min).

3.4. Spectral Data

(4→2)-abeo-Cleroda-2,13E-dien-2,14-dioic acid (1). Oil. [α] +2.0 (c 0.10, CHCl₃), UV λ_max (log ε) (MeOH) nm: 220 (3.24). IR ν_max (KBr) cm⁻¹: 3,361, 2,957, 2,922, 1,684, 1,654, 1,437, 1,208, 1,142. ESIMS (positive mode) m/z: 357.3 [M+Na]⁺. ESIMS (negative mode) m/z: 333.3 [M-H]⁻. HR-FAB-MS m/z: 335.2226 [M+H]⁺ (calcd. for C₂₀H₃₁O₄ 335.2222). ¹H-NMR (CD₃OD, 500 MHz) and ¹³C-NMR (CD₃OD, 125 MHz), see Table 1.

(4→2)-abeo-2,13-Diformyl-cleroda-2,13E-dien-14-oic acid (2). Oil. [α] +13.3 (c 1.0, CHCl₃), UV λ_max (log ε) (MeOH) nm: 219 (2.78). IR ν_max (KBr) cm⁻¹: 2,924, 2,853, 1,687, 1,676, 1,639, 1,564, 1,383, 1,208, 1,183. ESIMS (positive mode) m/z: 355.2 [M+Na]⁺. ESIMS (negative mode) m/z: 331.3 [M-H]⁻. HR-FAB-MS m/z: 355.1880 [M+Na]⁺ (calcd for C₂₀H₂₈O₄Na, 355.1885), 333.2060 [M+H]⁺ (calcd. for C₂₀H₂₉O₄⁺, 333.2066). ¹H-NMR (CDCl₃, 500 MHz) and ¹³C-NMR (CDCl₃, 125 MHz), see Table 1.

16(R&S)-Methoxycleroda-4(18),13-dien-15,16-olide (3). Oil. [α] −71.0 (c 0.1, MeOH), UV λ_max (log ε) (MeOH) nm: 207 (4.32). IR ν_max (KBr) cm⁻¹: 2,955, 2,924, 2,853, 1,758, 1,680, 1,639, 1,456, 1,206. ESIMS (positive mode) m/z: 355.24 [M+Na]⁺. ESIMS (negative mode) m/z: 331.20 [M-H]⁻. HR-FAB-MS m/z: 355.2251 [M+Na]⁺ (calcd. for C₂₁H₃₂O₃Na, 355.2249), 333.2427 [M+H]⁺ (calcd. for C₂₁H₃₃O₃⁺, 333.2430). ¹H-NMR (DMSO-d₆, 500 MHz) and ¹³C-NMR (DMSO-d₆, 125 MHz), see Table 1.

3.5. Cell Culture

Murine RAW 264.7 macrophages were obtained from the American Type Culture Collection (ATCC, Rockville, MD, USA). Cells were propagated in RPMI-1640 medium supplemented with 10% heated-inactivated FCS and 2 mM l-glutamine (Life Technologies, Inc., Gaithersburg, MD, USA), and incubated in a 5% CO₂ incubator at 37 °C [28].

3.6. Detection of Nitric Oxide Expression by Griess Reaction

RAW 264.7 cells were seeded in 24-well plate at a density of 5 × 10⁵ cells/mL, and then incubated with or without LPS (1 µg/mL) in the absence or presence of the tested compounds (1, 5, 10 µM) for 24 h. The effect of the tested compounds on NO production was measured indirectly by determining the nitrite levels using the Griess reaction [2,28].

3.7. Statistical Analysis

All tested compounds were re-purified by reversed-phase HPLC before the bioassay test (purity > 99%). All results are expressed as the means ± S.E. from three independent experiments. Data analysis involved one-way ANOVA with subsequent Scheffé test; p values < 0.05 were considered to be significant.

4. Conclusions

Chemical investigation of the unripe fruits of P. longifolia var. pendula resulted in the isolation of three new clerodane diterpenes: (4→2)-abeo-cleroda-2,13E-dien-2,14-dioic acid (1), (4→2)-abeo-2,13-diformyl-cleroda-2,13E-dien-14-oic acid (2), and 16(R&S)-methoxycleroda-4(18),13-dien-15,16-olide (3), together with five known compounds 4–8. The inhibitory activity of the isolated compounds on NO production in LPS-stimulated macrophage RAW 264.7 was evaluated utilizing the Griess reaction. Among the tested compounds, 16-hydroxycleroda-3,13-dien-15,16-olide (6) and 16-oxocleroda-3,13(14)E-diene-15-oic acid (7) reduced NO production in LPS-induced cells in a dose-dependent manner with 81.1% and 86.3%, inhibition, respectively. These results add weight to the accumulating evidence supporting the potential application of P. longifolia var. pendula as a potent anti-inflammatory remedy.