A new Butenolide Derivative from the Brown Alga Sargassum micracanthum

Abstract

A new butenolide derivative (1), along with three known compounds (2-4) were isolated from the MeOH extract of brown alga Sargassum micracanthum. The structures of 1 to 4 were determined by the analyses of 1D and 2D NMR and mass spectroscopic data. The known compounds (2-4) were identified as (5E,10Z)-6,10,14-trimethylpentadeca-5,10-dien-2,12-dione (2), (5E,9E)-6,10,14-trimethylpentadeca-5,9-dien-2,12-dione (3), and (-)-loliolide (4) by comparing with their published spectroscopic data. The antioxidant activities of compounds 1 to 4 were evaluated based on using 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities. Compounds 1 to 4 were inactive at the concentration of 200 μM.

Introduction

The brown alga Sargassum micracanthum belongs to the family Sargassaceae. ¹ Its genera are distributed in several regions around Korea, Japan, Thailand, and pacific coastal areas. The extract of S. micracanthum exhibits the diverse biological activities including antioxidant, ² antiinflammatory, ³ antimelanogenic, ⁴ and antiviral ⁵ activities. Previous studies of the brown alga, S. micracanthum have shown the various chemical structures including plastoquinones, ⁶ chromenols, ⁷ chromanes, ⁸ sargassumketone, ⁹ and farnesylacetones. ¹⁰ As part of our continued search for new secondary metabolites from brown algae, a new butenolide derivative (1), together with three known compounds (2-4) were isolated from the methanolic extract of S. micracanthum. Herein, we describe the isolation and structure elucidation of 1 to 4 (Figure 1).

Figure 1.

Structures of compounds 1 to 4.

Results and Discussion

The compound 1 was acquired as a colorless oil. The molecular formula of compound 1 was assigned as C₁₄H₂₀O₄ by HR-ESI-MS from the positive ion peak at m/z 275.1252 [M + Na]⁺ (calcd. for C₁₄H₂₀O₄Na, 275.1259). The ¹H NMR spectrum of 1 showed two olefinic methines at δ_H 5.76 (1H, s, H-2) and 5.13 (1H, t, J = 6.7 Hz, H-8), four methylenes at δ_H 2.45 (2H, t, J = 7.4 Hz, H-10), 2.23 (2H, m, H-9), 2.14/1.93 (2H, m, H-6), and 2.12/1.80 (2H, m, H-5), and three methyls at δ_H 2.11 (3H, s, H-12), 2.03 (3H, br, H-14), and 1.62 (3H, s, H-13). The ¹³C NMR spectrum of 1 exhibited fourteen carbon signals including two carbonyls at δ_C 208.9 (C-11) and 170.3 (C-1), two olefinic quaternary carbons at δ_C 166.9 (C-3) and 135.1 (C-7), one acetal at δ_C 108.1 (C-4), two olefinic methines at δ_C 124.5 (C-8) and 118.2 (C-2), four methylenes at δ_C 43.4 (C-10), 34.0 (C-5), 33.1 (C-6), and 22.5 (C-9), and three methyl carbons at δ_C 12.8 (C-14), 16.0 (C-13), and 30.1 (C-12) (Table 1). The HMBC correlations from H-2 to C-1, C-4, from H₃-14 to C-2, C-3, and C-4, and from H₂-5 to C-4 suggested the presence of 3-methylfuranone moiety in compound 1. Furthermore, the linear partial moiety of compound 1 was elucidated via ¹H-¹H COSY correlations of [H₂-5-H₂-6] and [H-8-H₂-9-H₂-10] and the HMBC correlations from H₃-12 to C-11, from H₂-10 to C-8, C-9, and C-11, from H₂-9 to C-7, C-8, C-10, and C-11, from H₃-13 to C-6, C-7, and C-8, and from H₂-6 to C-5, C-7, and C-8. The HMBC correlations from H-2 to C-4, from H₃-14 to C-4, and from H₂-5 to C-4 established the attachment between 3-methylfuranone moiety and linear partial moiety via the acetal carbon atom at C-4 (Figure 2). The geometry of double bond at C-7 was determined as E by comparing with methyl carbon (C-13, δ_C 16.0) chemical shifts in the literature (Z configuration: above δ_C 20.0). ¹¹ The absolute configuration of compound 1 was determined as R configuration ¹² by comparing the specific rotation data with similar structure in the literature. Consequently, the compound 1 was elucidated to be a new butenolide derivative and named Sargassumin A (1). Otherwise, the compound 3 was first reported by Kusumi. T. ¹³ However, the structure of 3 was only assigned by the ¹H NMR spectrum. The compound 3 was obtained as a colorless oil. The molecular formula of 3 was found to be C₁₈H₃₀O₂ by HR-ESI-MS ion peak at m/z 301.2134 [M + Na]⁺ (calcd. for C₁₈H₃₀O₂Na, 301.2143). The ¹³C NMR spectrum of 3 revealed two carbonyls at δ_C 209.6 (C-12) and 209.0 (C-2), two olefinic quaternary carbons at δ_C 136.2 (C-6) and 129.2 (C-10), two olefinic methines at δ_C 129.5 (C-9) and 123.0 (C-5), one methine at δ_C 24.6 (C-14), six methylenes at δ_C 54.6 (C-11), 50.7 (C-13), 43.9 (C-3), 39.4 (C-7), 26.7 (C-8), and 22.6 (C-4), and five methyls at δ_C 16.1 (6-CH₃), 16.6 (10-CH₃), 22.7 (C-15), 22.7 (14-CH₃), and 30.1 (C-1). These spectroscopic data were similar to those of compound 2. However, the key difference between 2 and 3 was E-configuration at C-10 (δ_C 16.6) in 3, instead of Z-configuration at C-10 in 2. Furthermore, the side chain of 3 was elucidated by HMBC correlations from H₃-15 to C-13 and C-14, from H₂-11 to C-13, C-12, C-10, and 10-CH₃. Therefore, compound 3 was identified as (5E,9E)-6,10,14-trimethylpentadeca-5,9-dien-2,12-dione.

Table 1.

¹H- and ¹³C-NMR Spectroscopic Data of 1 in CDCl₃ (δ in ppm and J values in [Hz] in parentheses).

	1 a
Position	δ H	δ C
1		170.3
2	5.76 (s)	118.2
3		166.9
4		108.1
5	2.12 (m)	34.0
	1.80 (m)
6	2.14 (m)	33.1
	1.93 (m)
7		135.1
8	5.13 (d, J = 6.7)	124.5
9	2.23 (m)	22.5
10	2.45 (t, J = 7.4)	43.4
11		208.9
12	2.11 (s)	30.1
13	1.62 (s)	16.0
14	2.03 (s)	12.8

^a

Measured at 500 MHz (¹H NMR) and 125 MHz (¹³C NMR).

Figure 2.

HMBC and ¹H-¹H COSY correlations of compound 1.

The known compounds 2 to 4 were identified as (5E,10Z)-6,10,14-trimethylpentadeca-5,10-dien-2,12-dione (2), ¹⁴ (5E,9E)-6,10,14-trimethylpentadeca-5,9-dien-2,12-dione (3) ¹³ , and (-)-loliolide (4), ¹⁵ respectively.

The antioxidant activities of compounds 1 to 4 were evaluated based on using 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities. ¹⁶ However, no antioxidant activity was observed at the concentration of 200 μm.

Experimental

General Experimental Procedures

The optical rotation was recorded on a JASCO P-2000 Digital Polarimeter (JASCO). The HR-ESI-MS spectrum was obtained using a SCIEX X500R Q-TOF LC-MS/MS spectrometer (SCIEX). The medium pressure liquid chromatography (MPLC) (Buchi) was performed using a C₁₈ column (40 g, Biotage). The HPLC (Waters Acquity Arc, Waters, Miliford) system was recorded using a C₁₈ column (Cosmosil, 4.6 μm × 250 mm) with a photodiode array (PDA) (Waters 2998) detector. The ¹H and ¹³C NMR spectra were measured with a JNM-ECZ500R FT-NMR spectrometer (JEOL).

Plant Material

The brown alga, S. micracanthum was collected from Udo-myeon, Jeju Island, Korea, in June 2018 and identified by Dr Jeong-Chan Kang. The voucher specimen (MABIK AL00079431) has been deposited with the National Marine Biodiversity Institute of Korea.

Extraction and Isolation

The dried and ground S. micracanthum (450 g) was extracted three times with MeOH at room temperature for 1 day. The methanolic extract was partitioned with n-hexane, CHCl₃, EtOAc, and butanol, respectively. We combined CHCl₃ (1.86 g) and EtOAc (84.4 mg)-soluble layers based on HPLC profiles. The mixture was fractionated by MPLC eluted with water: acetonitrile gradient system (10:90-0:100) to obtain six fractions (A-F). The fraction A (195.6 mg) was further fractionated by MPLC using increasing solvent gradients of MeOH (10% aq. MeOH-100% MeOH) to yield three subfractions (A1-A3). The A2 fraction (20.4 mg) was separated via reversed-phase HPLC eluted with the isocratic solvent system using 30% aq. ACN (flow rate: 3 mL/min) to obtain compound 1 (2.2 mg). The A1 fraction (36.5 mg) was isolated by reversed-phase HPLC eluted with 30% aq. ACN (flow rate: 3 ml/min) to yield compound 4 (5.9 mg). The E fraction (103.4 mg) was purified by reversed-phase HPLC eluted with 75% aq. ACN to provide compound 2 (5.0 mg). Compound 3 was acquired from the fraction D (175.4 mg) by reversed-phase HPLC using isocratic solvent conditions of 65% aq. ACN (flow rate: 3 mL/min).

Sargassumin A (1): Colorless oil; [a]_D²⁵: −9.7 (c 0.15, CHCl₃); ¹H (500 MHz, CDCl₃) and ¹³C NMR (125 MHz, CDCl₃) spectra see Table 1; HR-ESI-MS (positive): m/z 275.1251 C₁₄H₂₀O₄Na [M + Na]⁺ (calcd. for C₁₄H₂₀O₄Na, 275.1259).

(5E,9E)-6,10,14-trimethylpentadeca-5,9-dien-2,12-dione (3): Colorless oil; HR-ESI-MS (positive): m/z 301.2134 C₁₈H₃₀O₂Na [M + Na]⁺ (calcd. for C₁₈H₃₀O₂Na, 301.2142).¹ H (500 MHz, CDCl₃): δ_H 5.19 (1H, t, J = 6.8 Hz, H-9), 5.05 (1H, t, J = 6.8 Hz, H-5), 2.98 (2H, s, H-11), 2.43 (2H, t, J = 7.3 Hz, H-3), 2.26 (2H, m, H-13), 2.24 (2H, m, H-4), 2.11 (3H, s, H-1), 2.09 (1H, m, H-14), 1.98 (2H, t, J = 7.6 Hz, H-7), 1.59 (3H, s, 6-CH₃), 1.57 (3H, s, 10-CH₃), 0.87 (3H, s, 14-CH₃), and 0.86 (3H, s, H-15); ¹³C NMR (125 MHz, CDCl₃): δ_C 209.6 (C-12), 209.0 (C-2), 136.2 (C-6), 129.5 (C-9), 129.2 (C-10), 123.0 (C-5), 54.6 (C-11), 50.7 (C-13), 43.9 (C-3), 39.4 (C-7), 30.1 (C-1), 26.7 (C-8), 24.6 (C-14), 22.7 (14-CH₃ and C-15), 22.6 (C-4), 16.6 (10-CH₃), and 16.1 (6-CH₃).

Conclusions

In this study, a new butenolide derivative (1), along with three known compounds (2-4) were isolated from S. micracanthum. The known compounds (2-4) were identified as (5E,10Z)-6,10,14-trimethylpentadeca-5,10-dien-2,12-dione (2), (5E,9E)-6,10,14-trimethylpentadeca-5,9-dien-2,12-dione (3), and (-)-loliolide (4). All compounds revealed no antioxidant activities against ABTS and DPPH radical scavenging activities (>200 μm). This is the first report of the butenolide derivative from S. micracanthum.

Supplemental Material

sj-docx-1-npx-10.1177_1934578X211068606 - Supplemental material for A new Butenolide Derivative from the Brown Alga Sargassum micracanthum

Supplemental material, sj-docx-1-npx-10.1177_1934578X211068606 for A new Butenolide Derivative from the Brown Alga Sargassum micracanthum by Ji-Yul Kim, Jeong Min Lee, Hyun-Soo Kim, Dae-Won Ki, Mi-Jin Yim, Seok-Chun Ko, Jung Min Shin, Myeong Seok Lee, Yun Gyeong Park and Dae-Sung Lee in Natural Product Communications