Two New Compounds from Rhizomes of Musa basjoo

Abstract

Two new compounds, rel-(3S,4aR,10bR)-3-(4′-methoxyphenyl)-8-hydroxy-9-methoxy-4a,5,6,10b-tetrahydro-3H-naphtho[2,1-b]pyran (1), 7-hydroxy- 2-phenylnaphthalene-1-carboxylic acid ethyl ester (2), were isolated from the 70% ethanol extract of the rhizomes of Musa basjoo. Their structures were determined on the basis of spectroscopic analysis including 1-dimensional (1D), 2D nuclear magnetic resonance and high-resolution electrospray ionization mass spectroscopy.

The rhizomes of Musa basjoo are native to the East Asian tropics, which is mainly distributed in the south of the HuaiHe River in the QinLing Mountains in China. Previously, the isolation of triterpenes, alkaloids, polysaccharides, acenaphthenes, acenaphtylenes, phenylphenalenones, and bicyclic diarylheptanoids has been reported from phytochemical studies of this genus. ^{1 -4} The rhizomes of Musa basjoo have been demonstrated to exhibit diverse pharmacological activities like antibacterial, antiinflammatory, and anticancer. ^{5 -8} This paper describes the isolation and structural elucidation of 2 new compounds (Figure 1) from the 70% ethanol extract of the rhizomes of Musa basjoo.

Figure 1

The structure of compounds 1 - 2.

Compound 1 was isolated as a white, amorphous powder. As shown in Supplemental figure S1, the molecular formula was established as C₂₁H₂₂O₄ based on high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) [M + Na]⁺ ion at m/z 361.1415 (calculated for C₂₁H₂₂O₄Na, m/z 361.1410). The 1-dimensional (1D) spectrum showed the typical signals of the bicyclic diarylheptanoids. The ¹H-nuclear magnetic resonance (NMR) spectrum (Supplemental figure S2) showed characteristic signals for an hydroxy group of δ_H 5.46 (1H, s), a 1, 2, 4, 5-tetrasubstituted aromatic ring group of δ_H 6.74 (1H, s, H-10) and 6.60 (1H, s, H-7), 1 para substituted phenyl group of δ_H 7.35 (2H, d, J = 8.6 Hz, H-2′, 6′) and 6.90 (2H, d, J = 8.6 Hz, H-3′, 5′), and 2 olefinic protons of δ_H 6.19 (1H, m, H-1) and 5.91 (1H, dt, J = 10.3, 2.4 Hz, H-2). The ¹³C-NMR spectrum (Supplemental figure S3) showed characteristic signals for 2 methoxy carbonyl of δ_C 56.0 (9-OCH₃) and 55.3 (4′-OCH₃), 2 olefinic signal of δ_C 129.0 (C-1) and 127.8 (C-2), and aromatic carbon signals δ_C 159.2 (C-4′), 133.1 (C-1′), 129.2 (C-2′,6′), 113.7 (C-3′,5′), 145.3 (C-9), 143.8 (C-8), 129.1 (C-6a), 114.2 (C-7), 110.4 (C-10), and 129.0 (C-10a). One-dimensional NMR data (Table 1) (Supplemental figures S4-S5) indicated that compound 1 was a structural analog of the reported compound, rel-(3S,4aR,10bR)-8-hydroxy-3-(4′-hydroxyphenyl)−9-methoxy-4a,5,6,10b-tetrahydro-3H-naphtha[2,1-b]pyran, ⁹ with the only difference in compound 1 being the replacement of the hydroxyl group at C-4′ with a methoxy group. Heteronuclear multiple bond correlation (HMBC) spectra as shown in Supplemental table S1 were further used to confirm the structure and to determine the positions of the methoxy groups. The HMBC long-range correlations (Figure 2) from 4′-OCH₃ and H-2′, 6′ to C-4′ suggested that the methoxy groups (4′-OCH₃) were directly connected at the C-4′. Furthermore, in the HMBC spectrum the correlations from 9-OCH₃ and H-7 to C-9 indicated that the methoxy groups (9-OCH₃) were located at C-9. Therefore, compound 1 was determined to be rel-(3S,4aR,10bR)-3-(4′-methoxyphenyl)-8-hydroxy-9-methoxy-4a,5,6,10b-tetrahydro-3H-naphtho[2,1-b]pyran.

Figure 2

Key heteronuclear multiple bond (H→C) correlations of 1 and 2.

Table 1

¹H (400 MHz, CDCl₃) and ¹³C (100 MHz, CDCl₃) Nuclear Magnetic Resonance Data for 1 and 2.

Position	1		2
	δC (ppm)	δH (ppm, J in Hz)	δc (ppm)	δH (ppm, J in Hz)
1	129.0	6.19 m	128.2
2	127.8	5.91 dt (10.3, 2.4)	139.5
3	71.9	5.14 d (2.4)	125.2	7.35 d (8.4)
4			130.2	7.87 d (8.4)
4a	68.3	4.22 m	128.0
5	26.0	2.09 m, 1.89 m	130.4	7.79 d (8.8)
6	26.1	2.65 m, 2.90 m	118.7	7.13 dd (8.8,2.4)
6a	129.1
7	114.2	6.60 s	155.5
8	143.8		107.0	7.42 d (2.4)
8a			131.5
9	145.3		170.5
10	110.4	6.74 s	61.8	4.11 q (7.1)
10a	129.0
10b	36.9	3.45 s
11			13.7	0.91 t (7.1)
1′	133.1		141.5
2′, 6′	129.2	7.35 d (8.6)	128.7	7.45 m
3′, 5′	113.7	6.90 d (8.6)	128.5	7.45 m
4′	159.2		127.7	7.39 m
9-OCH3	56.0	3.88 s
4′-OCH3	55.3	3.81 s
8-OH		5.46 s

Compound 2 was isolated as a yellowish solid. As shown in Supplemental figure S1, the molecular formula was established as C₁₉H₁₆O₃ based on HR-ESI-MS [M-H]⁻ ion at m/z 291.1053 (calculated for C₁₉H₁₅O₃, m/z 291.1027). The ¹H-NMR spectrum (Supplemental figure S7) revealed an ABX spin system for trisubstituted aromatic ring of δ_H 7.42 (1H, d, J = 2.4 Hz, H-8), 7.13 (1H, dd, J = 8.8, 2.4 Hz, H-6), and 7.79 (1H, d, J = 8.8 Hz, H-5), a mono-substituted benzene ring of δ_H 7.39 (1H, m, H-4′) and7.45 (4H, m, H-2′, 3′, 5′, 6′), AB aromatic system for 2 protons as 2 doublets centered at δ_H 7.35 (1H, d, J = 8.4 Hz, H-3) and 7.87 (1H, d, J = 8.4 Hz, H-4), and a oxyethyl group H-OCH₂CH₃ of δ_H 4.11 (2H, q, J = 7.1 Hz, ROCH₂ CH₃) and 0.91 (3H, t, J = 7.1 Hz, ROCH₂ CH₃ ). The ¹³C-NMR spectrum (Table 1) revealed the presence of 19 distinct signals (Supplemental figure S8), including 1 oxyethyl group carbon atoms, an ester group, and 16 aromatic carbon signals. The key HMBC spectrum (Supplemental figure S10) that exhibited correlations from H-3 to C-1, C-1′, H-2′, 6′ to C-1′, C- 3′, 5′, C-2 and H-4 to C-2, C-5 confirmed the mono-substituted aromatic ring connected to C-2. The correlation (Figure 2) from H-5 to C-7, C-8a, H-6 to C-7, C-4a and H-8 to C-7, C-4a showed the hydroxy group connected to C-7. NMR spectrum (Supplemental figures S9-S10) indicated that compound 2 was determined to be 7-hydroxy-2-phenylnaphthalene-1-carboxylic acid ethyl ester.

Experimental

General

HR-ESI-MS was measured on a Bruker Daltonics micro TOF-Q II mass spectrometer (Bruker Daltoniks GmbH, Germany). NMR was recorded on a JEOL ECS 400 NMR spectrometer (at 400 MHz for ¹H and 100 MHz for ¹³C) (JEOL, Japan). Column chromatography was performed with silica gel (200-300 mesh and 300-400 mesh, Qingdao Haiyang chemical co., Ltd, China), Toyopearl HW-40F (Tocho corporation, Japan), and Sephadex LH-20 (Pharmacia Biotech, Switzerland).

Plant Material

Rhizomes of Musa basjoo (16040803) were purchased from Yunnan of China in 2016, which were identified as the dried rootstock of Musaceae Musa basjoo Sied.et Zu cc. by associate professor Qing de Long from Department of Medicinal Botany and Pharmacognosy, and the specimens (No. BJ001) were preserved in the Guizhou Provincial Key Laboratory of Pharmaceutical Preparations, Guizhou Medical University.

Extraction and Isolation

The dried rhizomes of Musa basjoo (20 kg) were extracted with 70% ethanol (3 times). After evaporation of the solvent under reduced pressure, the crude extract (1129.2 g) was dissolved in water and separated through a D101 macroporous resin, following elution in proper order by water, 50% ethanol, and 95% ethanol. The 50% ethanol extract (42 g) was chromatographed on silica gel column chromatography eluted gradiently with a gradient of dichloromethane–methanol (15:1, 10:1, 8:1, 6:1, 4:1, 2:1, 1:1, v/v) to give 10 fractions (Fr.1~10). The Fr.2 (1.51 g) was subjected to silica gel column chromatography eluted with petroleum ether–chloroform (5:1 v/v) to obtain 6 subfractions (Fr.2.1~2.6). Fr.2.2 was separated by silica gel column chromatography, eluted with petroleum ether–acetone (4:1 v/v) and further purified by Sephadex LH-20 column (chloroform–methanol, 1:1) and Toyopearl HW-40F column (methanol) to give 1 (13.9 mg) and 2 (4.0 mg).

Rel-(3S,4aR,10bR)-3-(4′-Methoxyphenyl)-8-Hydroxy-9-Methoxy-4a,5,6,10b-Tetrahydro-3H-Naphtho[2,1b]pyran (1)

White, amorphous powder.

¹H and ¹³C NMR (CDCl₃): Table 1.

HR-ESI-MS m/z: 361.1415 [M + Na]⁺ calculated for C₂₁H₂₂O₄Na, 361.1410.

7-Hydroxy-2-Phenylnaphthalene-1-Carboxylic Acid Ethyl Ester (2)

Yellowish solid.

¹H and ¹³C NMR (CDCl₃): Table 1.

HR-ESI-MS m/z: 291.1053 [M-H]⁻ calculated for C₁₉H₁₅O₃, 291.1027.

Supplemental Material

Supplementary material - Supplemental material for Two New Compounds from Rhizomes of Musa basjoo

Supplemental material, Supplementary material, for Two New Compounds from Rhizomes of Musa basjoo by Li Jiang, Yang Wang, Min-Hui Zhu, Lin Zheng, Lin-Zhen Li, Ai-Min Wang, Chang-Hu Lin, Ting Liu and Yong-Jun Li in Natural Product Communications