A New Coumaronochromone From Boerhaavia diffusa

Abstract

A new coumaronochromone derivative, boeravinone R (5,7-dihydroxy-3´-methoxy-coumaronochromone) (1), together with 4 known compounds: β-sitosterol (2), oleic acid (3), physcion (4), and glutinol (5), was isolated from Boerhaavia diffusa L. growing in Vietnam. Their structures were determined based on spectroscopic and physicochemical analyses.

Boerhaavia diffusa L., a member of the genus Boerhaavia (Nyctaginaceae), including about 40 species, is widely distributed in tropical and subtropical areas and warm climates. ¹ Boerhaavia diffusa is a perennial herbaceous plant which grows throughout the tropical regions of Africa and Asia. It has been used in folklore medicine of Vietnam, India, and Nepal for the treatment of various diseases such as jaundice, anemia, asthma, dyspepsia, nephrotic syndrome, convulsions, enlargement of spleen, abdominal pain, abdominal tumor, or urinary and kidney disorder. Previous phytochemical investigations indicated that B. diffusa L. extract contains a wide range of compounds including rotenoids, flavonoids, steroids, alkaloids, xanthones, lignans, and phenolic compounds.

Herein, we report the isolation and structural elucidation of a new coumaronochromone derivative, boeravinone R (5,7-dihydroxy-3′-methoxycoumaronochromone) (1), together with 4 known compounds: β-sitosterol (2), oleic acid (3), physcion (4), and glutinol (5) (Figure 1).

Figure 1

Chemical structure and key HMBC correlations of 1.

Boeravinone R (5,7-dihydroxy-3′-methoxycoumaronochromone) 1 was isolated as a yellow amorphous powder. Its molecular formula was determined as C₁₆H₁₀O₆ through the pseudomolecular ion peak m/z 343.0466 [M + HCOO]⁻ in the high-resolution electrospray ionization-mass spectrometry (HR-ESI-MS), calcd. 343.0459 for C₁₆H₁₀O₆ and supported by ¹³C nuclear magnetic resonance (NMR) and distortionless enhancement by polarization transfer (DEPT) spectra. The ¹H- and ¹³C-NMR spectra suggested an isoflavone structure of 1. A proton signal at 12.79 ppm indicated the presence of C-5 chelated hydroxyl group. A pair of meta-coupled doublets were assigned to the A-ring protons of a 5,7-dioxygenated isoflavone at δ_H 6.31 (d, J = 2.0 Hz, H-6) and 6.58 (d, J = 2.0 Hz, H-8), and an ABC system based on 3 aromatic protons at δ_H 7.14 (d, J = 8.0 Hz, H-4′), 7.40 (t, J = 8.0 Hz, H-5′), and 7.51 (d, J = 8.0 Hz, H-6′) belonged to the B-ring protons of the isoflavone with 2′,3′-dioxygenation. Moreover, the absence of a proton signal of H-2 at circa 8 ppm showed the coumaronochromone skeleton. The ¹H-NMR spectrum also indicated the presence of a methoxy group at δ_H 3.98 (3H, s, O-CH₃). The ¹³C-NMR spectra showed the presence of a conjugated carbonyl at δ_C 178.5 (C-4), 9 aromatic quaternary carbons at δ_C 164.7 (C-2), 163.9 (C-7), 162.2 (C-5), 154.9 (C-8a) , 144.7 (C-3′), 137.3 (C-2′), 123.5 (C-1′), 102.8 (C-4a), and 97.2 (C-3), 5 aromatic methine carbons at δ_C 126.5 (C-5′), 112.6 (C-6′), 109.0 (C-4′), 99.9 (C-6), and 95.2 (C-8), and a methoxy carbon at δ_C 56.2 (O-CH₃). The spectral pattern of 1 closely resembles coccineone A, ¹² which was isolated from Boerhaavia coccinea Mill., except for a hydroxyl group at the position C-3′ in coccineone A replaced by a methoxy group in compound 1. The series of ^2,3 J _H-C correlations detected in the HMBC spectra (Figure 1) confirmed the presence of a coumaronochromone skeleton and the location of the substituents. In particular, the correlation of the signal at δ_H 7.51 (H-6′) with δ_C 97.2 (C-3), 109.0 (C-4′), and with the oxygenated carbons δ_C 137.3 (C-2′), and the correlation of δ_H 7.14 (H-4′) with δ_C 112.6 (C-6′) and 137.3 (C-2′), 144.7 (C-3′). In addition, the correlation of the signal at δ_H 3.98 (3H, s, O-CH₃) with a carbon δ_C 144.7 (C-3′) confirmed that the methoxy group must be located at C-3′ (Figure 1). Thus compound 1 was identified as 5,7-dihydroxy-3′´-methoxy- coumaronochromone or boeravinone R.

Compounds 2 to 5 were identified by comparing their physical and spectral data with the reference β-sitosterol (2), ¹³ oleic acid (3), ¹⁴ physcion (4), ¹⁵ and glutinol (5). ¹⁶

Experimental

General Procedures

NMR experiments were performed on a Bruker AM500 FT-NMR Spectrometer (Bruker Biospin, Zurich, Switzerland) using tetramethyl silane as an internal standard. The HR-MS spectra were measured in the ESI mode on Agilent 6530 Accurate-Mass Q-TOF LC/MS spectrometer (Agilent Technologies, USA). Thin-layer chromatography was performed on precoated plates DC Alufolien 60 F254 and visualized by UV light (254 or 360 nm) and sprayed with 2% H₂SO₄ in EtOH. Column chromatography was carried out with normal phase (silica gel 0.04-0.063 nm, 0.063-0.200 nm, Merck Ltd.).

Plant Materials

The roots of B. diffusa L. were collected in Da Nang on May 2016 and identified by Phan Van Truong at National Institute of Medicinal Materials (NIMM). A voucher specimen has been deposited at NIMM under the voucher specimen number 10 591.

Extraction and Isolation

Dried and powdered roots of Boerhaavia diffusa L (1.75 kg) were extracted with ethanol 95% under reflux (3 × 3 hours). The ethanol extract was combined and concentrated under vacuum at 55°C. The obtained residue (146.86 g) was suspended in water and successively partitioned with n-hexane, EtOAc, and n-BuOH. The n-hexane residue (14.06 g) was subjected to silica gel column chromatography (CC) with a gradient solvent system (n-hexane/ethyl acetate 100:1; 50:1; 20:1; 10:1, v/v) to give 9 main fractions (H1-H9). The fraction H3 was subjected to repeated silica gel CC with a gradient solvent system (n-hexane/ethyl acetate 100:1; 50: 1) to yield compound 3 (16 mg). The fraction H4 was applied to repeated silica gel CC with a gradient solvent system (n-hexane/ethyl acetate 50: 1; 20:1; 9:1) to yield compound 2 (23 mg). The fraction H5 was purified by silica gel column with a gradient of n-hexane and ethyl acetate (50:1, 20:1 and 9:1) to yield compound 4 (14 mg). The fraction H7 was fractionated by a silica gel CC with a gradient solvent system (dichloromethane/methanol 50:1, 20:1; 10:1 and 5:1) to yield compounds 5 (11 mg). The fraction H8 was subjected to a silica gel CC with a gradient solvent system (dichloromethane/methanol 100:0; 50:1; 20:1 and 9:1) to yield compound 1 (8 mg).

Boeravinone R (1)

Yellow amorphous powder.

¹H-NMR (500 MHz, DMSO-d₆ ): δ 12.79 (s,-OH), 7.51 (1H, d, J = 8.0 Hz, H-6′), 7.40 (1H, t, J = 8.0 Hz, H-5′), 7.14 (1H, d, J = 8.0 Hz, H-4′), 6.58 (1H, d, J = 2.0 Hz, H-8), 6.31 (1H, d, J = 2.0 Hz, H-6), 3.98 (3H, s, OCH₃).

¹³C-NMR (125 MHz, DMSO-d₆ ): δ 178.5 (C-4), 164.7 (C-2), 163.9 (C-7), 162.2 (C-5), 154.9 (C-8a), 144.7 (C-3′), 137.3 (C-2′), 126.5 (C-5′), 123.5 (C-1′), 112.6 (C-6′), 109.0 (C-4′), 102.8 (C-4a), 99.9 (C-6), 97.2 (C-3), 95.2 (C-8), 56.2 (O-CH₃).

HR-ESI-MS: m/z 343.0466 [M + HCOO]⁻ (calcd. for C₁₆H₁₀O₆, 343.0459)