Two rare flavonoid-2-(2-phenylethyl)chromones and five new dimeric 2-(2-phenylethyl)chromones were isolated from ethanol extract of agarwood of Aquilaria walla by LC-MS-guided fractionation procedure. Their structures were established based on extensive spectroscopic methods including HRESIMS, 1D and 2D NMR, as well as by comparison with the literature. Compound 1 showed cytotoxic activity against five human cancer cell lines with IC50 values ranging from 13.40 to 28.96 μM with cisplatin as the positive control.
Introduction
Agarwood is the resinous heartwood containing fragrant resin obtained from the trees of Aquilaria and Gyrinops species after they get physical injury or microbial attack [1,2]. To date, twenty-one species in the genus Aquilaria and nine species in the genus Gyrinops have been documented scattered extensively in south China, Myanmar, Thailand, Vietnam, India, and other Southeast Asia countries [2,3]. Aquilaria walla is recorded mainly distributing in the wet and intermediate zones of Sri Lanka, and minority in southwest India [4]. Its agarwood appears usually black with small white spots and has unique cool fragrance [5].
Modern phytochemistry studies indicated that 2-(2-phenylethyl)chromones are one of the main characteristic components in agarwood exhibiting various of biological activities [[6], [7], [8], [9], [10]]. Previous chemical studies on A. walla have led to identify three new 2-(2-phenylethyl)chromones, especially including an unprecedented flavonoid-2-(2-phenylethyl)chromone, which displayed significant cytotoxicity versus five human cancer cell lines [5,[11], [12], [13]]. In order to look for more interesting structures from agarwood of A. walla, the continuing research was carried out, and two rare flavonoid-2-(2-phenylethyl)chromones (1,2) and five new dimeric 2-(2-phenylethyl)chromones (3–7) were further isolated. In vitro cytotoxic assessment using MTT method showed that compound 1 presented cytotoxic activity versus the same five human cancer cell lines with IC50 values ranging from 13.40 to 28.96 μM.
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Results and discussion
Compound 1 was obtained in the form of yellow gum. Its molecular formula was established as C35H32O12 based on the HRESIMS pseudomolecular ion peak at m/z 667.1791 [M + Na]+ (calcd. For C35H32NaO12, 667.1786) and 13C NMR data, indicating 20 indices of hydrogen deficiency. The 1H NMR data of 1 (Table 1) exhibited resonances characteristic for a set of para-substituted phenyl group [δH 6.89 (2H, d, J = 8.5 Hz, H-2′, 6′), δH 6.65 (2H, d, J = 8.6 Hz, H-3′, 5′)], a set of ABX coupling aromatic
General
The HRESIMS were measured with an API QSTAR Pulsar mass spectrometer (Bruker, Germany). 1H, 13C, and 2D NMR spectra were measured on Bruker AV III spectrometer (Bruker, Germany) at 500 MHz (1H) and 125 MHz (13C). Chemical shifts were referenced to the solvent residual peaks. Optical rotations were measured on a Modular Circular Polarimeter 500 polarimeter (Anton Paar, Austria). ECD and UV spectra were recorded on a MOS-500 spectrometer (Biologic, France). Analytic HPLC was performed with an
Conclusion and discussion
Two rare flavonoid-2-(2-phenethyl)chromones (1, 2) and five new dimeric 2-(2-phenylethyl)chromones (3–7) were identified from agarwood of A. walla. By review bi-2-(2-phenylethyl)chromone derivatives from agarwood from different original plants, only three flavonoid-2-(2-phenlethyl)chromone derivatives have been isolated from agarwood of A. walla including the one reported in our previous publication [5], and none of them have been reported from agarwood from other species so far. Accordingly,
CRediT authorship contribution statement
Leyao Chen was responsible for investigation and writing-original draft; Huiqin Chen was responsible for project administration and writing-review ans editing; Caihong Cai, Jingzhe Yuan and Cuijuan Gai were responsible for investigation; Shoubai Liu was responsible for project administration; Wenli Mei was responsible for funding acquisition, writing-review and editing; Haofu Dai was responsible for funding acquisition and supervision.
Declaration of Competing Interest
There are no conflicts of interest to declare.
Acknowledgments
This work was financially supported by National Key Research and Development Program (2018YFC1706404), and Agriculture Research System of China of MOF and MARA (cars-21).
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