Abstract
Four new dimeric 2-(2-phenylethyl)chromones (1–4) were isolated from the agarwood of Aquilaria crassna in Laos. All structures were determined based on extensive spectroscopic methods including IR, UV, 1D and 2D NMR, ECD, and HRMS. The structures of compounds 1–3 were characterized by two C-O-C linkages between the two chromone moieties, while compound 4 featured one C-O-C linkage and one C
C bond connecting with two chromone moieties. Compounds 1 and 3 showed weak cytotoxicity against human hepatocellular carcinoma BEL-7402 cell line with IC50 values of 44.68 and 42.10 μΜ.
Graphical abstract
The structures of four new compounds and the sample of agarwood agarwood of Aquilaria crassna in Laos.
Introduction
Agarwood is the resinous heartwood from Aquilaria or Gyrinops species (Thymelaeaceae), which also called eaglewood, gaharu, jinko, aloeswood, kanankoh, kyara, jinkoh, chenxiang or oud in different cultures [1,2]. Aquilaria crassna Pierre ex Lecomte, one of the important resources of agarwood in the world market, mainly grow in Laos, Thailand, Cambodia and Vietnam [3].
2-(2-Phenylethyl)chromone, an important type of characteristic component in agarwood, which was also the main class of active ingredient of agarwood [1,2,4]. So far, >150 2-(2-phenylethyl)chromone derivatives have been identified from agarwood of some Aquilaria species [[4], [5], [6], [7], [8]], while, vast majority of these 2-(2-phenylethyl)chromones were monomers, not polymers. Since Japanese researchers acquired a few 2-(2-phenylethyl)chromone polymers from agarwood in Kalimantan in the 1980s [[9], [10], [11], [12]], there haven't been identified any new 2-(2-phenylethyl)chromone polymer from agarwood until 2017. From the year 2017 to now, 30 new dimeric 2-(2-phenylethyl)chromones have been identified from agarwood of Aquilaria sinensis and Aquilaria crassna respectively [[5], [6], [7], [8]], which could be a breakthrough on the phytochemical study of agarwood. Due to the great separation potential of chromone polymer from agarwood, further studies on this type of compounds could be performed seriously in the future.
As an indispensable part of our study on bioactive chemical constituents of agarwood from A. crassna, previous investigation on agarwood of A. crassna from Laos, led to the isolation of a series of 2-(2-phenylethyl)chromone derivatives [5,6,13,14] and some eremophilane sesquiterpenes [[13], [14], [15]]. During our ongoing research for bioactive 2-(2-phenylethyl)chromone derivatives, four dimeric chromones (1–4) were obtained from agarwood of A. crassna. All dimeric chromones were evaluated for their cytotoxic activity against three human cancer cell lines in vitro. Compounds 1 and 3 showed weak cytotoxicity against human hepatocellular carcinoma BEL-7402 cell line with IC50 values of 44.68 and 42.10 μΜ. This paper described the structural elucidation of the four new dimeric 2-(2-phenylethyl)chromones (1–4), as well as their cytotoxic activities.
Section snippets
General
HRESIMS were measured with an API QSTAR Pulsar mass spectrometer (Bruker). 1D and 2D NMR spectra were recorded on Bruker AV III spectrometer (Bruker) (1H NMR at 500 MHz and 13C NMR at 125 MHz), TMS as the internal standard. Optical rotation was measured on a Rudolph Autopol III polarimeter. UV spectra were performed on a Shimadzu UV-2550 spectrometer (Beckman, America). IR absorptions were obtained on a Nicolet 380 FT-IR instrument (Thermo) using KBr pellets. CD data were collected using a
Results and discussion
The EtOAc extract of the agarwood A. crassna was separated successively by repeated column chromatography and semi-preparative HPLC purification to afford four new bi-2-(2-phenylethyl)chromone derivatives (1–4) (Fig.1).
Compound 1 was obtained as white amorphous powder. Its molecular formula was found to be C35H30O9 on the basis of HREIMS (m/z 617.1828 [M + Na]+, calcd for C35H30NaO9, 617.1782). Its IR spectrum showed absorptions of OH groups (3438 cm−1) and α, β-unsaturated carbonyl groups
Acknowledgements
This work was financially supported by Central Public-interest Scientific Institution Basal Research Fund for Innovative Research Team Program of CATAS (17 CXTD-15), Innovative Research Team Grant of the Natural Science Foundation of Hainan Province (No.2017CXTD020), and China Agriculture Research System (CARS-21).
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