Polyacetylenes from the Roots of Swietenia macrophylla King

A phytochemical investigation of the roots of Swietenia macrophylla led to the isolation of seven polyacetylenes, including five new compounds (1–5) and two known ones (6–7). Their structures were elucidated by extensive spectroscopic analysis and detailed comparison with reported data. All the isolates were tested for their cytotoxicity against the human hepatocellular carcinoma cell line BEL-7402, human myeloid leukemia cell line K562, and human gastric carcinoma cell line SGC-7901. Compounds 1 and 6 showed moderate cytotoxicity against the above three human cancer cell lines with IC50 values ranging from 14.3 to 45.4 μM. Compound 4 displayed cytotoxicity against the K562 and SGC-7901 cancer cell lines with IC50 values of 26.2 ± 0.4 and 21.9 ± 0.3 μM, respectively.

Swietenia macrophylla, a perennial deciduous timber tree that reaches a height of up to 50 m [26], is native to Central and South America [27] and widely distributed in West India, Malaysia, and southern China [28,29].Antecedent chemical investigations on S. macrophylla have focused mostly on the aboveground parts and their bioactive limonoids [30].It is necessary to expand the scope of research on S. macrophylla and discover or develop additional biologically active constituents of this plant genus [31].Our recent study on the roots of S. macrophylla led to the isolation of a series of xanthones, limonoids, and other chemical components [32,33].As a continuation of our studies on the biologically active agents from this plant, five new and two known acyclic C 18 -C 14 polyacetylenes have been further isolated here, and their cytotoxic activities against the human hepatocellular carcinoma cell line BEL-7402, human myeloid leukemia cell line K562, and human gastric carcinoma cell line SGC-7901 were investigated.In this paper, the isolation, structural elucidation, and cytotoxicity of these compounds are reported as follows.

General Experimental Procedures
The 1 H, 13 C, and 2D NMR spectra were recorded on a Bruker AV III spectrometer (Bruker, Bremen, Germany) at either 500 MHz ( 1 H) or 125 MHz ( 13 C) using TMS as an internal standard.The HRMS were measured with an API QSTAR Pulsar mass spectrometer (Bruker).The UV spectra were performed on a Shimadzu UV-2550 spectrometer (Beckman, Brea, CA, USA).The IR absorptions were obtained on a Nicolet 380 FT-IR instrument (Thermo, Pittsburgh, PA, USA) using KBr pellets.The optical rotation was measured on a Rudolph Autopol III polarimeter (Rudolph, Hackettstown, NJ, USA).Silica gel (60-80, 200-300 mesh, Qingdao Marine Chemical Co. Ltd., Qingdao, China), ODS gel (20-45 µm, Fuji Silysia Chemical Co. Ltd., Durham, NC, USA), and Sephadex LH-20 (Merck, Darmstadt, Germany) were used for column chromatography.The TLC was conducted on pre-coated silica gel G plates (Qingdao Marine Chemical Co.Ltd.), and spots were detected by spraying with 10% H 2 SO 4 in EtOH followed by heating.

Plant Material
The plant material was collected at the Chinese Academy of Tropical Agricultural Sciences, Haikou, China, in April 2014 and was identified as Swietenia macrophylla by Dr. Jun Wang (Institute of Tropical Bioscience and Biotechnology, Chinese Academy of Tropical Agricultural Sciences).The voucher specimen (No.DYTHXM201404) was deposited at the Institute of Tropical Bioscience and Biotechnology, Chinese Academy of Tropical Agricultural Science.

Bioassay of Cytotoxic Activity
MTT assay, originally described by Mosmann [39], was used to quantitate the cytotoxicity of compounds 1-7.The human hepatocellular carcinoma cell line BEL-7402, human myeloid leukemia cell line K562, and human gastric carcinoma cell line SGC-7901, which were obtained from the cell bank of type culture collection of the Chinese Academy of Sciences, Shanghai Institute of Cell Biology, were cultured in RPMI 1640 medium supplemented with 10% fetal bovine serum at the conditions of 37 • C, 5% CO 2 , and 90% humidity.Paclitaxel was used as the positive control and DMSO was used as the negative control.Different concentrations of the test sample (each had triplicate wells) were designed as 0.1, 0.4, 1.6, 6.3, 25, and 100 µM.The logarithmic phase cells (90 µL) were selected to seed onto the 96-well plates at a concentration of 5 × 10 4 cell/mL.Then, 15 µL of MTT dissolved in PBS at 5 mg/mL was added to each well, and the system was incubated at 37 • C for 4 h.After that, the supernatant was discarded, and 100 µL of DMSO was added into each well.Finally, the OD value was measured by a MK3 Microtiter plate reader at a wavelength of 490 nm.

Conclusions
Seven polyacetylenes were isolated from the roots of S. macrophylla.Their structures were determined by spectroscopic analysis and comparing data in the literature.Furthermore, compounds 1 and 6 displayed weak cytotoxicity in the BEL-7402, SGC-7901, and K562 cell lines, and compound 4 showed a weak cytotoxic effect in the SGC-7901 and K562 cell lines.

Table 3 .
Cytotoxicity of compounds 1-7 in the human cancer cell lines.
a Positive control.