Four New Sesquiterpenoids from the Roots of Diarthron Tianschanica with Their Antineoplastic Activity

Four new sesquiterpenoids, known as diarthronchas A–D (1–4), and one known daphnauranol B (5) were isolated from the methanol extract of the roots of Diarthron tianschanica. The compounds structures were determined on the basis of spectroscopic data. All of the isolated compounds were profiled for their antineoplastic activity.


Introduction
Diarthron tianschanica (Pobed.) Kit Tanis, a member of the genus Diarthron (Thymelaeaceae), is only observed in the Zhaosu County of Xinjiang Uygur Autonomous Region, China [1]. The roots of D. tianschanica have been used in folk medicine to cure a wide variety of ailments, including coughs; asthma [2]; bronchitis [3]; and tuberculosis of the skin, bone, and epididymis [4]. Previously, our group examined the chemical constituents of D. tianschanica, resulting in the identification of lignans, coumarins, and diarylpentanols [5,6]. As part of our ongoing phytochemical study, we further investigated the chemical constituents from the roots of D. tianschanica and obtained four new sesquiterpenoids, known as diarthronchas A-D (1-4), and one known daphnauranol B (5) [7] (Figure 1). In this paper, we elucidate the structure of these new compounds and their antineoplastic activity.

Antineoplastic Activity of Compounds 1-5
All of the isolated compounds were tested in vitro for their cytotoxic activity against HepG-2, MCF-7, and HeLa human cancer cell lines, with paclitaxel serving as a positive control. The results ( Table 3) showed that compounds 1, 3, and 5 were moderately cytotoxic against HepG-2 cells with inhibitory concentration 50% (IC 50 ) values at 18. 9, 22.5, and 20.3 µM, respectively, while compound 2 was weakly cytotoxic with an IC 50 value of 41.3 µM. Furthermore, compounds 2 and 5 exhibited weak cytotoxicity against HeLa cells with IC 50 values of 39.6 and 29.6 µM, respectively. None of the compounds had activity against MCF-7 cell lines.

General Experimental Procedures
Optical rotation data were obtained using a Perkin-Elmer 341 digital polarimeter (PerkinElmer, Norwalk, CT, USA). CD spectra were obtained using a JASCO J-815 spectropolarimeter (JASCO, Easton, Md., USA). UV and IR spectra were obtained using Shimadzu UV2550 and FTIR-8400S spectrometers (Shimadzu, Kyoto, Japan), respectively. nmR spectra were obtained using a Bruker AV III 600 nmR spectrometer (Bruker, Billerica, German) with chemical shift values presented as δ values and TMS (Tetramethylsilane) as the internal standard. HRESIMS was performed using an LTQ-Orbitrap XL spectrometer (Thermo Fisher Scientific, Boston, MA, USA). Column-chromatography (CC) was performed using silica gel (100-200 mesh, Qingdao Marine Chemical Plant, Qingdao, China) and Sephadex LH-20 (Pharmacia, Uppsala, Sweden). Precoated silica gel GF254 plates (Zhi Fu Huang Wu Pilot Plant of Silica Gel Development, Yantai, China) were used for TLC. All of the solvents used were of analytical grade (Beijing Chemical Plant, Beijing, China).

Plant Material
The roots of Diarthron tianschanica were collected in September 2013 from Zhaosu city, Xinjiang Autonomous Region, China, and were identified by Prof Xiao-Guang Jia, Department of Pharmaceutical Chemistry, Xinjiang Institute of Chinese and Ethnic Medicine. A voucher specimen (NO. 13094) was deposited at the Xinjiang Institute of Chinese and Ethnic Medicine.

Isolation and Purification of Compounds 1-5
The roots of D. tianschanica (10 kg) were soaked with MeOH at room temperature (3 × 40 L, 3 h each) and were extracted three times under reflux. Removal of the MeOH under reduced