Two New Aryltetralin Lignans from the Roots of Dolomiaea souliei

Two new aryltetralin-type lignans, dolomiaeasin A (1) and dolomiaeasin B (2), were isolated from the roots of Dolomiaea souliei. Their structures were elucidated by means of various spectroscopic analyses. The cytotoxicities of 1 and 2 were tested by the MTT method, and both compounds showed no significant cytotoxic activities against the A549 and A2780 human cancer cell lines. This is the first time that aryltetralin-type lignans were isolated from the genus Dolomiaea.


Introduction
Dolomiaea souliei (Franch.) Shih belongs to the Dolomiaea genus in the family Compositae, and is mainly distributed in western Sichuan and eastern Tibet [1]. D. souliei is a traditional Chinese medicine which is well known for its medicinal uses in relieving pain and different indigenous diseases [2]. Previous studies indicated that D. souliei is a rich source of sesquiterpenes, triterpenes and lignans, some of which have been reported to exhibit anti-tumor, anti-ulcer and anti-inflammatory activities [3,4]. In our search for biologically active compounds, we investigated the chemical constituents of this plant. In this study, two new aryltetralin-type lignans, dolomiaeasin A (1) and dolomiaeasin B (2), were isolated from the roots of D. souliei. Their structures were elucidated using UV, IR, 1D, 2D NMR and HR-ESI-MS experiments, while the configurations of both compounds were deduced by comparison of their CD data with those reported in the literature. This is the first report of aryltetralin-type lignans isolated from the genus Dolomiaea. Finally, the cytotoxicities of 1 and 2 were tested against the A549 and A2780 human cancer cell lines. indicated that 1 resembled the structure of (+)-cycloolivil [5].

Structural Identification
The disappearance of H-8', sharp downfield shift of C-8' (δ: 75.1) and obvious change of H-7' (a singlet) in 1 indicated that H-8' of (+)-cycloolivil was substituted by a group. When combined with HR-ESI-MS data, this group was inferred as a hydroxyl. A negative Cotton effect at 290 nm suggested that H-7' was α (S configuration at C-7') [6]. The remaining chiral centers at C-8' and C-8 were assigned as 8'R and 8R configurations, for the CD data of 1 [(290 (−1.8), 271 (+0.5), 237 (+0.7)] being very similar to that of (+)-cycloolivil 6-O-β-D-glucoside which has the same chiral centers [7]. The results were in good accordance with the energy minimized conformation, which was obtained from a molecular modeling program in Discovery Studio 3.1. On basis of the above evidence, compound 1 was inferred as a structure with 7'S, 8'R and 8R configurations, and named dolomiaeasin A (Figure 1 HMBC, H to C Differences in chemical shift values and CD signals suggested a different stereochemistry of 2. A positive Cotton effect at 291 nm revealed that H-7' was β (R configuration at C-7') [6]. An opposite configuration of 7'-phenyl and 8'-CH 2 OH was inferred for there was no NOE correlation observed between H-9' and H-2'/H-6', i.e., the configuration at C-8' was 8'S. Differences in rotation values, CD and NMR revealed that these two compounds were not enantiomers. Thus, the remaining chiral centre at C-8 was inferred as 8R. On basis of the above deductions, the elucidation of compound 2 was characterized as 7'R, 8'S and 8R, and named dolomiaeasin B (Figure 2). HMBC, H to C

Cytotoxic Activity
While studies have indicated that an aryltetralin lactone (e.g., podophyllotoxin) and its derivatives were potent anticancer agents [8], compounds 1 and 2 showed no significant cytotoxic activities, with IC 50 values exceeding 20 μM, when assessed against the A549 and A2780 human cancer cell lines.

Plant Material
The

Bioassays
Compounds 1 and 2 were assessed by the MTT method using the A549 and A2780 human cancer cell lines. Cells were seeded in 96-well plates and incubated at 37 °C, 5% CO 2 for 24 h. Then 150 μL of five different concentrations (0.2, 0.5, 1, 2, 5, 10 μM) for each compound (dissolved in DMSO) were added to each well and incubated for another 24 h. After removing the supernatant, 150 μL of MTT (0.5 mg/mL) were added to each well and incubated for 4 h. Finally, the liquid in the wells was removed, DMSO (150 μL) was added, and the absorbance at 570 nm was recorded on a microplate reader (Wellscan MK3, Labsystems Dragon, Helsinki, Finland).

Conclusions
Two new aryltetralin-type lignans, dolomiaeasin A (1) and dolomiaeasin B (2), were isolated from the roots of Dolomiaea souliei. Both compounds showed no significant cytotoxicities against the A549 and A2780 human cancer cell lines. To the best of the authors' knowledge, this is the first report of aryltetralin-type lignans from the genus Dolomiaea.