*3.2. Plant Material*

The plant was collected from Loushanguan in Zunyi City, People's Republic of China, in October 2013, and identified as *Mahonia imbricata* by Prof. Jian-Wen Yang, Zunyi Medical University. A voucher specimen (20131020) was deposited with the Herbarium of the School of Pharmacy, Zunyi Medical University.

#### *3.3. Extraction and Isolation*

The air-dried and finely ground stems of *Mahonia imbricata* (30 kg) were extracted with 90% ethanol for three times at room temperature (5 days each). The ethanol extract was evaporated under reduced pressure to get a crude extract (2 kg), which was further suspended in acidic water (pH 3–4) and then filtered to obtain the filtrate. The filtrate was alkalized to pH 9–10 and extracted successively with CHCl3 and n-BuOH (each 3 times), respectively. The CHCl3 extract was subjected to silica gel column chromatography (80 mm × 600 mm, 1400 g, 300–400 mesh), eluted with a gradient of CHCl3/MeOH (*v*/*v* = 100:1–1:100) to afford ten fractions (A–J). Fraction C was separated by Sephadex LH-20 column chromatography (CHCl3/MeOH = 1:1) and divided into 15 subfractionses (fraction A.1–A.15). Mahimbrine A (3.8 mg) was obtained from fraction A.6 by sempipreparative HPLC (MeOH/H2O = 55:45, flow rate 4 mL/min).

#### *3.4. Spectral Data*

Mahimbrine A: brown gum; UV (CH3OH): 230 (2.68), 288 (2.43); IR (KBr): 3434, 2920, 1633, 1590, 1512, 1384, 1292, 1026, 810; 1H and 13C NMR data, Table 1. HR-ESI-MS: *m*/*z* 406.1632 [M + H]+, C24H24NO5 +, calcd. 406.1649; *m*/*z* 811.3226 [*2*M + H]+, calcd. 811.3225.

#### **4. Conclusions**

In conclusion, mahimbrine A has been isolated from the endemic plant of *Mahonia imbricate* and its chemical structure has been elucidated. It displays an interesting structure with a dihydroisoquinoline moiety bound to a rare benzotropolone ring system. The structure of mahimbrine A was elucidated by HR-ESI-MS, 1D, 2D NMR and a biosynthetic pathway was proposed. Mahimbrine A did not show antimicrobial activity at the tested concentration of 1 mg/mL. The interesting structural architecture of this natural product might find further applications.

**Supplementary Materials:** 1D, 2D NMR spectra and HR-ESI-MS of mahimbrine A are available online.

**Author Contributions:** M.-S.Z. conceived and designed the experiments; Y.D. isolation and purification of mahimbrine A from *Mahonia imbricata*; S.-B.F. performed the biological activity tests. D.-L.G. characterization of 2D NMR spectra; S.-J.X. identification of mahimbrine A and writing of the manuscript.

**Acknowledgments:** This work was financially supported by the National Natural Sciences Foundation of China (No. 31560102), and the United Fund of Guizhou Province, Zunyi Medical University and Zunyi City (QKHJ-LKZ [2010] 26).

**Conflicts of Interest:** The authors declare no conflict of interest.
