Chemical Constituents from the Flower of Hosta plantaginea with Cyclooxygenases Inhibition and Antioxidant Activities and Their Chemotaxonomic Significance

Two new phenolic glucosides, hostaflavanone A (1) and anti-1-phenylpropane-1,2-diol-2-O-β-d-glucopyranoside (2), together with six known compounds, anti-1-phenylpropane-1,2-diol (3), phenethyl-O-β-d-glucopyranoside (4), phenethanol-β-d-gentiobioside (5), phenethyl-O-rutinoside (6), (1S, 3S)-1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (7), and (1R, 3S)-1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (8), were isolated from the flower of Hosta plantaginea, and their structures were elucidated by nuclear magnetic resonance (NMR), high resolution electrospray ionization mass spectroscopy (HRESIMS), and circular dichroism (CD) analyses. The cyclooxygenases (COX-1 and COX-2) inhibition and antioxidant activities of compounds 1 and 4–6 were investigated, and they showed moderate cyclooxygenases inhibition activities. Moreover, only compound 1 exhibited moderate antioxidant activity, with an IC50 value of 83.2 μM, while 4–6 showed insignificant activity with IC50 values of 282, 257, and 275 μM, respectively. This is the first report of compounds 3 and 5–8 from the Liliaceae family. The chemotaxonomic significance of the isolated compounds was also summarized.


Introduction
The genus Hosta belongs to the family Liliaceae, with approximately 40 species distributed in the temperate and subtropical zones of Asia [1].The ethnopharmacological and chemotaxonomic significance of the genus Hosta led us to investigate the chemical constituents of one of its species, namely Hosta plantaginea (Lam.)Aschers, which was a medicinal and ornamental plant in China.Its flowers are commonly used as a traditional Mongolian medicine in China for the treatment of sore throat, mute, lung heat, and toxic heat [2].Previous phytochemical studies on H. plantaginea afforded structurally-diverse and biologically-active compounds, such as steroidals, alkaloids, flavonoids, and monoterpenes, some of them showed potent anti-inflammatory, cytotoxic, antibacterial, antiviral, and antioxidant activities [3][4][5][6][7][8].These facts encouraged us to investigate new and bioactive secondary metabolites from H. plantaginea.In the present study, we had isolated and elucidated two new phenolic glucosides (1 and 2), and six known ones from the ethanol extract of the flowers of H. plantaginea.
of the flowers of H. plantaginea.Herein, we report the isolation, structure elucidation, as well as the cyclooxygenases' (COX-1 and COX-2) inhibition and antioxidant activities of compounds 1-8 (Figure 1).This is the first report of compounds 3 and 5-8 from the Liliaceae family.The chemotaxonomic significance of the isolated compounds was also summarized.

Biological Activities
Compounds 1 and 4-6 exhibited moderate activity to that of the standard reference drug, and were tested for their inhibitory activity against ovine COX-1 and COX-2 (Table 3), with IC 50 values of 15.5-41.2 and 31.7-45.4µM, while the IC 50 values of the positive control celecoxib were 9.0 and 1.0 µM, respectively.While compounds 1 and 4 were more active against COX 1 (SI values <<1), compounds 5 and 6 were about equally potent against both COX enzymes with SI values of about 1.
The antioxidant activity of compounds 1, and 4-6 was measured by the 1,1-diphenyl-2picrylhydrazyl (DPPH) method and the results are summarized in Table 3.Only compound 1 exhibited moderate antioxidant activity, with an IC 50 value of 83.2 µM, while 4-6 showed insignificant activity with IC 50 values of 282, 257, and 275 µM, respectively.The IC 50 value of the positive control L-ascorbic acid was 33.9 µM.These compounds may thus, possibly together with further constituents, contribute to the biological activity of H. plantaginea.
The phenylpropanoids have been previously isolated from the Hosta species, including trans-p-hydroxy-cinnamic acid from H. ventricosa [23], p-coumaramide, trans-N-p-coumaroyltyramine, and cis-N-coumaroyltyramine from H. longipes [24], feruloyltyramine, and lyciumide A from H. ensata [25].In addition, the phenethanols 4 and α-hydroxyacetovanillone were isolated from H. plantaginea and H. ventricosa [23], respectively.Thus, compounds 2-6 from H. plantaginea, suggesting that their occurrence could be used to verify the chemotaxonomic relationship of H. plantaginea and other species of Hosta, and also might sever as valuable chemotaxonomic makers for the identification of H. plantaginea.Further comprehensive phytochemical investigations involving an expand series of compounds could help define the chemotaxonomic significance of species belonging to genus Hosta.

Plant Materials
The flowers of Hosta plantaginea (Lam.)Aschers were collected in Shanquan town, Nanchuan district, Chongqing, People's Republic of China, in September 2014, and were identified by one of authors (Guo-yue Zhong).A voucher specimen (no.YZH201409) was deposited at the Research Center of Natural Resources of Chinese Medicinal Materials and Ethnic Medicine, Jiangxi University of Traditional Chinese Medicine, Nanchang, China.

Acid Hydrolysis and HPLC Analysis
The absolute configurations of the sugar moieties in the structures were determined by the previously described method with minor modifications [9].Compound 1 (3 mg) was hydrolyzed with 2 mL of 2 M HCl for 3 h at 90 • C. The mixture was evaporated to dryness in vacuo, and the residue was dissolved in H 2 O and extracted with CHCl 3 .After the aqueous layer was dried in vacuo, the residue was dissolved in pyridine (1 mL) containing L-cysteine methyl ester (1 mg) and heated at 60 • C for 1 h.o-Tolyl isothiocyanate (5 µL) was added, and the mixture was heated at 60 • C for 1 h and directly analyzed by HPLC.Analytical HPLC was performed on a reversed-phase C18 column (5 µm, 4.60 × 250 mm; Intertsutain, Shimadzu) at 30 • C with isocratic elution using 25% CH 3 CN containing 0.1% formic acid for 40 min at a flow rate 0.8 mL/min.The peaks were detected with a UV detector at 250 nm.The standard monosaccharides, D-glucose, and L-glucose, were subjected to the same process.

Figure 3 .
Figure 3. CD spectrum of compound 1 in CH 3 OH.
a not assigned; b no signal.