1. Introdution

The aerial parts of Epimedium koreanum Nakai(Berberidaceae)have been used in China for over 2000 years for various medicinal treatments，in particular as tonic，antirheumatic， and aphrodisiac(Pharmacopoeia Committee of P. R. China，2010). Up to now，many studies were focused on the flavonoids in plants of Epimedium Linn. for their various bioactivities in vivo or in vitro(Ma et al，2011; Wang et al，2010; Zhang et al， 2008; Sun et al，1996; Li et al，1995). In our previous study，the isolation and structural elucidation of two new prenylflavonols were reported from Epimedium brevicornum Maxim(Luo et al，2009). As the continuation of our search for more new secondary metabolites，a new furanflavonol glycoside(1)was obtained from E. koreanum.

2. Materials and methods 2.1 General and plant materials

UV spectrum was run on a UV 210A Spectrophotometer. 1D NMR and 2D NMR spectra were recorded on Bruker DRX−400 Instrument with TMS as an internal st and ard. ESI-MS was taken on an API Qstar Pulsar Instrument， and HR-EI-MS was performed on a VG Autospec−3000 Spectrometer. Column chromatography(CC)was performed with silica gel(100−200 mesh，Qingdao Marine Chemical Inc.，Qingdao，China) and Lichroprep RP-18 gel(40−63 μm，Merck，Darmstadt，Germany). Fractions were monitored by TLC， and spots were visualized by heating silica gel plates sprayed with 5% H₂SO₄ in EtOH.

The leaves of Epimedium koreanum Nakai were purchased from the market of Medical Building of Jilin province，China，in November 2010， and identified by Dr. Hai-zhou Li. A voucher specimen(KMUST 20101109)was deposited at the Laboratory of Phytochemistry，Faculty of Life Science and Technology，Kunming University of Science and Technology.

The air-dried and powdered leaves of E. koreanum(1 kg)were extracted with 75% aqueous acetone(for three times). After removal of the solvent in vacuo，the residue was suspended in water and fractionated with CHCl₃，EtOAc， and n-BuOH，successively. The EtOAc extract(10.5 g)was purified by Sephadex LH-20，eluted with MeOH-H₂O(3:7→6:4→9:1)，to give six fractions(Frs. A−F). Fr. C(600 mg)was subjected to silica gel column chromatography eluted with CHCl₃-MeOH gradients(30:1 and 20:1)，to give eleven subfractions(Frs. C-1−C-11). Fr. C-7(44.7 mg)was then purified on semipreparative HPLC(flow rate of 3 mL/min)with 40% MeOH in H₂O as mobile phase to give compound 1(8 mg).

A solution of compound 1(1 mg)in 0.5 mL HCl(1 mol/L)was heated at 90−100 ^oC in a screw-capped vial for 5 h. The mixture was partitioned with CHCl₃(0.5 mL)， and the HCl layer was compared with the st and ard sample of rhamnose on TLC(EtOAc-MeOH-AcOH-H₂O 11:2:2:2)by visualizing the spots.

Compound 1: yellow powder; ESI-MS: m/z 479 [M + Na]⁺; HR-EI-MS: m/z 456.1049([M]⁺，C₂₃H₂₀O₁₀⁺，calcd. 456.1056); ¹H-NMR(DMSO-d₆，400 MHz) and ¹³C-NMR(DMSO-d₆，100 MHz)data are shown in Table 1.

Table 1

Table 1 1H-NMR and 13C-NMR data of compound 1(400 and 100 MHz，in DMSO-d6) No. δC δH(mult，J，Hz) 2 159.4 3 137.2 4 180.6 5 159.5 12.46(s，5-OH) 6 95.6 6.91(1H，s) 7 160.6 8 109.9 9 150.5 10 108.7 1′ 122.3 2′ 132.1 7.90(1H，d，8.0) 3′ 116.7 7.01(1H，d，8.0) 4′ 161.9 5′ 116.7 7.01(1H，d，8.0) 6′ 132.1 7.90(1H，d，8.0) 2″ 146.1 7.77(1H，d，2.0) 3″ 104.6 7.12(1H，d，2.0) Rha 1′′′ 103.6 5.47(1H，brs) 2′′′ 72.2 4.29(1H，brs) 3′′′ 73.1 3.78(1H，dd，9.0，3.0) 4′′′ 72.1 3.76(1H，m) 5′′′ 71.9 3.41(1H，m) 6′′′ 17.7 0.97(3H，d，6.0)

Table 1 1H-NMR and 13C-NMR data of compound 1(400 and 100 MHz，in DMSO-d6)

3. Results and discussion

Compound 1 was obtained as yellow powder， and its molecular formula was established as C₂₃H₂₀O₁₀ by the ESI-MS ion peak at m/z 479 [M + Na]⁺ as well as the HR-EI-MS data(m/z 456.1049 [M]⁺，calcd. for C₂₃H₂₀O₁₀，456.1056)(Figure 1). Compound 1 gave a positive reaction with Mg-HCl reagent， and its UV spectrum gave absorption maximum at 267 and 348 nm，which indicated the presence of flavonol skeleton in its structure(Tu et al，2011).

Figure 1 Structure of compound 1

A group of ¹H-NMR data at δ_H 5.47(1H，brs，H-1′′′)，4.29(1H，brs，H-2′′′)，3.78(1H，dd，J = 9.0，3.0 Hz，H-3′′′)，3.76(1H，m，H-4′′′)，3.41(1H，m，H-5′′′) and 0.97(3H，d，J = 6.0 Hz，H-6′′′)，as well as corresponding ¹³C-NMR data at δ_C 103.6(C-1′′′，d)，72.2(C-2′′′，d)，73.1(C-3′′′，d)，72.1(C-4′′′，d)，71.9(C-5′′′，d)， and 17.7(C-6′′′，q)，indicated the presence of α-L-rhamnose(Tu et al，2011)，which was further confirmed by the acid hydrolysis of compound 1. The correlation peak from the anomeric proton H-1′′′ of rhamnose to C-3(137.2，s)was observed in the HMBC spectrum，indicating that the rhamnose was located at C-3. Therefore，compound 1 was 5，4′-dihydroxyfurano [2″，3″:7，8] flavonol 3-O-α-L-rhamnoside.

A group of signals at δ_H 7.77(d，J = 2.0 Hz，H-2″)/ δ_C 146.1(C-2″，d) and δ_H 7.12(d，J = 2.0 Hz，H-3″)/ δ_C 104.0(C-3″)，in combination with the HMBC correlations(Figure 2)of H-3″ with C-2″，C-7(δ_C 160.6)， and C-8(δ_C 109.9)，as well as H-2″ with C-3″，C-7， and C-8，indicated that a furan ring was fused at C-7(oxygenated) and C-8 of ring A(Yadav，Ahmad， and Maurya，2004). In addition，the signal at δ_H 12.46 in the ¹H-NMR spectrum was assigned as the chelated C₅-OH group. Therefore，the singlet signal at δ_H 6.91(1H，s)was unambiguously assignable to H-6 of ring A，which was further proved by the HMBC correlations from H-6 to C-5(δ_C 159.5)，C-7， and C-10(δ_C 108.7). Moreover，four low-field aromatic protons at δ_H 7.01(2H，d，J = 8.0 Hz) and 7.90(2H，d，J = 8.0 Hz)forming an AA′BB′ coupling system elucidated that only C-4′ was substituted in ring B. The low-field signal of C-4′ at δ 161.9 combined with the molecular formula C₂₃H₂₀O₁₀ revealed that C-4′ was replaced by a hydroxyl group，which was further supported by the HMBC correlations from H-3′ and H-5′ to C-4′.

Figure 2 Key HMBC correlations of compound 1

Therefore，based upon the above cumulative evidences，the structure of compound 1 could be explicitly determined as 5，4′-dihydroxyfurano [2″，3″:7，8] flavonol 3-O-α-L- rhamnoside.