Pterocarpadiols A-D, Rare 6a, 11b-Dihydroxypterocarpans from Derris robusta

1 Introduction

The genus Derris (Leguminosae) contains about 800 species, widely distributed in tropical and subtropical parts of the world [1]. The bark and leaves of Derris species are commonly utilized as folk medicine to treat human diseases such as arthritis and eczema [2]. The extracts and reported chemical constituents exhibit antioxidant, antibacterial and pesticidal activities [2]. Previous studies show the genus is a rich source of flavonoids, isoflavonoids, pterocarpans and rotenoids [2]. As part of a BioBioPha [http://www.chemlib.cn] objective to assemble a large-scale natural product library very valuable in the discovery of new drug leads from nature [3-7], the phytochemical investigation on the twigs and leaves of Derris robusta led to the isolation of four new 6a, 11b-dihydroxypterocarpans, namely pterocarpadiols A-D (1-4), together with 10 known pterocarpans: 1, 11b-dihydro-11b-hydroxymaackiain (5) [8], 1, 11b-dihydro-11b-hydroxymedicarpin (6) [8], pisatin (7) [9], variabilin (8) [10], 6a-hydroxymaackiain (9) [11], 6a-hydroxymedicarpin (10) [12], maackiain (11) [13], medicarpin (12) [14], 3-hydroxy-8, 9-methylenedioxypterocarpene (13) [15], and anhydroglycinol (14) [16]. Among the known pterocarpans, compounds 5-10 and 14 were isolated from the genus for the first time. This paper reports the isolation and structure elucidation of pterocarpadiols A-D.

2 Results and Discussion

Compound 1, obtained as amorphous powder, had a molecular formula of C₁₆H₁₂O₇ as deduced from its positive-ion HRESIMS at m/z 339.0462 [M+Na]⁺ (calcd for C₁₆H₁₂O₇Na, 339.0475), requiring 11 degrees of unsaturation. The ¹H NMR spectrum (Table 1) showed three characteristic aliphatic protons of 6a-hydroxypterocarpan skeleton at δ_H 4.99, 4.37 (each 1H, d, J = 10.5 Hz, H-6), and 4.73 (s, H-11a) [17]. The ¹³C NMR spectrum (Table 3) displayed a total of 16 carbon resonances, including three typical oxygen-bearing carbons of 6a-hydroxypterocarpan at δ_c 70.2 (t, C-6), 78.7 (s, C-6a) and 91.4 (d, C-11a). Three olefinic protons at δ_H 6.80 (d, J = 10.0 Hz, H-1), 6.09 (dd, J = 10.0, 1.8 Hz, H-2), and 5.41 (d, J = 1.8 Hz, H-4) were assignable to A-ring by comparison with hydroxycristacarpone (15) (Fig. 1) [18], and their spectral difference was almost completely rooted in the D ring. Two aromatic singlets at δ_H 6.81 (s, H-7), 6.26 (s, H-10) and a methylenedioxy signal at δ_H 5.90, 5.89 (each 1H, d, J = 1.0 Hz) were newly detected, while the prenyl and methoxy signals disappeared, which suggested that the methylenedioxy group should be connected to C-8 and C-9. The inference was confirmed by the HMBC correlations from the proton at δ_H 6.81 (s, H-7) to the carbons at δ_c 78.7 (s, C-6a), 144.2 (s, C-8), and 151.6 (s, C-9), and from the methylenedioxy protons to the carbons at δ_c 144.2 (s, C-8), and 151.6 (s, C-9). Regrettably, it was inconclusive to establish relative configurations at C-6a, C-11a and C-11b by ROESY analysis, since the pivotal hydroxy signals were undetectable in CD₃OD. As we know, hydroxy proton signals were observable and often appeared as sharp peaks in DMSO-d₆, and their HMBC and ROESY correlations often played an important role in structure elucidation, especially the determination of relative configuration [19]. The clear ROESY correlation (DMSO-d₆, Fig. 2) of 6a-OH ↔ H-11a revealed a cis fusion of the B/C ring junction, while the correlations of 11b-OH ↔ H-11a and H-6α indicated α-orientation of the hydroxy group at C-11b. Accordingly, the structure of 1 was established and named as pterocarpadiol A.

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Compound 2, white amorphous powder, had a molecular formula of C₁₆H₁₄O₆ based on the positive-ion HRESIMS at m/z 325.0674 [M+Na]⁺ (calcd for C₁₆H₁₄O₆Na, 325.0683). The NMR spectroscopic data (Tables 1, 3) were similar to those of pterocarpadiol A (1), and the major difference was that its NMR spectra newly displayed a methoxy group (δ_H 3.72; δ_c 56.0) instead of the methylenedioxy. And then three aromatic protons at δ_H 7.25 (d, J = 8.5 Hz, H-7), 6.55 (dd, J = 8.5, 2.4 Hz, H-8), and 6.29 (d, J = 2.4 Hz, H-10) were assignable to an ABX-type aromatic D-ring. The methoxy group was linked to C-9 on the basis of the HMBC correlations from the proton at δ_H 7.25 (d, J = 8.5 Hz, H-7) to the carbons at δ_c 78.0 (s, C-6a), 164.2 (s, C-9), and 162.9 (s, C-10a), and from the methoxy protons at δ_H 3.72 (s) to the carbon at δ_c 164.2 (s, C-9). Therefore, the structure of 2 was established and named as pterocarpadiol B.

Compound 3, white amorphous powder, possessed a molecular formula of C₁₆H₁₄O₇ according to its positive-ion HRESIMS at m/z 341.0620 [M+Na]⁺ (calcd for C₁₆H₁₄O₇Na, 341.0632), which was 2 m.u. higher than that of 1. Signals of an oxymethylene at δ_H 4.49, 4.28 (each 1H, d, J = 10.0 Hz, H-6), an oxymethine at δ_H 4.44 (s, H-11a), an olefinic proton at δ_H 5.22 (s, H-4), two aromatic singlets at 6.93 (s, H-7), 6.49 (s, H-10) and a methylenedioxy at δ_H 5.96, 5.94 (each 1H, s) were observed in the ¹H NMR spectrum (Table 2). By comparison of the NMR spectra (Tables 2, 3) of 3 and 1, two ortho-coupled doublets and the corresponding olefinic carbons were absent whereas two sp³ carbons at δ_c 31.2 (t) and δ_c 31.8 (t) were newly detected, which hinted that 3 should be 1, 2-dihydropterocarpadiol A. The inference was confirmed by the HMBC correlations from the proton at δ_H 2.43 (td, 14.1, 4.5, H-1β) to the carbon at δ_c 89.9 (d, C-11a) and from the proton at δ_H 2.16 (ddd, 16.0, 4.5, 2.8, H-2β) to the carbon at δ_c 107.5 (d, C-4). The clear ROESY correlations (DMSO-d₆) of 6a-OH/11b-OH ↔ H-11a indicated that 3 possessed the same stereochemistry as 1. Thus, the structure of 3 was established and named as pterocarpadiol C.

Compound 4, white amorphous powder, had a molecular formula of C₁₆H₁₆O₆ determined by the positive-ion HRESIMS at m/z 327.0819 [M+Na]⁺ (calcd for C₁₆H₁₆O₆Na, 327.0839). The NMR spectroscopic data (Tables 1, 3) were similar to those of pterocarpadiol C (3), and the obvious difference was that a methoxy signal (δ_H 3.74; δ_c 56.0) replaced the methylenedioxy in 3. According to the HMBC correlations from the proton at δ_H 7.25 (d, J = 8.0 Hz, H-7) to the carbons at δ_c 78.1 (s, C-6a), 164.1 (s, C-9), and 162.2 (s, C-10a), and from the methoxy protons at δ_H 3.74 (s) to the carbon at δ_c 164.1 (s, C-9), the methoxy group was positioned at C-9 as with the previous structure. Thus, the structure of 4 was established and named as pterocarpadiol D.

Pterocarpans isolated in our current research such as pisatin (7), variabilin (8), and maackiain (11) exhibited without exception negative optical rotation values (-286°, -304°, and -260°, respectively), and their absolute configurations had been established as 6aS, 11aS (7), 6aS, 11aS (8), and 6aR, 11aR (11) [20]. As their related co-constituents, pterocarpadiols A-D (1-4) also gave large negative optical rotation values (-484.0°, -397.0°, -507.0° and -476.0°, respectively), thereupon we assumed that the absolute configurations of 1-4 could be assigned as 6aS, 11aR, 11bS depicted in Fig. 1. Nonetheless, this issue deserved further studies in the future. Until now, only two 6a, 11b-dihydroxypterocarpans: hydroxytuberosone [21] and hydroxycristacarpone [18], were reported and only from the family Leguminosae, therefore pterocarpadiols A-D as chemical markers in Derris species may be helpful in chemotaxonomical classification.

3 Experimental Section

3.1 General Experimental Procedures

Optical rotations were measured on a Jasco P-1020 automatic digital polarimeter. UV data were obtained from HPLC online analysis. NMR spectra were carried out on a Bruker AV-400, Bruker DRX-500 or Bruker AV-800 spectrometer with deuterated solvent signals used as internal standards. ESI and HRESIMS were performed with a Shimadzu LC-IT-TOF mass spectrometer equipped with an ESI interface (Shimadzu, Kyoto, Japan). Silica gel 200-300 mesh (Qingdao Marine Chemical Inc., Qingdao, China), Chromatorex C-18 (40-75 μm, Fuji Silysia Chemical Ltd., Japan) and Sephadex LH-20 (Amersham Biosciences, Uppsala, Sweden) were used for normal pressure column chromatography (CC). Fractions were monitored and analyzed by TLC, in combination with Agilent 1200 series HPLC system equipped by Extend-C18 column (5 μm, 4.6 × 150 mm).

3.2 Plant Material

The twigs and leaves of D. robusta were collected from the Pu'er region of Yunnan Province, China, in May 2011, and identified by Mr. Yu Chen of Kunming Institute of Botany. A voucher specimen (BBP0350021DR) was deposited at BioBioPha Co., Ltd.

3.3 Extraction and Isolation

The air-dried and powdered twigs and leaves (12.0 kg) of D. robusta were extracted with 95 % EtOH at room temperature, and the solvent was removed under reduced pressure to give crude extract (ca. 870 g), which was fractionated by silica gel CC successively eluted with petroleum ether (PE)/acetone gradient and then MeOH to yield nine fractions A-I. Fraction C (PE/acetone, 6:1) was subjected to silica gel CC (CHCl₃/MeOH, 100:0 → 100:2) and Sephadex LH-20 (CHCl₃/MeOH, 1:1; or MeOH) to give 5 (51 mg), 6 (185 mg), 7 (138 mg), 8 (262 mg), 11 (665 mg), and 12 (608 mg), and the remainder was separated by RP-18 (40 % MeOH/H₂O) and Sephadex LH-20 (MeOH) to yielded 9 (661 mg) and 10 (349 mg). The fraction E (PE/acetone, 4:1) was repeatedly applied to silica gel CC (CHCl₃/MeOH, 30:1 → 15:1) and Sephadex LH-20 (MeOH) to yiele 13 (23 mg) and 14 (114 mg). The fraction F (PE/acetone, 3:1) was repeatedly separated on silica gel CC (CHCl₃/MeOH, 10:1), Sephadex LH-20 (CHCl₃/MeOH, 1:1) and RP-18 (40 % MeOH/H₂O) to yield 1 (233 mg) and 2 (33 mg), and the remainder was further isolated on silica gel (CHCl₃/MeOH, 10:1), Sephadex LH-20 (MeOH) and RP-18 (45 % MeOH/H₂O) to yield 3 (47 mg) and 4 (28 mg). The retention times (t_R) of 1-4 on an analytical HPLC Extend-C18 column (20 % → 100 % MeOH in H₂O over 8.0 min followed by 100 % MeOH to 13.0 min, 1.0 ml/min, 25 ℃) were 6.03, 6.16, 6.50, and 6.60 min, respectively.

3.4 Pterocarpadiol A (1)

White amorphous powder; UV (MeOH) λ_max: 235, 306 nm; $ \left[\alpha \right]_{\rm{D}}^{23} $ -484.0 (c 0.5, MeOH); ¹H NMR data: see Tables 1 and 2; ¹³C NMR data: see Table 3; ESIMS (pos.): m/z 339 [M+Na]⁺; HRESIMS (pos.): m/z 339.0462 [M+Na]⁺ (calcd for C₁₆H₁₂O₇Na, 339.0475).

3.5 Pterocarpadiol B (2)

White amorphous powder; UV (MeOH) λ_max: 230, 285, 305 nm; $ \left[\alpha \right]_{\rm{D}}^{22} $ -397.0 (c 0.2, MeOH); ¹H NMR data: see Table 1; ¹³C NMR data: see Table 3; ESIMS (pos.): m/z 325 [M+Na]⁺; HRESIMS (pos.): m/z 325.0674 [M+Na]⁺ (calcd for C₁₆H₁₄O₆Na, 325.0683).

3.6 Pterocarpadiol C (3)

White amorphous powder, UV (MeOH) λ_max: 260, 308 nm; $ \left[\alpha \right]_{\rm{D}}^{23} $ -507.0 (c 0.2, MeOH); ¹H NMR data: see Tables 1 and 2; ¹³C NMR data: see Table 3; ESIMS (pos.): m/z 341 [M+Na]⁺; HRESIMS (pos.): m/z 341.0620 [M+Na]⁺ (calcd for C₁₆H₁₄O₇Na, 341.0632).

3.7 Pterocarpadiol D (4)

White amorphous powder, UV (MeOH) λ_max: 232, 261 nm; $ \left[\alpha \right]_{\rm{D}}^{23} $ -476.0 (c 0.2, MeOH); ¹H NMR data: see Table 1; ¹³C NMR data: see Table 3; ESIMS (pos.): m/z 327 [M+Na]⁺; HRESIMS (pos.): m/z 327.0819 [M+Na]⁺ (calcd for C₁₆H₁₆O₆Na, 327.0839).

Notes

Acknowledgments

This work was financially supported by the "Large-scale Compound Library" project of National Development and Reform Commission of China.

Compliance with Ethical Standards

Conflict of Interest

The authors declare no conflict of interest.