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

Abstract Four hitherto unknown 6a,11b-dihydroxypterocarpans, namely pterocarpadiols A–D (1–4), were isolated from the ethanol extract of the twigs and leaves of Derris robusta. Their structures were elucidated on the basis of extensive spectroscopic analysis. Pterocarpadiols A–D are a kind of very rare 6a,11b-dihydroxypterocarpans, and their presence as markers may be helpful in chemotaxonomical classification. Graphical Abstract Electronic supplementary material The online version of this article (doi:10.1007/s13659-015-0078-y) contains supplementary material, which is available to authorized users.


Results and Discussion
Compound 1, obtained as amorphous powder, had a molecular formula of C 16 (Fig. 1) [18], and their spectral difference was almost completely rooted in the D ring. Two aromatic singlets at d H 6.81 (s, H-7), 6.26 (s, H-10) and a methylenedioxy signal at d 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 d H 6.81 (s, H-7) to the carbons at d 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 d 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 CD 3 OD. As we know, hydroxy proton signals were observable and often appeared as sharp peaks in DMSO-d 6 , 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-d 6 , 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-6a indicated a-orientation of the hydroxy group at C-11b. Accordingly, the structure of 1 was established and named as pterocarpadiol A. Compound 2, white amorphous powder, had a molecular formula of C 16 (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 ( (Table 2). By comparison of the NMR spectra   1 (s, C-6a), 164.1 (s, C-9), and 162.2 (s, C-10a), and from the methoxy protons at d H 3.74 (s) to the carbon at d 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.

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 lm, 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 lm, 4.6 9 150 mm).

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.

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