Subject Areas: Plant Science
The genus Amorpha (Fabaceae) including 16 species is native to North America with a center of diversity in the southeastern United States  . Among them, A. fruticosa (Figure 1) which has been used as a Chinese folk medicine for the treatment of burn, ambustion, carbuncle and eczema  is the only one species introduced into China (Zhao, 1982)  . Up to now, chemical investigations of genus Amorpha mainly focused on A. fruticosa, which resulted in the isolation of more than forty compounds, including stilbenes  , rotenoids  and flavanones
Figure 1. The photos of A. fruticosa (left) and its seeds (right).
 , as well as isoflavones  . The three former groups are also found in the other species of this genus  -  . Our previous study on the EtOAc partition of 95% EtOH extract of the seeds of A. fruticosa has resulted in the isolation of eight rotenoid glycosides  . In this paper, the isolation and characterization of seventeen compounds from its n-hexane partition were reported. Among them, one compound was firstly purified as a natural product, nine were isolated from the genus Amorpha for the first time, and one was obtained from the Fabaceae family initially. The presence of these compounds suggests that genus Amorpha and Dalbergia may have very close chemotaxonomic relationship, and shows the relationship between this plant and other species from the Fabaceae family.
2. Materials and Methods
2.1. Plant Material
The seeds of A. fruticosa were collected in Shangqiu City, Henan Province of China. After authenticated by Professor Zhangpin Gou (Department of Pharmacology, Guangdong Medical College), a voucher specimen (No. 20130910) was deposited in the Guangdong Key Laboratory for Research and Development of Natural Drugs, Guangdong Medical University, Zhanjiang, China.
1D and 2D NMR spectra were recorded on Bruker AV-500 spectrometer. CD spectra were measured on a Bio-Logic MOS-450 circular dichroism spectrometer. Column chromatographies (CC) were carried out using silica gel (Qingdaohaiyang, China) and Sephadex LH-20 (Pharmacia Biotech AB, Sweden). Analytical high- performance liquid chromatography (HPLC) was carried out on a Agilent 1200 series and a C18 reversed-phase column (Cosmosil, 4.6 mm × 250 mm , 5.0 μm). Preparative HPLC were carried out on a Gilson 305 pump, a Varian Prostar 345 UV detector and a C18 reversed-phase column (Cosmosil, 20 mm × 250 mm , 5.0 μm).
2.3. Extraction and Isolation
Fresh and powdered seeds of A. fruticosa (4.0 kg) were extracted three times with 95% EtOH at room temperature (3 × 40 L). The solution was removed under vacuum at 50˚C to yield the residue (495 g). The residue was suspended in distilled water, and then successively partitioned with n-hexane, EtOAc and n-BuOH respectively. After removing the solvent, then-hexane partition (120 g) was chromatographied by silica gel column using gradient mixtures of cyclohexane-acetone (92:8 → 50:50) as eluants to yield eleven fractions (Fr. H1-Fr. H11). Fr. H3 (3.3 g) was separated by Sephadex LH-20 column using CH3OH as eluent and further purified by preparative HPLC (CH3OH-H2O, 15:85) to yield compounds 15 (1210.0 mg) and 16 (1040.0 mg), 17 (110.0 mg). Fr. H4 (4.2 g) was separated by Sephadex LH-20 column using CH3OH as eluent to give four subfractions (Fr. H4a-Fr. H4d). Then, Fr. H4a (132.0 mg) was further purified by Sephadex LH-20 column as CH3OH to yield compounds 13 (5.0 mg) and 6 (18.0 mg). Fr. H4b (1550.0 mg) was purified by preparative HPLC (CH3OH-H2O, 45:55) to yield compounds 14 (190.1 mg), 2 (324.0 mg), 1 (740.0 mg) and 3 (66.0 mg). Fr. H4c (250.0 mg) was purified by preparative HPLC (CH3OH-H2O, 60:40) to yield compound 5 (100.0 mg). Fr. H5 (500.0 mg) was separated as the same method as Fr. H4to yield three subfractions (Fr. H5a-Fr. H5c), which were purified by preparative HPLC (CH3OH-H2O, 45:55) to give compounds 4 (10.1 mg), 10 (5.3 mg) and 11 (6.2 mg), respectively. The same procedure was applied to Fr. H6 (430.0 mg) to get four subfractions (Fr. H6a-Fr. H6d). Fr. H6b, Fr. H6c and Fr. H6d were purified by preparative HPLC (CH3OH-H2O, 40:60) to yield compounds 7 (10.1 mg), 8 (5.5 mg) and 9 (10.4 mg), respectively. Fr. H9 (536.0 mg) was isolated by preparative HPLC (CH3OH-H2O, 25:75) to yield compound 12 (200.5 mg).
3. Results and Discussion
The isolated compounds were identified as 6aR, 12aR-dalbinol (1)  , amorphigenin (2)  , 12a-hydrox- ydalpanol (3)  , 6aS, 12aS-dalbinol (4)  , 12a-hydroxymunduserone (5)  , amorphispironone (6)  , ononin (7)  , isoformonentin (8)  , daidzein (9)  , prunetin (10)  , pratensein (11) (Dixit et al., 2012)  , 7-O-β-D-glucopyranosyl-7-hydroxy-2’,4’,5’-trimethoxyisoflavone (12)  , 7-hydroxy-2’,4’,5’-trimetho- xyisoflavone (13)  , amorfrutin B (14)  , gallic acid (15)  , 4-methoxygallic acid (16)  and 3,4-dimethoxygallic acid (17)  , respectively, on the basis of their 1H NMR and 13C NMR spectra analysis and comparison with those reported data in the related literatures (Figure 2). The absolute configurations of 6a and 12a of 1-3 and 5 were assigned as R,R based on the CD data which revealed a negative Cotton effect at 326 nm, while 4 was S,S with mirror image CD spectrum of 1 (Figure 3)  .
The present study reported the isolation and structure elucidation of seventeen secondary metabolites from the seeds of A. fruticosa, including six rotenoids (1-6), seven isoflavones (7-13), one stilbene (14) and three benzoic acid derivatives (15-17). Among them, compound 4 was firstly purified as a natural product, compounds 5, 8-12 and 15-17 were isolated from the genus Amorpha for the first time, and compound 17 was obtained from the
Figure 2. The structures of compounds 1-17.
Figure 3. The CD spectra of compounds 1-5.
Fabaceae family initially. Although compound 12 has been once reported, it has no CAS number yet. Compounds 1-3, along with the previous reported compounds amorphin and 8’-O-β-D-glucopyranosyl-amorphigenin, have been isolated or detected in ten species of the genus Amorpha (A. fruticosa, A. angustifolia, A. canescens, A. frangrans, A. californica, A. glabra, A. mierophylla, A. nana, A. croceotanata, and A. caroliniana)  at the same time, which provides a chemotaxonomic evidence to support the morphological classification of this genus.
Rotenoids 1-2 and isoflavones 12-13, as well as our previous reported rotenoid glucosides 8’-O-β-D-gluco- glucopyranosyl-amorphigenin and dalbin  , were also isolated from Dalbergia monnetaria (Fabaceae) simultaneously  . Such a coincidence should be explained by the fact that the isoflavone 13 was the key intermediate during the biosynthesis of its corresponding rotenoids  . In addition, some compounds obtained in this studyalso exist in other species of genus Dalbergia, as follows: compound 1 in D. nitidula  ; compound 7 in D. paniculata  and D. odorifera  ; compound 9 in D. odorifera  , D. parviflora  , D. frutescens  , D. ecastophyllum  , D. stevensonii  and D. ecastophylla  ; compound 10 in D. odorifera  , D. sympathetic  , D. sissoo  and D. spinosa  ; compound 11 in D. sissoo  , D. parviflora  and D. odorifera  ; compound 13 in D. vacciniifolia  . All these suggested that genus Amorpha and Dalbergia may have very close chemotaxonomic relationship.
Furthermore, compounds 1 and 2 were also synchronally isolated from Berchemia discolor  , compound 3 was once obtained from Aeschynomene indica  , and compound 5 was obtained from Pachyrrhizus erosus  , Tephrosia fulvinervis  and Tephrosia pentaphylla  , which exhibit the relationships between A. fruticosa and these species, all belonging to family Fabaceae except Berchemia discolor (Rhamnaceae). Compound 4 was isolated naturally for the first time, while compounds 6 and 14 both only existed in A. fruticosa. Compounds 15 and 9 widely distributed in the family Fabaceae, and compound 16 has only been isolated from one species of it (Canavalia gladiate)  , while it is the first time to obtain compound 17 from this family. Therefore, compounds 4, 6, 14 and 17 should be served as chemotaxonomic markers for A. fruticosa.
According to the above results, our present study extends the knowledge about the compounds of genus Amorpha and confirms Dalbergia as its chemotaxonomic related genus.
This work was supported by the National Natural Science Foundation of China (81503226) and the Doctoral Fund of Guangdong Medical College (XB1301).
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