Bioactive Secondary Metabolites from Medicinal Plants

Natural products have been a reservoir of diverse structures that lead to pharmacologically active compounds, lead compounds or new chemical entities in drug discovery. At present, one quarter of all approved small molecular drugs are based on natural products or natural product derivatives. Terrestrial plants are a major source of natural products. Especially plants used in traditional medicine systems are mostly chosen as material for chemical investigation to discover pharmacologically active compounds. This dissertation describes secondary metabolites isolated from four medicinal plants including Sarcotheca griffithii and Piper retrofractum collected from Indonesia, Talinum triangulare collected from Nigeria and Amorpha fruticosa collected from Germany. The structure of the isolated compounds was unambiguously elucidated by one- and two-dimensional NMR spectroscopy, mass spectroscopy and by comparison with literature data. The absolute configurations were determined by ECD calculations or by comparison of optical rotation and CD with literature data. The isolated compounds were tested for their antifungal, antibacterial and cytotoxic activities. All obtained results were published or submitted for publication in respective international journals. This dissertation consists of the following parts: Sarcotheca griffithii This is the first report of secondary metabolites from the genus Sarcotheca (Oxalidacae). From the methanolic extract of leaves of S. griffithii, six flavonoid C-glucosides were isolated. Five of them are new compounds including chrysin 6-C-(2″-O--L-rhamnopyranosyl)--D-glucopyranoside (1), chrysin 6-C-(2″-O-α-L-rhamnopyranosyl)--D-glucopyranosyl-7-O--D-glucopyranoside (2), chrysin 6-C-(2″-O-α-L-rhamnopyranosyl)-β-L-fucopyranoside (3), chrysin 6-C-(2″-O-α-L-rhamnopyranosyl)-6′-deoxy-ribo-hexos-3-uloside (4), chrysin 6-C-β-boivinopyranosyl-7-O-β-L-glucopyranoside (5). The known compound isovitexin 2″-O-α-L-rhamnopyranoside (6) is the major compound. Their structures were elucidated by analysis of NMR and MS data as well as by comparison with literature data. The absolute configuration of glucose and rhamnose moieties were determined by acid hydrolysis and ester derivatization followed by TLC analysis and comparison of its Rf values with those of the authentic standards. Chemical investigation of S.griffithii led to the suggestion that chrysin C- glycosides seemed to be the chemotaxonomic marker of this species. The crude extract and fractions were found to not be active against the L5178Y mouse lymphoma cell line. Piper retrofractum The genus Piper (Piperaceae) is known worldwide as an important spice and medicinal plant. For this study, the fruits of P.retrofractum were collected in Indonesia. Chromatographic separation of the methanolic extract gave three new amides, namely (2E,14Z)-N-isobutyleicosa-2,14-dienamide (1), dipiperamide F (2) and G (3), together with 30 known compounds. Their structures were determined by analysis of the obtained 1D and 2D NMR spectra as well as MS data and by comparison with literature data. The isolated compounds were classified as long chain alkylamides, methylenedioxyphenyl amides, cyclobutanamides and cyclohexenamides. Plausible biosynthetic pathwaysare discussed in this study. Preliminary assays of all isolated compounds against the fungus Cladosporium cladosporioides revealed that piperanine was active. All isolated compounds were also tested for their cytotoxicity against the L5178Y mouse lymphoma cell line. Three cyclobutanamides, dipiperamide F and G (2 and 3), nigramide R (31); one cyclohexenamide,chabamide (30); four methylenedioxyphenyl amide, dehydropipernonaline (25), guineensine (22), brachystamide B (23), retrofractamide C (20), and two alkylamides, pellitorine (13), pipericine (14) displayed considerable cytotoxicity with IC50 values of 10.0, 13.9, 11.6, 9.3, 8.9, 17.0, 16.4, 13.4, 28.3, and 24.3 μM, respectively. Structure-activity relationships of cytotoxic amides are discussed. Talinum triangulare Chromatographic investigation of the methanolic extract from the roots of Talinum triangulare (Portulacaceae) from Nigeria yielded two new C-methylated flavonoids, 5,6-dimethoxy-7-hydroxy-8-methyl-flavone (1), 5,6-dimethoxy-8-methyl-2-phenyl-7H-1-benzopyran-7-one (2), and one new α-pyrone derivative, 4-methoxy-6-(2-hydroxy-4-phenylbutyl)-2H-pyran-2-one (3), together with thirteen known compounds including nine amides (4-12), indole-3-carboxylic acid (13), p-hydroxy benzoic acid (14), and two steroids (15-16). Extensive analysis of NMR and MS data as well as comparison with literature data was performed to determine their structures. All isolated compounds were subjected to cytotoxic assay against the mouse lymphoma L5178Y cell line and antifungal assay against Cladosporium cladosporioides. However, none of them showed significant activity. Amorpha fruticosa The dichloromethane-methanol extract of fruits of Amorpha fruticosa (Fabaceae) was separated using various chromatography techniques. Fourteen new natural products including 2-[(Z)-styryl]-5-geranyl-resorcin-1-carboxylic acid (1), amorfrutin D (2), demethoxyamorfrutin D (3), 8-geranyl-3,5,7-trihydroxyflavanone(13), 8-geranyl-3′,5,7-trihydroxy-4′-methoxyisoflavone(14), 6-geranyl-3′,5,7-trihydroxy-4′-methoxyisoflavone (15), 8-geranyl-3′,7-dihydroxy-4′-methoxyisoflavone (16), 3-demethoxydalbinol (42), 6a,12a-dehydro-3-demethoxyamorphigenin (43), (6aR,12aR,5′R)-amorphigenin (44), amorphispironone B−C (45−46), resokaempferol 3-O-β-D-glucopyranosyl-(12)-β-D-glucopyranoside-7-O-α-L-rhamnopyranoside (51), and daidzein 7-O-β-D-glucopyranosyl-(12)-β-D-glucopyranoside (52) as well as 40 known compounds were isolated. Their structures were unambiguously established by extensive 1D and 2D NMR spectroscopic analyses as well as by mass-spectrum data. CD calculations or comparison of CD with literature data were involved in determination of the absolute configuration of 11, 42, 44 and 45. The sugar moieties were characterized using acid hydrolysis of the respective glycosides, derivatization, and HPLC analysis with comparison to authentic standards. All isolated compounds were tested for their antibacterial and cytotoxic activities. Compounds 1, 4, 5, 7, 8, 12−15 displayed potent to modest antibacterial activities against several gram-positive bacteria. In addition, compounds 10, 13, 25, 27, 30, 32, 34, 35, 37, 41, 45, 48 and 53 exhibited significant cytotoxicity against the L5178Y mouse lymphoma cell line. The structure-activity relationships of respective antibacterial and cytotoxic compounds are discussed. Plausible biosynthetic pathways leading to bibenzyl and rotenoids isolated in this study were proposed.