The isolation and characterization of triterpene saponins from Silphium and the chemosystematic and biological significance of saponins in the Asteraceae
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My dissertation studies involved the isolation and characterization of triterpene saponins from eleven species of Silphium L. (Asteraceae). Saponins are an economically important group of natural products with a wide range of biological activities and a broad distribution in the plant kingdom. Typically, saponins consist of one or more sugar moieties attached to either a steroid or a triterpene aglycone. In the course of these investigations, nine new triterpene saponins were identified. The compounds were identified as 3β,6β,16β-trihydroxyolean-12-en-23-al-3-O-β-glucopyranosyl-16-O-β- glucopyranoside (1), urs-12-ene-3β,6β,16β-triol-3-O-β-galactopyranosyl-(1 " 2)-β- glucopyranoside (2), 3β,6β,16β-trihydroxyolean-12-en-23-oic acid-3-O-β- glucopyranosyl-16-O-β-glucopyranoside (3), urs-12-ene-3β,6β,16β,21V-tetraol-3-O-β- glucopyranoside (4), olean-12-ene-3β,6β,16β,21β-tetraol-3-O-β-glucopyranoside (5), olean-12-ene-3β,6β,16β,21β,23-pentaol-3-O-β-glucopyranosyl-16-O-β-glucopyranoside (6), olean-12-ene-3β,6β,16β-triol-3-O-β-glucopyranosyl-16-O-α-arabinopyranosyl-(1 " 2)-β-glucopyranoside (7), olean-12-ene-3β,6β,16β,23-tetraol-3-O-β-glucopyranosyl-16- O-α-arabinopyranosyl-(1 " 2)-β-glucopyranoside (8) and 3β,6β,16β,21β- tetrahydroxyolean-12-en-23-al-3-O-β-glucopyranoside (9). Moreover, the cytotoxic activities of the isolated compounds were tested against human breast cancer cell line MDA-MB-231. Results showed that compound 2 decreased breast cancer cell proliferation in a statistically significant manner at 25 "g/ml. Several other known saponins were also isolated and identified. The structures of these compounds were established by chemical methods and spectral analyses including 1 H-NMR, 13 C NMR, HMBC, NOESY DEPT and TOSCY experiments as well as high resolution and low resolution ESI-MS analyses. In addition to the full structural characterization of the above-mentioned compounds, a comprehensive genus-wide HPLC/ESI-MS analysis of Silphium leaf extracts revealed the presence of over 90 additional saponins in the Silphium genus and in the closely related Lindheimera texana. To complement my chemical studies of Silphium, I also present a phylogenetic analysis of the entire Asteraceae based on phytochemical data and a detailed overview of the chemistry of the Asteraceae using a large-scale chemical database to view the phytochemical data within the framework of the most recent molecular phylogeny for the Asteraceae. Finally, I comment on the results of my studies in the context of the chemosystematic and medicinal significance of triterpene saponins from the Asteraceae.