| Respiratory viral infections have long been recognized as important causes of morbidity and mortality in young children and older adults. However, effective chemotherapeutic agents are still very limited. Thus the search for natural products as antiviral agents against respiratory viruses has attracted considerable attention in recent years. In the work described in this thesis, three medicinal plants were studied. The structures of the isolated compounds were characterized by a combination of spectroscopy and X-ray crystallography. Their antiviral activities were evaluated by in vitro cytopathogenic effects (CPE) reduction assay.; Caesalpinia minax Hance is a plant belonging to the Fabaceae family. The seeds (“ku-shi-lian” in Chinese) of this plant have been used as a folk remedy for some diseases including fever, the common cold and dysentery. Investigation of the active fractions from the seeds (brown precipitate fraction after defatting with hexane) and chloroform fraction led to the isolation of five new tetracyclic furanoditerpenoids (caesalmin C, D, E, F and H), three new (caesalmin A, B and G) and a known (bonducellpin D) pentacyclic furanoditerpenoid lactones, a new macrocyclic furanoditerpenoid (macrocaesalmin), a new rearranged furanoditerpenoid (spirocaesalmin) and a known sterol (stigmasterol).; In order to compare the chemical components of the seeds and the stem of the same plant, investigation was extended to the components in the stem which led to the isolation of a known triterpenoid, namely friedelin. In order to investigate the influence of molecular polarity on antiviral activity, reductive cleavage of the naturally abundant caesalmin D, E and F and reduction of friedelin were performed to afford three new furanoditerpenoid derivatives and a triterpenoid derivative. The bioassay results showed that caesalmin C, D, E and F exhibited potent activity against parainfluenza virus type 3 (PIV3), and an increase in polarity led to a decrease in activity.; Mikania micrantha H.B.K. (Compositae) is a fast-growing, perennial creeping vine that has been used as an alternative medicine to treat skin itches and athlete's foot in Jamaica. Phytochemical investigation of the ethanol extract of the dried aerial part led to the isolation of two new (mikacranolide A, mikacranolide B) and a known (dihydromikanolide) sesquiterpene lactones and a new (potassium mikanin 3-O-sulfate) and two known (mikanin and alpinetin) flavonoids and a known sterol (ergosta-7,22-dien-3β-ol). The bioassay results showed that mikacranolide A exhibited potent activity against both respiratory syncytial virus (RSV) and PIV3 and potassium mikanin 3-O-sulfate exhibited potent activity against PIV3 virus.; The herb Radix Stemonae, known as ‘Bai-Bu’ in Traditional Chinese Medicine, is mainly derived from the root of Stemona tuberosa Lour (Stemonaceae family). It is often used as an antitussive drug to treat respiratory disorders, e.g., bronchitis, pertussis and tuberculosis, and also as an anthelmintic agent for domestic animals. Phytochemical studies on this herb from resulted in the isolation of a known stemona alkaloid (neotuberostemonine) and two new stenine-type alkaloids, namely neotuberostemonol and neotuberostemoninol. The bioassay results showed that the major component neotuberostemonine was not active against RSV and PIV3 viruses.; Although the mechanisms of action of the above compounds are still under investigation, the results achieved so far showed good prospects in the search for new antiviral agents against respiratory viruses from medicinal plants. |
