Chemical Characterisation of Novel Metabolites from Bioactive Herbs

One reality in the pharmaceutical and medical industries is the need to discover new lead compounds to allow for the development of new drug targets. In this endeavour, science is increasingly turning to natural sources in order to increase the pool of active compounds to enter development. Herbal medicine has a long and established history in the treatment of disease, stretching back to very early civilisations. Still utilised today, herbal medicines provide vital health care for much of the world’s population, and are increasingly popular in affluent countries despite access to clinical-based medicine. Herbal medicine utilises active ingredients, typically novel metabolites, from plants to provide clinical effect. This makes the investigation of herbal remedies important for the continued development of Western medicine. In order to prevent the adulteration, misidentification or inactivity of herbal medicines, chemical profiling and identification of the active constituents of these medicines is necessary to guarantee their efficacy and safety. One of the most powerful tools available to chemists undertaking this task is Nuclear Magnetic Resonance Spectroscopy (NMR), which has enabled the identification of increasing numbers of natural products of high complexity. The aims of this project1 were to phytochemically characterise unidentified medicinal plants, with the subsidiary aim of searching for compounds with potential antihelmintic activity. Plants from two different families, Liliaceae and Stemonaceae, were investigated in this study, with the first species Chamaelirium luteum, a member of Liliaceae and the second species, an unknown Stemona species, a member of Stemonaceae. Three structurally unique steroidal saponins 1-3 were isolated and found to be the major constituents of the medicinal herb Chamaelirium luteum. Absolute structural determination of the compounds was accomplished through extensive 1D and 2D NMR experiments, MS spectral analysis and comparison to synthetic structural analogues, which revealed the presence of an unusual erythro diol moiety in Saponins 1 and 2. An investigation of an unknown Stemona species led to the isolation of two previously unreported alkaloids 84-85 and one recently reported Stemona alkaloid, which are a class of plant alkaloids characterised by a pyrrolo[1,2-a]azepine nucleus. Profiling of the total alkaloids present in this species revealed a trace that does not correspond to any Stemona species reported to date. The profiling of the alkaloid content of an Australian species of Stemona was also performed, which indicated that further unreported alkaloids are likely to be contained in this species. As NMR was extensively used as a structural tool in the assignment of the complex natural products isolated in this study, this acquired knowledge was also applied to solve various structural questions in the fields of enzymatic mechanisms and in herbal medicine quality control. These investigations proved fruitful, and allowed certain aspects of the mechanism of cytochrome P450 enzymes to be elucidated through (NMR) identification of the structure of a product from oxidation of a novel mechanistic probe. The use of NMR to identify an unknown adulterant in an herbal extract was also successful, resulting in the identification of the unknown adulterant as Amaranth, a synthetic azo sulfonic acid containing naphthyl dye