Part I. Chemoenzymatic synthesis of usnic acid and analogues of usnic acid. Part II. The isolation of citric acid derivatives from Aspergillus niger.

摘要

Usnic acid 1, a highly functionalized dibenzofuran, is a polyketide secondary metabolite produced by several genera of lichens such as Usnea, Cladonia, Alectoria, Evernia, Ramalina, and Lecanora. Usnic acid (1) was first described in 1844 and has seen extensive use in traditional folk medicine systems as an antibiotic. More recently, anticancer, antiviral, antioxidant, anti-inflammatory, and analgesic properties have also been demonstrated. The proposed biosynthesis of usnic acid (1) has been advanced through detailed labeling studies. However, the actual enzymes and genes responsible for production of 1 in lichens have not been identified or characterized. Work presented in this thesis is directed at the synthesis of a key biosynthetic precursor to usnic acid (1), methylphloracetophenone ( 2). Methylphloracetophenone (2) has been synthesized using two separate routes. The first involved the methylation of trihydroxyacetophenone (3) with iodomethane. The second involved the acylation of trihydroxytoluene (4) with acetic anhydride using boron trifluoride diethyl etherate as a catalyst. This work will lay the foundation for further biosynthetic studies on usnic acid (1).As part of a natural products screening program, soil fungi have been cultured and grown in fermentation broths. Soil for this program was provided from the University of Manitoba cryptogam herbarium which houses lichen samples. From one strain of fungus, dimethyl oxalate (9), trimethyl citrate (10) and dimethyl citrate (11) are isolated. This fungus has been identified as Aspergillus niger both morphologically and genetically. A. niger is responsible for the production of millions of tonnes of citric acid annually, but this is the first report of methylated citric acid derivatives or dimethyl oxalate (9) from a filamentous fungus.Using horseradish peroxidase as a model enzyme system, we demonstrate that methylphloracetophenone (2) was oxidized to usnic acid ( 1). Analogues of 2 have been synthesized and include an ethyl group (5) and a propionyl group (6). These analogues were also subject to oxidation by horseradish peroxidase and we demonstrate that analogues of usnic acid (7 and 8) are formed. Although usnic acid (1) has potential as a pharmaceutical, it is hepatotoxic. These analogues of usnic acid (1) may be able to improve its pharmacological profile.