Studies in the chemistry of fungal natural products (2006)
Authorsvan der Sar, Soniashow all
Natural products as sources of novel therapeutic agents experienced a steady increase from around the turn of the twentieth century until it peaked in the 1970s and 1980s. However since this time pharmaceutical research in natural products has experienced a decline. Despite this trend the natural products industry now seems to be experiencing a revival of sorts. This thesis represents a continuation of the work on the isolation and structure elucidation of potential drug leads from terrestrial fungal sources that the natural products group at the University of Canterbury is engaged in. The known compound, pseurotin A (2.7) and two novel diastereomers, pseurotin A2 (2.8) and pseurotin A3 (2.9) were isolated from the extract of a Penicillium sp. of fungus collected from the foreshore of a beach in Vancouver, Canada. The absolute stereochemistry of pseurotin A2 and proposed absolute stereochemistry for A3 were elucidated using a combination of X-ray crystallography (A2 only), circular dichrosim, oxidative cleavage reactions, and J2-resoved 2D NMR experiments. The extract of an as yet unidentified endophytic fungus has yielded eight novel compounds related to the spirobisnaphthalene class of compounds. These eight compounds fall into to distinct groupings. The spiro-mamakones, distinguished by a structurally unprecedented oxygenated spiro-nonene skeleton, comprise five compounds, spiro-mamakones A-E (3.11, 3.15-3.18). In addition to these naturally occurring compounds, the semi-synthetic compounds, 4-oxo-spiro-mamakone A (3.12) and O-acetyl-spiro-mamakone A (3.21), were also synthesised. spiro-Mamakone A was found to be racemic, while X-ray crystallography and optical rotation revealed spiro-mamakone C (3.15) to be present as an enantiomeric mixture (4S*, 5S*, 9R*). Unfortunately the enantiomeric excess was unable to be elucidated. NOE experiments revealed spiro-mamakone B (3.16) to have the relative stereochemistry 4S*, 5S*, 9S*. The relative stereochemistry of spiro-mamakones D (3.17) (4S*, 5S*, 8S*, 9S*) and E (3.18) (4S*, 5S*, 8S*, 9R*) was proposed from comparison of coupling constant calculations from energy-minimised models with those of the experimentally determined values. The second group, comprising three novel compounds named the mamakunoic acids, mamakunoic acid A-C (3.8, 3.7, 3.10), are characterised by their acid substituted dihydro benzofuran system. The low yield obtained of these compounds, unfortunately prevented their stereochemical elucidation. In addition to structure elucidation, biosynthetic studies on spiro-mamakone A and mamakunoic acid B were also carried out. Analysis of the NMR spectra derived from spiro-mamakone A, labelled with isotopic acetate, revealed a situation complicated by the presence of isotopomers and racemisation, resulting in NMR spectra that were somewhat anomalous in appearance. These irregularities however, were resolved leading to the proposal that spiro-mamakone A was derived from a dihydroxynaphthalene (DHN) intermediate, which proceeds through to spiro-mamakone via an epoxide intermediate. Despite problems with purity and low yields of isotopically labelled mamakunoic acid B, it was proposed that like spiro-mamakone A, it proceeded via a DHN intermediate. The extract derived from a Malaysian Scleroderma sp. was found to contain a new dichlorinated pulvinic acid derivative, methyl-3',5'-dichloro-4,4'-di-O-methylatromentate (4.14), the structure of which was confirmed by X-ray crystallography. In addition three previously reported compounds, 4,4'-dimethoxyvulpinic acid (4.11), methyl-3'-chloro-4,4'-di-O-methylatromentate (4.12) and methyl-4,4'-dimethoxyvulpinate (4.13), were also isolated. The extract of another, as yet unidentified endophytic fungus was found to contain the new acetogenin, 1,5-dihydroxy-6-(2-hydroxyethyl)-3-methoxyacetophenone (5.7), differing from the known compound, 2,4-dihydroxy-6-(2-hydroxyethyl)-3-methoxyacetophenone (5.8) only by virtue of the substitution pattern. The structure of 5.7 was confirmed by X-ray crystallography. The implementation of efficient dereplication procedures is paramount for those working in the field of natural products. The recent advances that have been made in the dereplication process in the natural products group at the University of Canterbury are given using examples from this research and where necessary from other group members.