Antibacterial and antifungal screening of natural products sourced from Australian fungi and characterisation of pestalactams D-F.

Eighteen natural products sourced from Australian micro- or macro-fungi were screened for antibacterial and antifungal activity. This focused library was comprised of caprolactams, polyamines, quinones, and polyketides, with additional large-scale isolation studies undertaken in order to resupply previously identified compounds. Chemical investigations of the re-fermented culture from the endophytic fungus Pestalotiopsis sp. yielded three caprolactam analogues, pestalactams D-F, along with larger quantities of the known metabolite pestalactam A, which was methylated using diazomethane to yield 4-O-methylpestalactam A. The chemical structures of the previously undescribed fungal metabolites were determined by analysis of 1D/2D NMR and MS data. The structure of 4-O-methylpestalactam A was confirmed following single crystal X-ray diffraction analysis. The antibacterial and antifungal activity of all compounds was assessed, which identified three compounds, (1S,3R)-austrocortirubin, (1S,3S)-austrocortirubin, and 1-deoxyaustrocortirubin with mild activity (100 µM) against Gram-positive isolates and one compound, 2-hydroxy-6-methyl-8-methoxy-9-oxo-9H-xanthene-1-carboxylic acid, with activity against Cryptococcus neoformans and Cryptococcus gattii at 50 µM.

Entonalactams A-C: Isoindolinone derivatives from an Australian rainforest fungus belonging to the genus Entonaema.

Bioassay-guided fractionation of an antimalarial DCM/MeOH extract derived from the Australian rainforest fungus Entonaema sp. resulted in the isolation of three new isoindolinone derivatives, entonalactams A-C (1-3), along with the known natural products 3-methoxy-5-methylbenzene-1,2-diol (4), daldinal B (5), and ergosta-4,6,8(14),22-tetraen-3-one (6). The chemical structures of the new secondary metabolites were determined following extensive 1D/2D NMR and MS data analysis. A single crystal X-ray structure for entonalactam A (1) confirmed the NMR-based structure assignment. Entonalactams A-C (1-3) were all determined to be racemic based on chiro-optical data. All secondary metabolites were tested in vitro against Plasmodium falciparum malaria parasites, and ergosta-4,6,8(14),22-tetraen-3-one (6) was identified as the most active compound with 66% inhibition at 50 µM.

Chemical investigation of an antimalarial Chinese medicinal herb Picrorhiza scrophulariiflora.

An antimalarial medicinal plant Picrorhiza scrophulariiflora was chemically investigated as part of our ongoing research in traditional chinese medicines (TCM). Mass directed fractionation of the active part of the crude extract led to the isolation of ten main components, three new compounds (1-3) and seven known compounds (4-10). Compound 10 inhibited the growth of the Plasmodium falciparum 3D7 malarial parasite line, with an IC50 value of 8.3µM. This compound accounted for ∼95% of P. falciparum growth inhibitory activity in the crude extract confirming, for this TCM, that a single compound was responsible for the antimalarial activity.

Pyridocoumarin, aristolactam and aporphine alkaloids from the Australian rainforest plant Goniothalamus australis.

Chemical investigation of the CH(2)Cl(2)/CH(3)OH extracts from aerial parts of the Australian plant Goniothalamus australis has resulted in the isolation of two pyridocoumarin alkaloids, goniothalines A (1) and B (2) as well as eight known natural products, aristolactam AII (3), enterocarpam II (4), caldensine (5), sauristolactam (6), (-)-anonaine (7), asimilobine (8), altholactone (9) and (+)-goniofufurone (10). The chemical structures of all compounds were determined by extensive spectroscopic and spectrometric analysis. Methylation of 2 using TMS-diazomethane afforded 1, which unequivocally established that both 1 and 2 possessed a 10-methyl-2H-pyrano[2,3-f]quinolin-2-one skeleton. These pyridocoumarin alkaloids are putatively proposed to arise biosynthetically from an aporphinoid precursor. Compounds 1-10 were evaluated for in vitro antimalarial activity against a chloroquine-sensitive Plasmodium falciparum line (3D7). Sauristolactam (6) and (-)-anonaine (7) exhibited the most potent antiparasitic activity with IC(50) values of 9 and 7 µM, respectively.

Synthesis and antimalarial evaluation of a screening library based on a tetrahydroanthraquinone natural product scaffold.

As part of a research program aimed at discovering new antimalarial leads from Australian macrofungi a unique fungi-derived prefractionated library was screened against a chloroquine-sensitive Plasmodium falciparum line (3D7) using a radiometric growth inhibition assay. A library fraction derived from a Cortinarius species displayed promising antimalarial activity. UV-guided fractionation on the CH(2)Cl(2)/MeOH extract from this fungus resulted in the isolation of four known compounds: (1S,3R)-austrocortirubin (1), (1S,3S)-austrocortirubin (2), 1-deoxyaustrocortirubin (3), and austrocortinin (4). Compound 2 was used as a natural product scaffold in the parallel solution-phase synthesis of a small library of N-substituted tetrahydroanthraquinones (5-15). All compounds (1-15) were tested in vitro against P. falciparum 3D7 parasites and (1S,3S)-austrocortirubin (2), the major fungal constituent, was shown to be the most active compound with an IC(50) of 1.9 µM. This compound displayed moderate cytotoxicity against neonatal foreskin fibroblast (NFF) cells with an IC(50) of 15.6 µM.

Design and synthesis of screening libraries based on the muurolane natural product scaffold.

The plant-derived natural product 14-hydroxy-6,12-muuroloadien-15-oic acid (1) was identified as a unique scaffold that could be chemically elaborated to generate novel lead- or drug-like screening libraries. Prior to synthesis a virtual library was generated and prioritised based on drug-like physicochemical parameters such as log P, log D(5.5), hydrogen bond donors/acceptors, and molecular weight. The natural product scaffold (1) was isolated from the endemic Australian plant Eremophila mitchellii and then utilised in the parallel solution-phase generation of two series of analogues. The first library consisted of six semi-synthetic amide derivatives, whilst the second contained six carbamate analogues. These libraries have been evaluated for antimalarial activity using a chloroquine-sensitive Plasmodium falciparum line (3D7) and several compounds displayed low to moderate activity with IC(50) values ranging from 14 to 33 µM.

Synthesis and antiplasmodial evaluation of novel chromeno[2,3-b]chromene derivatives.

5-Methoxyflavenes and 6-methoxyflavenes were found to undergo stereoselective acid-catalyzed rearrangement to generate a range of novel chromeno[2,3-b]chromene derivatives. When subjected to an in vitro antiplasmodial growth inhibition assay using Plasmodium falciparum (3D7 line) the chromene analogues were shown to display IC(50) values ranging from 6.8 to 39.8 µM.