Four new 1,4-dihydroxy-5-phenyl-2-pyridinone alkaloids (1-4) were isolated from an EtOAc extract of a culture medium of Septoria pistaciarum. The structures of these compounds were determined by spectroscopic methods, and the absolute configuration of the major compound (1) by X-ray crystallographic analysis. Compound 1 exhibited moderate in vitro antiplasmodial (antimalarial) activity against chloroquine-sensitive (D6) and -resistant (W2) strains of Plasmodium falciparum and cytotoxic activity to Vero cells. Compound 2 was moderately active against both methicillin-sensitive and methicillin-resistant strains of Staphylococcus aureus.Plant-like metabolic pathways found in the apicoplast,1 a chloroplast-like organelle of Plasmodium species, have been identified as suitable targets in malaria drug discovery. A number of herbicides and phytotoxins with known molecular targets in the plastid have been evaluated for antimalarial activity.2 -4 Various phytotoxins released by plant pathogenic fungi are known 5 -7 to inhibit metabolic pathways in the apicoplast.8 As part of our ongoing search for potential sources of new antimalarial compounds, we have screened a number of plant pathogenic fungi for antiplasmodial activity.Septoria pistaciarum (Ascomycetes) is the causative agent of Septoria leaf and fruit spot disease in pistachio (Pistachia vera) in the US9 and Mediterranean countries.10 An EtOAc extract of this fungus showed potent herbicidal activity and moderate antiplasmodial activity against Plasmodium falciparum albeit with low selectivity index. No previous chemical work has been reported for this species. Bioassay-guided fractionation of the EtOAc extract led to the isolation of a new 1,4-dihydroxy-5-phenyl-2-pyridinone analogue as the active compound. Three additional new but inactive analogues of the same class were also isolated and identified. Several compounds of this class have previously been isolated from a number of fungal species and have shown antibacterial,11 , 12 antitumor,12 antifungal,11 , 13 and neurotrophic14 activities.* To whom correspondence should be addressed. Supporting Information available: NMR spectra of compounds 1-4, single-crystal X-ray structure of compound, and cif files of the X-ray data for compound 1.
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Results and DiscussionCompound 1 was isolated as the major active constituent of the EtOAc extract of S. pistaciarum. The molecular formula of 1 was determined as C 22 H 29 NO 4 by HRESIMS. The 13 C NMR spectra confirmed the presence of 22 carbon atoms, which comprised five quaternary, 11 methine, two methylene, and four methyl carbons. The 1 H NMR and COSY spectra displayed six hydrogens in the aromatic region consisting of a one-proton singlet and an A 2 B 2 C system due to a monosubstituted phenyl group. NMR signals in the aromatic region showed close resemblance to those of the 1,4-dihydroxy-5-phenyl-2-pyridinone moiety of the antitumor agent, TMC-69-6H, and its analogues.12 IR absorptions (3109, 1642, and 1556 cm −1 ) and UV maxima (207.1, 241.0, a...