Bioactive 1,4-Dihydroxy-5-phenyl-2-pyridinone Alkaloids from Septoria pistaciarum

Abstract

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. NIH Public Access 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...

Antiprotozoal and Antimicrobial Compounds from the Plant Pathogen Septoria pistaciarum

“…2 The aromatic regions of the 1 H NMR and COSY spectra were similar to those observed for 5-phenyl-2-pyridinones previously isolated from this species, and consisted of a one-proton singlet and an A 2 B 2 C spin system due to a monosubstituted phenyl group. 2 The aliphatic region exhibited two oxymethine and two methyl doublets indicating the presence of a 2,4-dimethyltetrahydropyran moiety and an N -methoxy group. HMBC correlations of H-2′ and H-6′ ( δ H 7.43) with C-3′ and C-5′ ( δ C 128.6), C-4′ ( δ C 127.8), C-1′ ( δ C 133.3), and C-5 ( δ C 114.5), and H-6 ( δ H 7.44) with C-2 ( δ C 158.0), C-4 ( δ C 161.9), C-5 ( δ C 114.5), and C-1′ ( δ C 133.3) supported the partial structure of the substituted pyridinone ring moiety.…”

“…ROESY correlations of compound 1 (Figure 1) and 2 were identical to those observed for septoriamycin A (8) suggesting that these two compounds had the same relative configurations. Since we have previously assigned the absolute configuration of septoriamycin A on the basis of X-ray diffraction data, 2 and all these compounds presumably share a common biosynthetic origin, compounds 1 and 2 also have 7 R , 8 R , 10 S , 11 R , and 12 R absolute configuration. It is further supported by their dextrorotatory specific rotations.…”

“…Fractionation of an EtOAc extract of a culture medium of S. pistaciarum by Sephadex LH-20 gel column chromatography followed by purification using silica gel and RP C 18 chromatography afforded four minor 2-pyridinone alkaloids 1–4 in addition to the known septoriamycin A ( 8 ) and its three derivatives, 2 three known perylenequinones (+)-cercosporin ( 5 ), (+)-14- O -acetylcercosporin ( 6 ), (+)-di- O -acetyl-cercosporin ( 7 ), lumichrome, and brassicasterol.…”

“…6 This suggested that the selective inhibition of the plant-like metabolic pathways in the apicoplast of malaria parasite 1 is not responsible for the observed antimalarial activity of compounds 5 – 7 . Septoriamycin A ( 8 ), with demonstrated antiplasmodial and antifungal activities, 2 exhibited significant antileishmanial activity with an IC 50 of 0.11 μM and an IC 90 of 0.29 μM (Table 5) and was more potent than the positive controls pentamidine and amphotericin B. Compounds 5 – 7 also showed significant antileishmanial activity with IC 50 values of 1.14 μM, 1.7 μM, and 3.1 μM, respectively (Table 5).…”

“…(ATCC 22201) was obtained from the American Type Culture Collection, Manassas, VA and it was grown as previously reported. 2 …”