The first synthesis of 2-methyl-6-pentadecyl-Δ1,6-piperideine (1), a major alkaloid of the piperideine chemotype in fire ant venoms, and its analogues, 2-methyl-6-tetradecyl-Δ1,6-piperideine (2) and 2-methyl-6-hexadecyl-Δ1,6-piperideine (3), was achieved by a facile synthetic method starting with glutaric acid (4) and urea (5). Compound 1 showed in vitro antifungal activity against Cryptococcus neoformans and Candida albicans with IC50 values of 6.6 and 12.4 μg/mL, respectively, and antibacterial activity against vancomycin-resistant Enterococcus faecium with an IC50 value of 19.4 μg/mL, while compounds 2 and 3 were less active against these pathogens. All three compounds strongly inhibited the parasites Leishmania donovani promastigotes and Trypanosoma brucei with IC50 values in the range of 5.0–6.7 and 2.7–4.0 μg/mL, respectively.

Thirty-three natural-product-based acylphloroglucinol derivatives were synthesized to identify antifungal compounds against Cryptococcus spp. that cause the life-threatening disseminated cryptococcosis. In vitro antifungal testing showed that 17 compounds were active against C. neoformans ATCC 90113, C. neoformans H99, and C. gattii ATCC 32609, with minimum inhibitory concentrations (MICs) in the range 1.0–16.7 μg/mL. Analysis of the structure and antifungal activity of these compounds indicated that the 2,4-diacyl- and 2-acyl-4-alkylphloroglucinols were more active than O-alkyl-acylphloroglucinols. The most promising compound found was 2-methyl-1-(2,4,6-trihydroxy-3-(4-isopropylbenzyl)phenyl)propan-1-one (11j), which exhibited potent antifungal activity (MICs, 1.5–2.1 μg/mL) and low cytotoxicity against the mammalian Vero and LLC-PK1 cell lines (IC50 values >50 μg/mL). This compound may serve as a template for further synthesis of new analogues with improved antifungal activity. The findings of the present work may contribute to future antifungal discovery toward pharmaceutical development of new treatments for cryptococcosis.

Bioassay-guided fractionation of the organic extracts of a Bacillus amyloliquefaciens strain (AP183) led to the discovery of a new macrocyclic polyene antibiotic, bacillusin A (1). Its structure was assigned by interpretation of NMR and MS spectroscopic data as a novel macrodiolide composed of dimeric 4-hydroxy-2-methoxy-6-alkenylbenzoic acid lactones with conjugated pentaene-hexahydroxy polyketide chains. Compound 1 showed potent antibacterial activities against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium with minimum inhibitory concentrations in a range of 0.6 to 1.2 μg/mL. The biosynthetic significance of this unique class of antibiotic compounds is briefly discussed.

Antifungal screening of small-molecule natural product libraries showed that a column fraction (CF) derived from the plant extract of Sagittaria latifolia was active against the fungal pathogen Cryptococcus neoformans. Dereplication analysis by liquid chromatography–mass spectrometry (LC-MS) and proton nuclear magnetic resonance spectroscopy (1H NMR) indicated the presence of new compounds in this CF. Subsequent fractionation of the plant extract resulted in the identification of two new isopimaradiene-type diterpenoids, 1 and 2. The structures of 1 and 2 were determined by chemical methods and spectroscopic analysis as isopimara-7,15-dien-19-ol 19-O-α-l-arabinofuranoside and isopimara-7,15-dien-19-ol 19-O-α-l-(5′-acetoxy)arabinofuranoside, respectively. Compound 1 exhibited IC50 values of 3.7 and 1.8 μg/mL, respectively, against C. neoformans and C. gattii. Its aglycone, isopimara-7,15-dien-19-ol (3), resulting from acid hydrolysis of 1, was also active against the two fungal pathogens, with IC50 values of 9.2 and 6.8 μg/mL, respectively. This study demonstrates that utilization of the combined LC-MS and 1H NMR analytical tools is an improved chemical screening approach for hit prioritization in natural product drug discovery.

Psorothatins A–C (1–3), three antibacterial prenylated acylphloroglucinol derivatives, were isolated from the native American plant Psorothamnus fremontii. They feature an unusual α,β-epoxyketone functionality and a β-hydroxy-α,β-unsaturated ketone structural moiety. The latter forms a pseudo-six-membered heterocyclic ring due to strong intramolecular hydrogen bonding, as indicated by the long-range proton–carbon correlations in the NMR experiments. Psorothatin C (3) was the most active compound against methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium, with IC50 values in the range 1.4–8.8 μg/mL. The first total synthesis of 3 described herein permits future access to structural analogues with potentially improved antibacterial activities.

The generation of natural product libraries containing column fractions, each with only a few small molecules, using a high-throughput, automated fractionation system, has made it possible to implement an improved dereplication strategy for selection and prioritization of leads in a natural product discovery program. Analysis of databased UPLC-MS-ELSD-PDA information of three leads from a biological screen employing the ependymoma cell line EphB2-EPD generated details on the possible structures of active compounds present. The procedure allows the rapid identification of known compounds and guides the isolation of unknown compounds of interest. Three previously known flavanone-type compounds, homoeriodictyol (1), hesperetin (2), and sterubin (3), were identified in a selected fraction derived from the leaves of Eriodictyon angustifolium. The lignan compound deoxypodophyllotoxin (8) was confirmed to be an active constituent in two lead fractions derived from the bark and leaves of Thuja occidentalis. In addition, two new but inactive labdane-type diterpenoids with an uncommon triol side chain were also identified as coexisting with deoxypodophyllotoxin in a lead fraction from the bark of T. occidentalis. Both diterpenoids were isolated in acetylated form, and their structures were determined as 14S,15-diacetoxy-13R-hydroxylabd-8(17)-en-19-oic acid (9) and 14R,15-diacetoxy-13S-hydroxylabd-8(17)-en-19-oic acid (10), respectively, by spectroscopic data interpretation and X-ray crystallography. This work demonstrates that a UPLC-MS-ELSD-PDA database produced during fractionation may be used as a powerful dereplication tool to facilitate compound identification from chromatographically tractable small-molecule natural product libraries.

Miltefosine is an alkylphosphocholine that shows broad-spectrum in vitro antifungal activities and limited in vivo efficacy in mouse models of cryptococcosis. To further explore the potential of this class of compounds for the treatment of systemic mycoses, nine analogs (3a−3i) were synthesized by modifying the choline structural moiety and the alkyl chain length of miltefosine. In vitro testing of these compounds against the opportunistic fungal pathogens Candida albicans, Candida glabrata, Candida krusei, Aspergillus fumigatus, and Cryptococcus neoformans revealed that N-benzyl-N,N-dimethyl-2-{[(hexadecyloxy)hydroxyphosphinyl]oxy}ethanaminium inner salt (3a), N,N-dimethyl-N-(4-nitrobenzyl)-2-{[(hexadecyloxy)hydroxyphosphinyl]oxy}ethanaminium inner salt (3d), and N-(4-methoxybenzyl)-N,N-dimethyl-2-{[(hexadecyloxy)hydroxyphosphinyl]oxy}ethanaminium inner salt (3e) exhibited minimum inhibitory concentrations (MIC) of 2.5–5.0 μg/mL against all tested pathogens, when compared to miltefosine with MICs of 2.5–3.3 μg/mL. Compound 3a showed low in vitro cytotoxicity against three mammalian cell lines similar to miltefosine. In vivo testing of 3a and miltefosine against C. albicans in a mouse model of systemic infection did not demonstrate efficacy. The results of this study indicate that further investigation will be required to determine the potential usefulness of the alkylphosphocholines in the treatment of invasive fungal infections.

Seven 6-alkyl-2,3,4,5-tetrahydropyridines (5a–5g) that mimic the natural piperideines that were recently identified in fire ant venom have been synthesized. Compounds 5c–5g with C-6 alkyl chain lengths from C14 to C18 showed varying degrees of antifungal activities, with 5e (6-hexadecyl-2,3,4,5-tetrahydropyridine) and 5f (6-heptadecyl-2,3,4,5-tetrahydropyridine) being the most active. Compound 5e exhibited minimum fungicidal concentrations of 3.8, 15.0, 7.5, and 7.5 μg/mL against Cryptococcus neoformans, Candida albicans, Candida glabrata, and Candida krusei, respectively. The antifungal activities of these compounds appear to be associated with the C-6 side chain length. This study represents the first effort to evaluate antifungal activities of synthetic analogues of the newly identified fire ant venom alkaloids.

Synthetic analogues of the marine-derived class of natural products phloeodictines have been prepared and exhibited potent in vitro fungicidal activities against a broad array of fungal pathogens including drug-resistant strains. The 6-hydroxy-2,3,4,6-tetrahydropyrrolo[1,2-a]pyrimidinium structural moiety with a C12 to C16 aliphatic side chain at C-6 has been shown to be the antifungal pharmacophore and may serve as a new antifungal template for further lead optimization.Keywords: antifungal template; fungicidal; mycosis; Phloeodictines

Density functional theory/B3LYP has been employed to optimize the conformations of selected 4-arylflavan-3-ols and their phenolic methyl ether 3-O-acetates. The electronic circular dichroism spectra of the major conformers have been calculated using time-dependent density functional theory to validate the empirical aromatic quadrant rule applied to the assignment of the absolute configuration of this class of compounds. The modest 6-31G* basis set was sufficient to produce reasonable spectra. The calculated Cotton effects at 220−240 nm, crucial for the assignment of the C-4 absolute configuration, result from electronic transitions of the molecular orbitals involving the π-electrons of the spatially close aromatic A-ring and 4-aryl moieties. The sign of this Cotton effect is determined by the orientation of the 4-aryl substituent: the negative and positive Cotton effects are associated with 4α- and 4β-aryl substituents, respectively.

Two new eudesmane derivatives were isolated from the leaves and flowers of Verbesina virginica, along with the known 6-O-β-E-p-coumaroyl-4α-hydroxyeudesmane (1). Their structures were determined as 6-O-β-Z-p-coumaroyl-4α-hydroxyeudesmane (2) and 6-O-α-E-p-coumaroyl-1β-4α-dihydroxyeudesmane (3) by spectroscopic methods.

Bioassay-guided fractionation of the extract of Topsentia sp. led to the identification of two new sulfated sterols, geodisterol-3-O-sulfite (1) and 29-demethylgeodisterol-3-O-sulfite (2), the active constituents reversing efflux pump-mediated fluconazole resistance. Both compounds enhanced the activity of fluconazole in a Saccharomyces cerevisiae strain overexpressing the Candida albicans efflux pump MDR1, as well as in a fluconazole-resistant Candida albicans clinical isolate known to overexpress MDR1. These results provide insight into the clinical utility of combining efflux pump inhibitors with current antifungals to combat the resistance associated with opportunistic fungal infections caused by C. albicans.

Time-dependent density functional theory (TDDFT) was employed for theoretical calculation of electronic circular dichroism (ECD) of a hexahydroxydiphenoyl (HHDP)-containing flavanone glycoside, mattucinol-7-O-[4′′,6′′-O-(aS)-hexahydroxydiphenoyl]-β-d-glucopyranoside (2). It identified the roles of the (2S)-flavanone and (aS)-HHDP moieties in generating the ECD spectrum of 2 and provided theoretical evidence for the empirical ECD rules applicable to monomeric flavanones and HHDP-containing compounds. The experimentally observed high-amplitude positive Cotton effect (CE) around 240 nm in 2 is derived from the (aS)-HHDP chromophore, while the low-amplitude negative CE in the 260−300 nm region is contributed by both the (aS)-HHDP and (2S)-flavanone moieties. The “linker” glucosyl moiety has little effect on the overall ECD. It appears that the respective chromophores in 2 contribute additively to the overall ECD, and the empirical rules are applicable for configurational assignment. However, if an (aR)-HHDP chromophore is present, as shown in mattucinol-7-O-[4′′,6′′-O-(aR)-hexahydroxydiphenoyl]-β-d-glucopyranoside (3), the dominant role of the (aR)-HHDP and interaction between the (aR)-HHDP and (2S)-flavanone moieties in its overall ECD may be confusing when applying the empirical rules to experimental ECD interpretation. Thus, theoretical calculation of the ECD that quantifies the contributions and interactions of different chromophores is essential for the assignment of the absolute configuration of such molecules.

Verbesinosides A−F (1−6), six new 15,27-cyclooleanane-type triterpenoid saponins carrying different aromatic acyl moieties on the aglycon, were isolated from the leaves and flowers of Verbesina virginica. Their structures were established by interpretation of spectroscopic data and chemical methods. The representative major saponin, verbesinoside A (1), has the structure 21-trimethoxybenzoyl 15α,27-cycloolean-12-en-3β,21β-diol-28-oic acid 3-O-β-d-xylopyranosyl(1→4)-β-d-xylopyranosyl(1→2)-β-d-glucopyranoside. This is the first report of triterpenoid saponins possessing the unique 15,27-cyclooleanane skeleton. The anisotropic effects of the aromatic acyl moieties on the triterpenoid skeleton are discussed.

Puupehanol (1), a new sesquiterpene-dihydroquinone derivative, was isolated from the marine sponge Hyrtios sp., along with the known compounds puupehenone (2) and chloropuupehenone (3) that are responsible for the antifungal activity observed in the extract. The structure of 1 was established as (20R,21R)-21-hydroxy-20,21-dihydropuupehenone by extensive spectroscopic and computational methods. Compound 2 exhibited potent activity against Cryptococcus neoformans and Candida krusei with MFCs of 1.25 and 2.50 μg/mL, respectively.Puupehanol (1), a new sesquiterpene-dihydroquinone derivative, was isolated from the marine sponge Hyrtios sp., along with the known antifungal compounds puupehenone (2) and chloropuupehenone (3).