Synthesis of and tautomerism in 3-acyltetramic acids

Total Synthesis of Aflastatin A

The total syntheses of aflastatin A and its C3-C48 degradation fragment (6a, R = H) have been accomplished. The syntheses feature several complex diastereoselective fragment couplings, including a Felkin-selective trityl-catalyzed Mukaiyama aldol reaction, a chelate-controlled aldol reaction involving soft enolization with magnesium, and an anti-Felkin-selective boron-mediated oxygenated aldol reaction. Careful comparison of the spectroscopic data for the synthetic C3-C48 degradation fragment to that reported by the isolation group revealed a structural misassignment in the lactol region of the naturally derived degradation product. Ultimately, the data reported for the naturally derived aflastatin A C3-C48 degradation lactol (6a, R = H) were attributed to its derivative lactol trideuteriomethyl ether (6c, R = CD₃). Additionally, the revised absolute configurations of six stereogenic centers (C8, C9, and C28-C31) were confirmed.

Mediation of metal chelation in cysteine-derived tetramate systems

A study of bicyclic tetramates modified with a bulky ester, which leads to steric hindrance of distal chelating atoms as a route for the alteration of metal binding ability is reported. This approach required the development of a direct method for the synthesis of different esters of cysteine from cystine, which then provided access to bicyclic tetramates by Dieckmann cyclisation. Further derivation to ketones and carboxamides by Grignard addition and transamination reactions respectively provided rapid access to a chemical library of tetramates with diverse substitution. Of interest is that bicyclic tetramate ketones and carboxamides showed different tautomeric and metal binding behaviour in solution. Significantly, in both systems, the incorporation of bulky C-5 esters at the bridging position not only reduced metal binding, but also enhanced antibacterial potencies against Gram-positive MRSA bacteria. Those tetramates with antibacterial activity which was not metal dependent showed physiochemical properties of MSA of 559-737 Ų, MW of 427-577 Da, clogP of 1.8-6.1, clogD_7.4 of -1.7 to 3.7, PSA of 83-109 Ų and relative PSA of 12-15% and were generally Lipinski rule compliant. A subset of tetramates exhibited good selectivity towards prokaryotic bacterial cells. Given that the work reported herein is synthesis-led, without the underpinning detailed mechanistic understanding of biological/biochemical mechanism, that the most active compounds occupy a small region of chemical space as defined by MW, clogP, PSA and %PSA is of interest. Overall, the bicyclic tetramate template is a promising structural motif for the development of novel antibacterial drugs, with good anti-MRSA potencies and appropriate drug-like physiochemical properties, coupled with a potential for multi-targeting mechanisms and low eukaryotic cytotoxicity.

Metal Binding and Its Amelioration in Tetramates

Metal chelation in tetramates may be ameliorated by changing the ligating group and by steric blocking, which in turn leads to a change in their antibacterial properties; the former was achieved by replacement of an amide with a C-9 C═N bond and the latter by the synthesis of cysteine-derived tetramates with functionalization at the C-6 or C-9 enolic groups. In both cases, the metal-chelating ability was weak, and a loss of antibacterial activity was observed. Tetramate alkylations with an extended tricarbonyl-conjugated system could be achieved under Mitsunobu conditions which led to regioisomers, distinguishable by careful heteronuclear multiple bond coherence correlation and carbonyl carbon chemical shift analysis. C-9 and C-6 O-alkylation were observed but not C-8 O-alkylation for tetramate carboxamides; interestingly, C-7 alkylation with allyl and prenyl derivatives was also observed, and this arose by the rearrangement of initially formed O-alkyl products. Only the C-7 alkylated tetramate derivatives 13a and 13d with no metal-chelating ability demonstrated promising antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), with the most active analogue exhibiting a minimum inhibitory concentration of ≤ 1.95 μg/mL against MRSA, suggesting a mechanism of action independent of metal chelation. Otherwise, modifications at C-6/C-9 of tetramates led to a complete loss of metal-chelating ability, which correlated with the loss of antibacterial activity. This work further confirms that the metal-chelating capability is of fundamental importance in the biological activity of tetramates.

NMR of Natural Products as Potential Drugs

This review outlines methods to investigate the structure of natural products with emphasis on intramolecular hydrogen bonding, tautomerism and ionic structures using NMR techniques. The focus is on 1H chemical shifts, isotope effects on chemical shifts and diffusion ordered spectroscopy. In addition, density functional theory calculations are performed to support NMR results. The review demonstrates how hydrogen bonding may lead to specific structures and how chemical equilibria, as well as tautomeric equilibria and ionic structures, can be detected. All these features are important for biological activity and a prerequisite for correct docking experiments and future use as drugs.

Discovery of γ-Lactam Alkaloid Derivatives as Potential Fungicidal Agents Targeting Steroid Biosynthesis

Biological control of plant pathogens is considered as one of the green and effective technologies using beneficial microorganisms or microbial secondary metabolites against plant diseases, and so microbial natural products have played important roles in the research and development of new and green agrochemicals. To explore the potential applications for natural γ-lactam alkaloids and their derivatives, 26 γ-lactams that have flexible substituent patterns were synthesized and characterized, and their in vitro antifungal activities against eight kinds of plant pathogens belonging to oomycetes, basidiomycetes, and deuteromycetes were fully evaluated. In addition, the high potential compounds were further tested using an in vivo assay against Phytophthora blight of pepper to verify a practical application for controlling oomycete diseases. The potential modes of action for compound D1 against Phytophthora capsici were also investigated using microscopic technology (optical microscopy, scanning electron microscopy, and transmission electron microscopy) and label-free quantitative proteomics analysis. The results demonstrated that compound D1 may be a potential novel fungicidal agent against oomycete diseases (EC₅₀ = 4.9748 μg·mL^-1 for P. capsici and EC₅₀ = 5.1602 μg·mL^-1 for Pythium aphanidermatum) that can act on steroid biosynthesis, which can provide a certain theoretical basis for the development of natural lactam derivatives as potential antifungal agents.

Synthetic Access to Hydrophilic Tetramate Derivatives of Cysteine

The synthesis, structural, and antibacterial evaluation of bicyclic tetramate derivatives of cysteine rendered hydrophilic with pendant heterocyclic substituents is reported; effective synthetic protocols and antibacterial activity for a small library of polar derivatives were found, and direct evidence for strong metal chelation in these systems was obtained. A computational study has developed a detailed understanding of the controlling factors of the key Dieckmann cyclization step.

Total Synthesis of Cladosins B and C

A convergent synthetic route to the fungal metabolites cladosins B and C has been developed, affording these natural products in 29% and 27% overall yield, respectively. The cladosins are rare examples of hybrid polyketides featuring a 3-enamine tetramic acid group derived from l-valine. Key steps in this modular six-step sequence include a DMAP-mediated O- to C-acyl rearrangement to unite the side chains with the tetramic acid core and subsequent amine incorporation using either ammonium acetate or HMDS.

Synthesis of Tetramic Acid Fragments Derived from Vancoresmycin Showing Inhibitory Effects towards S. aureus

An efficient route to various vancoresmycin‐type tetramic acids has been developed. The modular route is based on an effective Fries‐type rearrangement to introduce various appending acetyl residues. The minimum inhibitory concentration (MIC) values of the new tetramic acids against Staphylococcus aureus and Escherichia coli were determined, revealing that three of the new compounds exhibit antimicrobial activity against S. aureus. These bioactive compounds were structurally most closely related to the authentic vancoresmycin building block. Additionally, the compounds induced a lial‐lux bioreporter, which responds to cell wall stress induced by antibiotics that interfere with the lipid II biosynthesis cycle. These data suggest the tetramic acid moiety to be a part of the vancoresmycin pharmacophore.

Structure-Elucidating Total Synthesis of the (Polyenoyl)tetramic Acid Militarinone C§

The (polyenoyl)tetramic acid militarinone C (1) heads a family of seven members. Before our work, the configuration of C-5 was unknown whereas the configurations of C-8' and C-10' were either (R,R) or (S,S). We synthesized the four stereoisomers of constitution 1, which conform with these insights. This included cross-coupling both enantiomers of the western building block (8) with both enantiomers of the eastern building block (9). The specific rotations of the resulting 1 isomers suggested that natural 1 is configured like the coupling partners (S)-8 and (R,R)-9. This conclusion was corroborated by degrading natural 1 to alcohol 35 and by proving its configurational identity with synthetic (R,R)-35.

Hybrid Polyketides from a Hydractinia-Associated Cladosporium sphaerospermum SW67 and Their Putative Biosynthetic Origin

Five hybrid polyketides (1a, 1b, and 2–4) containing tetramic acid core including a new hybrid polyketide, cladosin L (1), were isolated from the marine fungus Cladosporium sphaerospermum SW67, which was isolated from the marine hydroid polyp of Hydractinia echinata. The hybrid polyketides were isolated as a pair of interconverting geometric isomers. The structure of 1 was determined based on 1D and 2D NMR spectroscopic and HR-ESIMS analyses. Its absolute configuration was established by quantum chemical electronic circular dichroism (ECD) calculations and modified Mosher’s method. Tetramic acid-containing compounds are reported to be derived from a hybrid PKS-NRPS, which was also proved by analyzing our ¹³C-labeling data. We investigated whether compounds 1–4 could prevent cell damage induced by cisplatin, a platinum-based anticancer drug, in LLC-PK1 cells. Co-treatment with 2 and 3 ameliorated the damage of LLC-PK1 cells induced by 25 μM of cisplatin. In particular, the effect of compound 2 at 100 μM (cell viability, 90.68 ± 0.81%) was similar to the recovered cell viability of 88.23 ± 0.25% with 500 μM N-acetylcysteine (NAC), a positive control.

Bioactivity and Mode of Action of Bacterial Tetramic Acids

Microbially produced 3-acyltetramic acids display a diverse range of biological activities. The pyreudiones are new members of this class that were isolated from bacteria of the genus Pseudomonas. Here, we performed a structure-activity relationship study and determined their mode of action. An efficient biomimetic synthesis was developed to synthesize pyreudione A. Pyreudiones and synthetic analogs thereof were tested for their amoebicidal, antibacterial, antiproliferative, and cytotoxic activities. The length of the alkyl side chain and the nature of the amino acid residues within the tetramic acid moiety strongly affected activity, in particular against mycobacteria. The mode of action was shown to correlate with the ability of pyreudiones to act as protonophores. Removal of the acidic proton by methylation of pyreudione A resulted in a loss of bioactivity.

Functionalised bicyclic tetramates derived from cysteine as antibacterial agents

Routes to bicyclic tetramates derived from cysteine permitting ready incorporation of functionality at two different points around the periphery of a heterocyclic skeleton are reported. This has enabled the identification of systems active against Gram-positive bacteria, some of which show gyrase and RNA polymerase inhibitory activity. In particular, tetramates substituted with glycosyl side chains, chosen to impart polarity and aqueous solubility, show high antibacterial activity coupled with modest gyrase/polymerase activity in two cases. An analysis of physicochemical properties indicates that the antibacterially active tetramates generally occupy physicochemical space with MW of 300-600, clog D7.4 of -2.5 to 4 and rel. PSA of 11-22%. This work demonstrates that biologically active 3D libraries are readily available by manipulation of a tetramate skeleton.

3-Acyltetramic acids as a novel class of inhibitors for human kallikreins 5 and 7

Human kallikreins 5 and 7 (KLK5 and KLK7) exhibit trypsin- and chymotrypsin-like activities and are involved in pathologies related to skin desquamation process. A series of new 3-acyltetramic acids were developed as a novel class of inhibitors of KLK5, KLK7 and trypsin enzymes. The nature and length of the acyl chain is crucial to the KLK5, KLK7 and trypsin inhibition activities, and the most potent compounds (but not the most selective) 2b, 2c and 2g showed low micromolar IC₅₀ values. While very few of the compounds were selective for KLK5, the selective inhibition of trypsin against chymotrypsin was achieved. Our molecular modelling studies revealed that the double bond in 2g provided the best fit in the binding site of KLK5, while the hydrogen bonding interactions modulated the best fit of 2c in the binding site of KLK7 due to the hydrophobicity of the cavity.

Aniline-Tetramic Acids from the Deep-Sea-Derived Fungus Cladosporium sphaerospermum L3P3 Cultured with the HDAC Inhibitor SAHA

Four new tetramic acids, cladosins H-K (1-4), and a related known compound, cladodionen (5), were isolated from the culture of the Mariana Trench (depth 6562 m) sediment-derived fungus Cladosporium sphaerospermum L3P3 treated with the histone deacetylase inhibitor SAHA (suberanilohydroxamic acid). Interestingly, compounds 1-5 existed as equilibrium E/ Z mixtures and 1-4 were the first cases of tetramic acids containing aniline moieties. Their structures including absolute configurations were elucidated through a combination of NMR, MS, and Mosher's method, together with the consideration of biogenetic origins. Incubation experiments of exogenous aniline and N-phenyloctanamide revealed that the aniline moiety in cladosins H-K (1-4) is probably derived from the degradation of SAHA, indicating that the well-known histone deacetylase inhibitor SAHA could be metabolized by L3P3 and provide aniline as a precursor for biotransformation of chemically reactive polyketides. The cytotoxicity of 1-5 was evaluated against the PC-3, MGC-803, SH-SY5Y, HCT-116, K562, and HL-60 cell lines, and compound 2 showed promising cytotoxicity against the HL-60 cell line with an IC₅₀ value of 2.8 μM.

New β-lactam - Tetramic acid hybrids show promising antibacterial activities

β-Lactams are the most important class of antibiotics, for which the emergence of resistance threatens their utility. As such, we explored the extent to which the tetramic acid motif, frequently found in naturally occurring antibiotics, can be used to generate novel β-lactam antibiotics with improved antibacterial activity. We synthesized new ampicillin - tetramic acid, cephalosporin - tetramic acid, and cephamycin - tetramic acid analogs and evaluated their activities against problematic Gram-positive and Gram-negative pathogens. Amongst the analogs, a 7-aminocephalosporanic acid analog, 3397, and a 7-amino-3-vinyl cephalosporanic acid, 3436, showed potent activities against S. aureus NRS 70 (MRSA) with MICs of 6.25 μg/mL and 3.13 μg/mL respectively. These new analogs were ≥16-fold more potent than cefaclor and cephalexin. Additionally, a Δ² cephamycin - tetramic acid analog 3474 which contained a basic guanidinium substituent at the 5-position of the tetramic acid core displayed potent activity against several clinical strains of K. pneumoniae and E. coli.

A Method for C2 Acylation of 1,3-Indandiones

The 1,3-indandione scaffold is an important structural motif used in the preparation of a large number of industrial chemical and pharmaceutical compounds. However, few approaches allow for the direct C2 acylation on these building blocks. A method was developed using DMAP and EDCI, which is mild in reactivity, covers a diverse range of carboxylic acid acylating agents, is compatible with electron releasing and withdrawing substituents on the 1,3-indandione partner, and performs well in a polar aprotic solvent (for solubility reasons) This method cleanly afforded twenty five different products in yields of 32-96%.

Diastereoselective reduction of the tricarbonyl moiety in bicyclic tetramates giving pyroglutamates

The reduction of C(6)-acyl bicyclic tetramic acids has been achieved with complete diastereoselectivity via catalytic hydrogenation using PtO2. The resulting pyroglutamates had potent antibacterial and anticancer properties and will allow the preparation of simple mimics of pyroglutamate-containing natural products.

Chemoselectivity and stereoselectivity of cyclisation pathways leading to bicyclic tetramates controlled by ring-chain tautomerisation in thiazolidines

Chemoselective Dieckmann cyclisation reactions on N-malonyl thiazolidine templates derived from cysteine and pivaldehyde or aromatic aldehydes may be used to access bicyclic tetramates, for which different pathways operate as a result of differing ring-chain tautomeric behaviour of the respective intermediate imines.

Nematicidal Activity of 3-Acyltetramic Acid Analogues Against Pine Wood Nematode, Bursaphelenchus xylophilus

Among 98 3-acyltetramic acid analogues, compounds 1c, 2c, 2f and 2g, showed >90% nematicidal activity against the pine wood nematode Bursaphelenchus xylophilus at a 10 μg/mL concentration. The nematicidal activities of compounds 1d, 1h, and 2k were a little lower at 88.0%, 85.8%, and 57.2% at a 10 μg/mL concentration, respectively. The nematicidal activity of emamection benzoate, widely used in Korea for the prevention of pine wilt disease, was 32.3% at a 10 μg/mL concentration. Other 3-acyltetramic acid analogues showed less than 30% nematicidal activity. A structure-activity relationship study indicated that the chain length of the C-acyl substituent was very important for high nematicidal activity. All active compounds had C₁₃H₂₇ or C₁₁H₂₃ acyl substituents, in two closely related groups with the common physicochemical properties of a polar surface area 57.6A², PSA (polar surface area) 7.8–8.6% and ClogP (calculated partition coefficient) 5.1–5.9 and a polar surface area 75–84A², PSA 11.1–11.6% and ClogP 4.7–5.1, respectively. Our study indicates that active 3-acyltetramic acid analogues could have potential as lead compounds for developing novel pine wood nematode control agents.

Mimics of pramanicin derived from pyroglutamic acid and their antibacterial activity

Mono and dihydroxypyrrolidinones are readily available by direct oxygenation of a pyroglutamate-derived bicyclic lactam with high diastereoselectivity, and these may be manipulated further in protected or unprotected form by Grignard addition to a pendant Weinreb amide to give acylhydroxypyrrolidinones.

Facile synthesis of acacetin and its derivatives

Acacetin, a O-methylated bioflavonoid isolated from the traditional Chinese medicine Xuelianhua (Saussurea tridactyla), is a promising orally effective atrium-selective antiarrhythmic agent for the treatment of atrial fibrillation (AF). Here we describe an efficient two-component method for the synthesis of acacetin and its derivatives.

Thiolactomycin-Based Inhibitors of Bacterial β-Ketoacyl-ACP Synthases with in Vivo Activity

β-Ketoacyl-ACP synthases (KAS) are key enzymes involved in the type II bacterial fatty acid biosynthesis (FASII) pathway and are putative targets for antibacterial discovery. Several natural product KAS inhibitors have previously been reported, including thiolactomycin (TLM), which is produced by Nocardia spp. Here we describe the synthesis and characterization of optically pure 5R-thiolactomycin (TLM) analogues that show improved whole cell activity against bacterial strains including methicillin-resistant Staphylococcus aureus (MRSA) and priority pathogens such as Francisella tularensis and Burkholderia pseudomallei. In addition, we identify TLM analogues with in vivo efficacy against MRSA and Klebsiella pneumoniae in animal models of infection.

Design, synthesis and microbiological evaluation of ampicillin-tetramic acid hybrid antibiotics

Exploiting iron-uptake pathways by conjugating β-lactam antibiotics with iron-chelators such as catechol and hydroxamic acid is a proven strategy to overcome permeability-related resistance in Gram-negative bacteria. Since naturally occurring iron chelating tetramic acids have not been previously examined for this purpose, an exploratory series of novel ampicillin-tetramic acid hybrids that structurally resemble ureidopenicillins was designed and synthesized. The new analogs were evaluated for the ability to chelate iron and their MIC activities determined against a representative panel of clinically significant bacterial pathogens. The tetramic acid β-lactam hybrids demonstrated a high affinity to iron in the order of 10⁻³⁰ M³. The hybrids were less active against Gram-positive bacteria. However, against Gram-negative bacteria, their activity was species dependent with several hybrids displaying improved activity over ampicillin against wild-type Pseudomonas aeruginosa. The anti-Gram-negative activities of the hybrids improved in the presence of clavulanic acid revealing that the tetramic acid moiety did not provide added protection against β-lactamases. Additionally, the hybrids were found to be efflux pump substrates as their activities markedly improved against pump-inactivated strains. Unlike the catechol and hydroxamic acid siderophore β-lactam conjugates, the activities of the hybrids did not improve under iron-deficient conditions. These results suggest that the tetramic acid hybrids gain permeability via different membrane receptors, or they are out competed by native bacterial siderophores with stronger affinities for iron. This study provides a foundation for the further exploitation of the tetramic acid moiety to achieve novel β-lactam anti-Gram-negative agents, providing that efflux and β-lactamase mediated resistance is addressed.

Ravynic acid, an antibiotic polyeneyne tetramic acid from Penicillium sp. elucidated through synthesis

Ravynic acid, an unsaturated tetramic acid possessing antibiotic activity, has been isolated from the fungus Penicillium sp. and identified through synthesis.

Recent Advances in the Total Synthesis of Tetramic Acid-Containing Natural Products

With incredible bioactivities and fascinating structural complexities, tetramic acid- (TA-) containing natural products have attracted favorable attention among the organic chemistry community. Although the construction of the TA core is usually straightforward, the intricate C3-side chain sometimes asks for some deliberative strategy so as to fulfill an elegant total synthesis. This review mainly covers some exceptional synthetic examples for each type of natural product in recent years, showcasing the great achievements as well as unsettled obstacles in this area, in the hope of accelerating the synthetic and biological investigations for this unique type of natural product.

Equisetin, reutericyclin and streptolodygin as natural product lead structures for novel antibiotic libraries

The emergence of antimicrobial resistance has created a need for the development of novel antibacterial therapies to treat infection. Natural products that exhibit antibacterial activity offer validated starting points for library generation, and the authors report here that small molecule mimics of tetramate-containing natural products may show antibacterial activity and offer the potential for further optimization.

Gastrointestinal localization of metronidazole by a lactobacilli-inspired tetramic acid motif improves treatment outcomes in the hamster model of Clostridium difficile infection

OBJECTIVES

Metronidazole, a mainstay treatment for Clostridium difficile infection (CDI), is often ineffective for severe CDI. Whilst this is thought to arise from suboptimal levels of metronidazole in the colon due to rapid absorption, empirical validation is lacking. In contrast, reutericyclin, an antibacterial tetramic acid from Lactobacillus reuteri, concentrates in the gastrointestinal tract. In this study, we modified metronidazole with reutericyclin's tetramic acid motif to obtain non-absorbed compounds, enabling assessment of the impact of pharmacokinetics on treatment outcomes.

METHODS

A series of metronidazole-bearing tetramic acid substituents were synthesized and evaluated in terms of anti-C. difficile activities, gastric permeability, in vivo pharmacokinetics, efficacy in the hamster model of CDI and mode of action.

RESULTS

Most compounds were absorbed less than metronidazole in cell-based Caco-2 permeability assays. In hamsters, lead compounds compartmentalized in the colon rather than the bloodstream with negligible levels detected in the blood, in direct contrast with metronidazole, which was rapidly absorbed into the blood and was undetectable in caecum. Accordingly, four leads were more efficacious (P < 0.05) than metronidazole in C. difficile-infected animals. Improved efficacy was not due to an alternative mode of action, as the leads retained the mode of action of metronidazole.

CONCLUSIONS

This study provides the clearest empirical evidence that the high absorption of metronidazole lowers treatment outcomes for CDI and suggests a role for the tetramic acid motif for colon-specific drug delivery. This approach also has the potential to lower systemic toxicity and drug interactions of nitroheterocyclic drugs for treating gastrointestine-specific diseases.

Antibacterial barbituric acid analogues inspired from natural 3-acyltetramic acids; synthesis, tautomerism and structure and physicochemical property-antibacterial activity relationships

The synthesis, tautomerism and antibacterial activity of novel barbiturates is reported. In particular, 3-acyl and 3-carboxamidobarbiturates exhibited antibacterial activity, against susceptible and some resistant Gram-positive strains of particular interest is that these systems possess amenable molecular weight, rotatable bonds and number of proton-donors/acceptors for drug design as well as less lipophilic character, with physicochemical properties and ionic states that are similar to current antibiotic agents for oral and injectable use. Unfortunately, the reduction of plasma protein affinity by the barbituric core is not sufficient to achieve activity in vivo. Further optimization to reduce plasma protein affinity and/or elevate antibiotic potency is therefore required, but we believe that these systems offer unusual opportunities for antibiotic drug discovery.

Synthesis of 3-acyltetramates by side chain manipulation and their antibacterial activity

An efficient approach for the introduction of 3-acyl side chain groups onto a core tetramate system, which are suitable for further manipulation by nucleophilic displacement or Horner-Wadsworth-Emmons coupling, provides access to a diverse library of substituted tetramates related to two distinct classes of natural products, equisetin and pramanicin. Assessment against S. aureus and E. coli indicated that some compounds exhibit significant antibacterial activity, providing unusual leads for further optimisation in the drug discovery process.

Cladosins F and G, two new hybrid polyketides from the deep-sea-derived Cladosporium sphaerospermum 2005-01-E3

Two new fungal hybrid polyketides, cladosins F (1) and G (2), with rare 6(3)-enamino-8,10-dihydroxy-tetraketide system were discovered from the deep-sea-derived fungus Cladosporium sphaerospermum 2005-01-E3 guided by OSMAC approach. Their structures were elucidated on the basis of comprehensive spectroscopic analyses, and cytotoxicity, antitubercular, anti-influenza A H1N1 virus, and NF-κB inhibitory activities were evaluated.

Cladosins A-E, hybrid polyketides from a deep-sea-derived fungus, Cladosporium sphaerospermum

Five new fungal hybrid polyketides, cladosins A-D (1-4), that contain a novel linear 6(3)-enamino-8,10-dihydroxy-tetraketide (1 and 2) or 6-enamino-7(8)-en-10-ol (3 and 4) moiety, as well as the biogenetically related cladosin E (5), were isolated from the deep-sea-derived fungus Cladosporium sphaerospermum 2005-01-E3. Their structures (1-5) were elucidated through a combination of spectroscopic data, chemical conversion, and both Mosher's and Marfey's methods for stereochemical assignment. A plausible biogenetic pathway to 1-5 is proposed. Cladosin C (3) possesses mild anti-influenza A H1N1 virus activity.

New routes towards reutericyclin analogues

A range of N-acylpyrrolo[3,4-c]isoxazoles and derived N-acyltetramides has been prepared via a nitrile oxide dipolar cycloaddition approach, as analogues of the acyltetramic acid metabolite reutericyclin, of interest for its antibiotic potential against Gram-positive bacteria including hospital-acquired infections of resistant Clostridium difficile.

Synthesis and antibacterial activity of monocyclic 3-carboxamide tetramic acids

A chemical library of carboxamide-substituted tetramates designed by analogy with antibacterial natural products, a method for their rapid construction, and the evaluation of their antibacterial activity is reported.