In order to address the dire need for new antibiotics to treat specific strains of drug resistant Gram-negative bacterial infections, a mixed ligand analog of the natural Acinetobacter baumannii selective siderophore, fimsbactin, was coupled to daptomycin, a Gram-positive only antibiotic. The resulting conjugate 11 has potent activity against multidrug resistant strains of A. baumannii both in vitro and in vivo. The study also indicates that conjugation of siderophores to “drugs” that are much larger than the siderophore (iron transport agent) itself facilitates active uptake that circumvents the normal permeability problems in Gram-negative bacteria. The results demonstrate the ability to extend activity of a normally Gram-positive only antibiotic to create a potent and targeted Gram-negative antibiotic using a bacterial iron transport based sideromycin Trojan horse strategy.

The synthesis of a small set of β-lactams containing isocyanate precursors is described. The release of the isocyanate precursor in model hydrolysis experiments was substantiated by trapping experiments, thus confirming that β-lactams can be designed that are capable of releasing alternatively reactive species. Preliminary biological assessments are also briefly discussed.

Electron deficient nitroaromatic compounds such as BTZ043 and its closest congener, PBTZ169, and related agents are a promising new class of anti-TB compounds. Herein we report the design and syntheses of 1,3-benzothiazinone azide (BTZ-N3) and related click chemistry products based on the molecular mode of activation of BTZ043. Our computational docking studies indicate that BTZ-N3 binds in the essentially same pocket as that of BTZ043. Detailed biochemical studies with cell envelope enzyme fractions of Mycobacterium smegmatis combined with our model biochemical reactivity studies with nucleophiles indicated that, in contrast to BTZ043, the azide analogue may have a different mode of activation for anti-TB activity. Subsequent enzymatic studies with recombinant DprE1 from Mtb followed by MIC determination in NTB1 strain of Mtb (harboring Cys387Ser mutation in DprE1 and is BTZ043 resistant) unequivocally indicated that BTZ-N3 is an effective reversible and noncovalent inhibitor of DprE1.Keywords: 1,3-Benzothiazinone azide; BTZ-N3; BTZ043; DprE1; Mycobacterium smegmatis; Mycobacterium tuberculosis; tuberculosis

Correction for ‘Syntheses and biological evaluations of highly functionalized hydroxamate containing and N-methylthio monobactams as anti-tuberculosis and β-lactamase inhibitory agents.

Both the resurgence of tuberculosis (TB) and antibiotic resistance continue to threaten modern healthcare and new means of combating pathogenic bacterial infections are needed. The syntheses of monobactams possessing hydroxamate and N-methylthio functionality are described, as well as their anti-TB, in vitro β-lactamase inhibitory, and general antimicrobial evaluations. A number of compounds exhibited significant anti-TB and β-lactamase inhibitory activity, with MIC values in the range of 25 to <0.19 μM against Mycobacteria tuberculosis (M. tb), and Ki values in the range of 25–0.03 μM against purified NDM-1 and VIM-1 lystate metallo β-lactamases. This work suggests that these scaffolds may serve as promising leads in developing new antibiotics and/or β-lactamase inhibitors.

Increasing interest in the potent anti-tuberculosis activity and the novel target (QcrB) of imidazo[1,2-a]pyridine-3-carboxamides encouraged extended structure–activity relationship studies of additional scaffolds. This study reports on the in vitro profiling of the imidazo[2,1-b]thiazole-5-carboxamides as a new promising class of anti-tuberculosis compounds endowed with nanomolar potency against replicating and drug-resistant Mycobacterium tuberculosis (Mtb) as well as low toxicity to VERO cells. Compounds 6, 16, and 17 had MIC values <10 nM and toxicity >100 μM. On-target selectivity of this series was confirmed by cross-resistance of specific QcrB mutants as well as the hypersusceptibility of a mutant with a functional gene deletion of the alternative cytochrome bd oxidase. Additionally, to demonstrate selectivity, three analogues (6, 15, 17) were broadly screened against a diverse set of eight strains of bacteria, including both Gram-positive and Gram-negative as well as six disease-causing non-tuberculosis mycobacteria. Finally, compounds 16 and 17 were found to be active in macrophages infected with Mtb.Keywords: antituberculosis; imidazothiazoles; respiration target; SAR

The discovery of 1,3-benzothiazin-4-ones (BTZs), especially BTZ043 and PBTZ-169 as potent agents for the treatment of tuberculosis, prompted intensive research related to development of potential antituberculosis agents based on electron deficient nitroaromatic scaffolds. Herein we report the syntheses, computational and NMR studies and anti-TB activity of oxidation products, 1,3-benzothiazinone sulfoxide (BTZ-SO) and 1,3-benzothiazinone sulfone (BTZ-SO2) derived from BTZ043. The combined computational and NMR work revealed differences in the total charge densities and molecular shapes of the oxidation products. While docking studies still suggested similar interactions and binding patterns for both products with the target DprE1 enzyme, antituberculosis assays indicated remarkable differences in their activity. Interestingly, BTZ-SO possesses potent activity against nonpathogenic and pathogenic mycobacterial strains, but BTZ-SO2 is only weakly active.Keywords: BTZ043; DprE1; oxidation; sulfone; sulfoxide; tuberculosis

Several N-acyl ciprofloxacin quinone derivatives based on a trimethyl lock structure were synthesized, and their in vitro antibacterial activity against a panel of clinically relevant bacteria was evaluated. A few new analogues displayed enhanced activity against Gram-positive species compared to the parent drug. Additionally, studies of 8-Cip, which was the most potent compound tested, indicate that it may act through a dual-action mechanism.Keywords: antibacterial; Ciprofloxacin; drug release; trimethyl lock

Tuberculosis (TB) continues to remain one of the most threatening diseases in the world. With the emergence of multi-drug resistant (MDR) and extensively drug resistant (XDR) strains, the need to develop new therapies is dire. The syntheses of a focused library of hydroxamates and hydroxamic acids is described, as well as anti-TB activity in the microplate alamar blue assay (MABA). A number of compounds exhibited good activity against Mtb, with notable compounds exhibiting MIC values in the range of 20–0.71 μM. This work suggests that both hydroxamates and their free acids may be incorporated into more complex scaffolds and serve as potential leads for the development of anti-TB agents.

•Emerging resistance to antibiotics requires development of new antibiotic scaffolds.•Methodology is described for synthesis of a new bicyclic β-lactam core structure.•Oxidative amidation of a monocyclic β-lactam produces a new bicyclic core.With antibiotic resistance on the rise, the need for new medicinal scaffolds is needed. The synthesis of an aminal bicyclic β-lactam core is described. The key synthetic step is Pd(II)-catalyzed oxidative amidation. The product is a single diastereomer, confirmed by X-ray crystallography.

Syntheses of two Siderophore–fluoroquinolone conjugates with a potential reduction triggered linker for drug release are described. The “trimethyl lock” based linker incorporated in the conjugates was designed to be activated by taking advantage of the reductive pathway of bacterial iron metabolism. Electrochemical and LC–MS studies indicated that the linker is thermodynamically reducible by common biological reductants and the expected lactonization proceeds rapidly with concomitant release of the drug. Antibacterial activity assays revealed that conjugates with the reduction-triggered linker were more potent than their counterparts with a stable linker, which suggests that drug release occurs inside bacterial cells.

Zolpidem (Ambien, 1) is an imidazo[1,2-a]pyridine-3-acetamide and an approved drug for the treatment of insomnia. As medicinal chemists enamored by how structure imparts biological function, we found it to have strikingly similar structure to the antitubercular imidazo[1,2-a]pyridine-3-carboxyamides. Zolpidem was found to have antituberculosis activity (MIC of 10–50 μM) when screened against replicating Mycobacterium tuberculosis (Mtb) H37Rv. Manipulation of the Zolpidem structure, notably, to structural isomers (“anagrams”), attains remarkably improved potency (5, MIC of 0.004 μM) and impressive potency against clinically relevant drug-sensitive, multi- and extensively drug-resistant Mtb strains (MIC < 0.03 μM). Zolpidem anagrams and analogues were synthesized and evaluated for their antitubercular potency, toxicity, and spectrum of activity against nontubercular mycobacteria and Gram-positive and Gram-negative bacteria. These efforts toward the rational design of isomeric anagrams of a well-known sleep aid underscore the possibility that further optimization of the imidazo[1,2-a]pyridine core may well “put TB to rest”.Keywords: Ambien; anti-TB; imidazopyridine analogues; tuberculosis; zolpidem

This review describes the use of nitroso Diels–Alder reactions for the functionalization of complex diene-containing natural products in order to generate libraries of compounds with potential biological activity. The application of this methodology to the structural modification of a series of natural products (thebaine, steroidal dienes, rapamycin, leucomycin, colchicine, isocolchicine and piperine) is discussed using relevant examples from the literature from 1973 onwards. The biological activity of the resulting compounds is also discussed. Additional comments are provided that evaluate the methodology as a useful tool in organic, bioorganic and medicinal chemistry.

Tuberculosis (TB), a disease caused by Mycobacterium tuberculosis (Mtb), is a global public health concern because of the emergence of various resistant strains. Benzothiazin-4-ones (BTZs), represented by BTZ043, are a promising new class of agents for the treatment of tuberculosis and have been shown to kill Mtb in vitro, ex vivo, and in mouse models of TB. Herein we report the design and syntheses of nitroaromatic sulfonamide, reverse-amide, and ester classes of anti-TB agents using a scaffold simplification strategy based on BTZ043. The presented work explores the effect of functional groups such as sulfonamides, reverse-amides, and esters that are attached to the nitroaromatic rings on their anti-TB activity. The in vitro activity of the compounds evaluated against the H37Rv strain of Mtb show that nitroaromatic sulfonamides and nitrobenzoic acid esters with two nitro substituents were most active and highlights the importance of the electronic character (electron deficient aromatic ring) of the nitroaromatic ring as a central theme in these types of nitroaromatic anti-TB agents.Keywords: BTZ043; DprE1; Mycobacterium tuberculosis; nitroaromatics; scaffold simplification strategy;

Palladium-catalyzed oxidative amidation has been used to synthesize hydroxamic acid-containing bicyclic β-lactam cores. Oxidative cleavage of the pendant alkene provides access to the carboxylic acid in one step.

The development of multidrug resistant (MDR) and extensively drug resistant (XDR) forms of tuberculosis (TB) has stimulated research efforts globally to expand the new drug pipeline. Nitroaromatic compounds, including 1,3-benzothiazin-4-ones (BTZs) and related agents, are a promising new class for the treatment of TB. Research has shown that the nitroso intermediates of BTZs that are generated in vivo cause suicide inhibition of decaprenylphosphoryl-β-d-ribose 2′ oxidase (DprE1), which is responsible for cell wall arabinogalactan biosynthesis. We have designed and synthesized novel anti-TB agents inspired from BTZs and other nitroaromatic compounds. Computational studies indicated that the unsubstituted aromatic carbons of BTZ043 and related nitroaromatic compounds are the most electron-deficient and might be prone to nucleophilic attack. Our chemical studies on BTZ043 and the additional nitroaromatic compounds synthesized by us and others confirmed the postulated reactivity. The results indicate that nucleophiles such as thiolates, cyanide, and hydride induce nonenzymatic reduction of the nitro groups present in these compounds to the corresponding nitroso intermediates by addition at the unsubstituted electron-deficient aromatic carbon present in these compounds. Furthermore, we demonstrate here that these compounds are good candidates for the classical von Richter reaction. These chemical studies offer an alternate hypothesis for the mechanism of action of nitroaromatic anti-TB agents, in that the cysteine thiol(ate) or a hydride source at the active site of DprE1 may trigger the reduction of the nitro groups in a manner similar to the von Richter reaction to the nitroso intermediates, to initiate the inhibition of DprE1.

Herein two new and concise synthetic approaches for making an unsaturated bicyclic oxamazin core are reported. The first involves the use of an intramolecular Diels–Alder reaction to form both of the fused rings in one step. The second approach incorporates ring-closing olefin metathesis in the final step to form the second fused ring of the core. The scope of the second approach was also expanded further to afford larger ringed bicyclic systems.

Chemical syntheses and biological evaluation of biscatecholate–monohydroxamate mixed ligand sideromycins utilizing the carbacephalosporin β-lactam antibiotic loracarbef and the fluoroquinolone antibiotic ciprofloxacin are described. The mixed ligand β-lactam sideromycin (1b) had remarkably selective and extremely potent antibacterial activity against the Gram-negative pathogen Acinetobacter baumannii ATCC 17961 (MIC = 0.0078 μM). The antibacterial activity of the β-lactam sideromycin was inversely related to the iron(III) concentration in the testing media and was antagonized by the presence of the competing parent siderophore. These data suggested that active transport of the mixed ligand β-lactam sideromycin across the outer cell membrane of A. baumannii via siderophore-uptake pathways was responsible for the selective and potent antibacterial activity.

Siderophores are multidentate iron(III) chelators used by bacteria for iron assimilation. Sideromycins, also called siderophore–antibiotic conjugates, are a unique subset of siderophores that enter bacterial cells via siderophore uptake pathways and deliver the toxic antibiotic in a “Trojan horse” fashion. Sideromycins represent a novel antibiotic delivery technology with untapped potential for developing sophisticated microbe-selective antibacterial agents that limit the emergence of bacterial resistance. The chemical synthesis of a series of mono-, bis-, and trihydroxamate sideromycins are described here along with their biological evaluation in antibacterial susceptibility assays. The linear hydroxamate siderophores used for the sideromycins in this study were derived from the ferrioxamine family and inspired by the naturally occurring salmycin sideromycins. The antibacterial agents used were a β-lactam carbacepholosporin, Lorabid, and a fluoroquinolone, ciprofloxacin, chosen for the different locations of their biological targets, the periplasm (extracellular) and the cytoplasm (intracellular). The linear hydroxamate-based sideromycins were selectively toxic toward Gram-positive bacteria, especially Staphylococcus aureus SG511 (MIC = 1.0 μM for the trihydroxamate–fluoroquinolone sideromycin). Siderophore–sideromycin competition assays demonstrated that only the fluoroquinolone sideromycins required membrane transport to reach their cytoplasmic biological target and that a trihydroxamate siderophore backbone was required for protein-mediated active transport of the sideromycins into S. aureus cells via siderophore uptake pathways. This work represents a comprehensive study of linear hydroxamate sideromycins and teaches how to build effective hydroxamate-based sideromycins as Gram-positive selective antibiotic agents.

A set of 14 imidazo[1,2-a]pyridine-3-carboxamides was synthesized and screened against Mycobacterium tuberculosis H37Rv. The minimum inhibitory concentrations of 12 of these agents were ≤1 μM against replicating bacteria and 5 compounds (9, 12, 16, 17, and 18) had MIC values ≤0.006 μM. Compounds 13 and 18 were screened against a panel of MDR and XDR drug resistant clinical Mtb strains with the potency of 18 surpassing that of clinical candidate PA-824 by nearly 10-fold. The in vivo pharmacokinetics of compounds 13 and 18 were evaluated in male mice by oral (PO) and intravenous (IV) routes. These results indicate that readily synthesized imidazo[1,2-a]pyridine-3-carboxamides are an exciting new class of potent, selective anti-TB agents that merit additional development opportunities.Keywords: imidazo[1,2-a]pyridine-3-carboxamides; MDR-TB; Mycobacterium tuberculosis; pharmacokinetics; XDR-TB;

An artificial tris-catecolate siderophore with a tripodal backbone and its conjugates with ampicillin and amoxicillin were synthesized. Both conjugates exhibited significantly enhanced in vitro antibacterial activities against Gram-negative species compared to the parent drugs, especially against Pseudomonas aeruginosa. The conjugates appeared to be assimilated by an induced bacterial iron transport process as their activities were inversely related to iron concentration. The easily synthesized tris-catecolate siderophore has great potential for future development of various drug conjugates to target antibiotic-resistant Gram-negative bacteria.

A method was developed to synthesize macrocyclic trihydroxamate siderophores using optimized Yamaguchi macrolactonization conditions. The natural ability of siderophores to bind iron(III) was exploited to template the reactions and allowed for rapid reaction rates, high product conversions, and the formation of large macrolactone rings up to 35 atoms. An X-ray structure of a 33-membered macrolactone siderophore–Fe(III) complex is presented.

Three analogs of mycobactin T, the siderophore secreted by Mycobacterium tuberculosis (Mtb) were synthesized and screened for their antibiotic activity against Mtb H37Rv and a broad panel of Gram-positive and Gram-negative bacteria. The synthetic mycobactins were potent (MIC90 0.02–0.88 μM in 7H12 media) and selective Mtb inhibitors, with no inhibitory activity observed against any other of the microorganisms tested. The maleimide-containing analog 40 represents a versatile platform for the development of mycobactin-drug conjugates, as well as other applications.

Two desferrioxamine B-ciprofloxacin conjugates with ‘trimethyl-lock’ based linkers that are designed to release the antibiotic after esterase or phosphatase-mediated hydrolysis were synthesized. The potential esterase-sensitive conjugate 13 displayed moderate to good antibacterial activities against selected ferrioxamine-utilizing bacteria, although the activities were lower than the parent drug ciprofloxacin. However, the potential phophatase-sensitive conjugate 23 was inactive against the same panel of organisms tested. These properties appeared to be related to the activating efficiency of the linker by the enzyme and to the outer membrane protein recognition of the chemically modified siderophore used in the conjugate.

Tuberculosis (TB) is a devastating disease resulting in a death every 20 s. Thus, new drugs are urgently needed. Herein we report ten classes of compounds—oxazoline, oxazole, thiazoline, thiazole, pyrazole, pyridine, isoxazole, imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine and imidazo[1,2-c]pyrimidine—which have good (micromolar) to excellent (sub-micromolar) antitubercular potency. The 5,6-fused heteroaromatic compounds were the most potent with MIC’s as low as <0.195 μM (9 and 11). Overall, the imidazo[1,2-a]pyridine class was determined to be most promising, with potency similar to isoniazid and PA-824 against replicating Mtb H37Rv, clinically relevant drug sensitive, multi- and extensively resistant Mtb strains as well as having good in vitro metabolic stability.

Several novel oxazolidinone antibiotics with a spiropiperazinyl substituent at the 4′-position of the phenyl ring were synthesized through nitroso Diels–Alder chemistry and the in vitro antibacterial activities were evaluated against various Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus, Enterococcus faecalis), Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) and mycobacteria (Mycobacterium vaccae, Mycobacterium tuberculosis). Analogs (8a and 12) were active against selected drug resistant microbes, like methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) and had no mammalian toxicity in a Hep-2 cellular assay (CC50 >100 μM).

Although the antimalarial agent artemisinin itself is not active against tuberculosis, conjugation to a mycobacterial-specific siderophore (microbial iron chelator) analogue induces significant and selective antituberculosis activity, including activity against multi- and extensively drug-resistant strains of Mycobacterium tuberculosis. The conjugate also retains potent antimalarial activity. Physicochemical and whole-cell studies indicated that ferric-to-ferrous reduction of the iron complex of the conjugate initiates the expected bactericidal Fenton-type radical chemistry on the artemisinin component. Thus, this “Trojan horse” approach demonstrates that new pathogen-selective therapeutic agents in which the iron component of the delivery vehicle also participates in triggering the antibiotic activity can be generated. The result is that one appropriate conjugate has potent and selective activity against two of the most deadly diseases in the world.

The discovery, syntheses, and structure–activity relationships (SAR) of a new family of heterocyclic antibacterial compounds based on N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds are described. A structurally diverse library of ∼100 heterocyclic molecules generated from Lewis acid-mediated nucleophilic ring-opening reactions with nitroso Diels–Alder cycloadducts and nitroso ene reactions with substituted alkenes was evaluated in whole cell antibacterial assays. Compounds containing the N-alkyl-N-(pyridin-2-yl)hydroxylamine structure demonstrated selective and potent antibacterial activity against the Gram-positive bacterium Micrococcus luteus ATCC 10240 (MIC90 = 2.0 μM or 0.41 μg/mL) and moderate activity against other Gram-positive strains including antibiotic resistant strains of Staphylococcus aureus (MRSA) and Enterococcus faecalis (VRE). A new synthetic route to the active core was developed using palladium-catalyzed Buchwald–Hartwig amination reactions of N-alkyl-O-(4-methoxybenzyl)hydroxylamines with 2-halo-pyridines that facilitated SAR studies and revealed the simplest active structural fragment. This work shows the value of using a combination of diversity-oriented synthesis (DOS) and parallel synthesis for identifying new antibacterial scaffolds.

Acylnitroso cycloadducts have proven to be valuable intermediates in the syntheses of a plethora of biologically active molecules. Recently, organometallic reagents were shown to open bicyclic acylnitroso cycloadducts and, more interestingly, the prospect of highly regioselective openings was raised. This transformation was employed in the synthesis of a compound with excellent inhibitory activity against 5-lipoxygenase ((±)-4a, IC50 51 nM), an important mediator of inflammation intimately involved in a number of disease states including asthma and cancer. Optimization of the copper-mediated organometallic ring opening reaction was accomplished allowing the further exploration of the biological activity. Synthesis of a number of derivatives with varying affinity for metal binding as well as pendant groups in a range of sizes was accomplished. Analogues were tested in a whole cell assay which revealed a subset of the compounds to be inhibitors of enzyme translocation, a mode of action not previously known and, potentially, extremely important for better understanding of the enzyme and inhibitor development. Additionally, the lead compound was tested in vivo in an established colon cancer model and showed very encouraging anti-tumorogenic properties.

A set of nine 2,7-dimethylimidazo[1,2-a]pyridine-3-carboxamides and one 2,6-dimethylimidazo[1,2-a]pyrimidine-3-carboxamide were synthesized. The compounds were evaluated for their in vitro antituberculosis activity versus replicating, nonreplicating, multi- and extensive drug resistant Mtb strains. The MIC90 values of seven of these agents were ≤1 μM against the various tuberculosis strains tested. A representative compound of this class (1) was screened against seven nontubercular strains as well as other nonmycobacteria organisms and demonstrated remarkable microbe selectivity. A transcriptional profiling experiment of Mtb treated with compound 1 was performed to give a preliminary indication of the mode of action. Lastly, the in vivo ADME properties of compounds 1, 3, 4, and 6 were assessed. The 2,7-dimethylimidazo[1,2-a]pyridine-3-carboxamides are a druglike and synthetically accessible class of anti-TB agents that have excellent selective potency against multi- and extensive drug resistant TB and encouraging pharmacokinetics.Keywords: Antituberculosis; imidazo[1,2-a]pyridine-3-carboxamides; MDR-TB; XDR-TB

Regioselective ring opening of N-hydroxycarbamate-derived nitroso cycloadducts by a copper-catalyzed allylic alkylation reaction was achieved and applied to the synthesis of a set of substituted diaryl ether containing compounds. Use of protected 3-hydroxybenzyl bromide allowed access to a late stage phenol intermediate after protection of the N-hydroxy moiety that was generated from the ring opening reaction. The diaryl ethers were then formed by copper-mediated coupling with arylboronic acids. After selective deprotection, alumina-promoted transcarbamoylation provided the target compounds. Previous results indicate that the compounds may possess significant inhibitory potency against the proinflammatory enzyme 5-lipoxygenase.

A series of analogs of the amamistatin natural products was designed and synthesized to facilitate additional anticancer structure–activity relationships. The results indicate that the anticancer activity is relatively independent of stereochemistry, ester or amide linkage and replacement of the oxazoline/oxazole based iron-binding group with a catechol.

A resin-bound nitroso compound sequestered a single unexpected component from crude plant seed extracts. Several plants, including Piper nigrum, Eugenia caryophyllata, and Pimenta dioica, were extracted with organic solvent in the presence of a nitroso-containing resin. The nitroso resin selectively sequestered a single compound, β-caryophyllene, via a chemo- and regioselective ene reaction. The ene product was released from the resin, and proper selection of the solid-phase linker and cleavage cocktail allowed concomitant further transformation of the primary ene product to a novel functionalized polycycle. Preliminary studies indicate that the new hydroxylamine-containing natural product derivatives have antibiotic activity.

Nitrofuranyl benzimidazoles can be made in one synthetic step from commercially available starting materials. The compounds displayed unexpected antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci with MICs as low as 1 μg ml−1.

Exclusive nucleophilic aromatic substitution of hydrogen in conjunction with cleavage of the N−O bond was observed when nitrosobenzene-derived nitroso cycloadducts were treated with indium triflate in the presence of alcohols. Aromatic alkoxylated syn-1,4 aminocycloalkenol products with good to excellent regioselectivity (16:84 to 0:100, ortho:para) were obtained.

A series of N-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)arylamides was synthesized by copper-catalyzed azide−alkyne cycloaddition (CuAAC) and afforded inhibitors of cancer cell growth. For example, compound 13e had an IC50 of 46 nM against MCF-7 human breast tumor cells. Structure−activity relationship (SAR) studies demonstrated that (i) meta-phenoxy substitution of the N-1-benzyl group is important for antiproliferative activity and (ii) a variety of heterocyclic substitutions for the aryl group of the arylamide are tolerated. In silico COMPARE analysis of antiproliferative activity against the NCI-60 human tumor cell line panel revealed a correlation to clinically useful antimicrotubule agents such as paclitaxel and vincristine. This in silico correlation was supported by (i) in vitro inhibition of tubulin polymerization, (ii) G2/M-phase arrest in HeLa cells as assessed by flow cytometry, and (iii) perturbation of normal microtubule activity in HeLa cells as observed by confocal microscopy. The results demonstrate that N-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)arylamide is a readily accessible small molecule scaffold for compounds that inhibit tubulin polymerization and tumor cell growth.

A series of 10,13-disubstituted 16-membered macrolides was synthesized using nitroso Diels–Alder reactions of leucomycin A7. Despite the extensive constituent functionalities in leucomycin, the hetero cycloaddition reactions proceeded in a highly regio- and stereoselective fashion. Subsequent chemical modifications of the nitroso cycloadducts, including N–O bond reduction, were also conducted. Most leucomycin derivatives retained antibiotic profiles similar to leucomycin A7, and, in contrast to leucomycin itself, several exhibited moderate antiproliferative and cytotoxic activity.

During the syntheses and studies of natural iron chelators (mycobactins), we serendipitously discovered that a simple, small molecule, oxazoline-containing intermediate 3 displayed surprising anti-tuberculosis activity (MIC of 7.7 μM, average). Herein we report elaboration of SAR around this hit as well as the syntheses and evaluation of a hundred oxazoline- and oxazole-containing compounds derived from an efficient three step process: 1) formation of β-hydroxy amides with serine or threonine; 2) cyclization to afford oxazolines; and 3) dehydration to give the corresponding oxazoles. A number of compounds prepared by this method were shown to possess impressive activity against Mycobacterium tuberculosis, extremely low toxicity and therefore high therapeutic indexes, as well as activity against even the more recalcitrant non-replicating form of M. tuberculosis. The uniqueness of their structures and their simplicity should allow them to be further optimized to meet ADME (absorption, distribution, metabolism, excretion) requirements. The syntheses of eight of the most potent in vitro compounds were scaled up and the compounds were tested in an in vivo mouse infection model to evaluate their efficacy before engaging upon more elaborate compound design and optimization.A large panel of oxazoline (3–57 and 101–105) and oxazole (58–100 and 106–111) benzyl ester analogs were synthesized and screened against H37Rv TB and VERO kidney cells to determine their potency and toxicity, respectively.

Ring-C modified alkaloids were synthesized from colchicine using iminonitroso Diels–Alder reactions in a highly regio- and stereoselective fashion. Several analogs exhibited cytotoxic activity similar to that of colchicine itself against PC-3 and MCF-7 cancer cell lines, by serving as prodrugs of colchicine through retro Diels–Alder reactions under the assayed conditions. In vitro microtubule polymerization assays indicated that these modifications affected their interaction with tubulin.

Treatment of nitroso Diels–Alder cycloadducts 1 with diazomethane in the presence of palladium acetate gives synthetically useful exo-6-oxa-7-azatricyclo[3.2.1.02,4]octane derivatives 7 in good to excellent yield. Using this methodology, a conformationally restricted 2′,3′-methano carbocyclic nucleoside was efficiently synthesized from nitroso cycloadduct 1a in seven steps.

Carbocyclic aminonucleosides and epi-4′-carbocyclic puromycin were prepared from an acylnitroso-derived hetero Diels–Alder cycloadduct. Pd(0)/InI-mediated allylations of a formyl species were used to install the 4′-hydroxymethyl group. A tethered aminohydroxylation strategy was employed to install the cis-2′,3′-aminoalcohol moiety with complete regio- and diastereocontrol.

A formyl equivalent was generated in situ from Eschenmoser’s salt in aqueous THF and was reacted with an allylindium species. Acylnitroso-derived hetero-Diels–Alder adducts and related allyl acetates were shown to be substrates for Pd(0)/InI-mediated allylations of formaldehyde-related species to provide homoallylic alcohols. Hydroxymethyl groups were installed with regio- and diastereocontrol to provide relevant disubstituted carbocyclic scaffolds. Enantiopure anti-disubstituted cyclopentene products were prepared from a chiral allyl acetate.

Treatment of acylnitroso-Diels–Alder [2.2.1] bicyclic adducts 2a–b with indium triflate in an alcohol solvent induces ring-opening reactions to afford monocyclic anti-1,2-, anti-1,4-, and syn-1,4-hydroxamic acids with good to excellent regio- and stereoselectivity (up to 7:86:7). Treatment of [2.2.2] bicyclic nitroso adducts 2c–d under similar reaction conditions generates only anti-1,2- and anti-1,4-hydroxamic acids with anti-1,4-product being predominant (up to 17:83).

Ene reactions of iminonitroso agents with olefins were investigated in both solution and solid phase. Reactions afforded allyl hydroxylamine products in up to 99% yield and with high regioselectivity. A Cu(I)-mediated enantioselective nitroso ene reaction gave an ene product with up to 40% ee.

Two cephalosporin-oxazolidinone conjugates were synthesized by incorporation of a carbamate linker at the 3′-position of the cephalosporin. These compounds show stability in aqueous media until specifically activated by a β-lactamase, and retain antibacterial activities profiles reflecting both the individual cephalosporin and oxazolidinone components.

A series of novel sterol analogs was prepared using nitroso Diels−Alder reactions with ergosterol. Most cycloaddition reactions proceeded in an excellent regio- and stereoselective fashion. Further N−O bond cleavage of cycloadducts generated compounds with biological activity in PC-3 and MCF-7 cancer cell lines.

By employing an intramolecular Pd(0)-mediated ring opening of an acylnitroso-derived cycloadduct, new hydroxamic acid containing benzodiazepines have been synthesized and have demonstrated biological activity in MCF-7 and PC-3 tumor cell lines. Subsequent N−O bond reduction of the hydroxamate has provided access to amide analogues for SAR studies. During the course of our syntheses, an intermediate oxazoline N-oxide was isolated and gave insight into the mechanism of the key Pd(0)-mediated reaction.

Homoallylic esters are obtained in a single transformation from allyl 2,2,2-trifluoroethyl malonates by using a Pd(0) catalyst. Facile decarboxylation of allyl 2,2,2-trifluoroethyl malonates is attributed to a decrease in pKa compared to allyl methyl malonates. Subsequent reduction of the homoallylic 2,2,2-trifluoroethyl ester provides a (hydroxyethyl)cyclopentenyl derivative that represents a key intermediate in the synthesis of carbocyclic nucleosides. A select allyl 2,2,2-trifluoroethyl malonate undergoes a decarboxylative Claisen rearrangement to provide a regioisomeric homoallylic ester.

Pathogenic microbes rapidly develop resistance to antibiotics. To keep ahead in the “microbial war”, extensive interdisciplinary research is needed. A primary cause of drug resistance is the overuse of antibiotics that can result in alteration of microbial permeability, alteration of drug target binding sites, induction of enzymes that destroy antibiotics (ie., beta-lactamase) and even induction of efflux mechanisms. A combination of chemical syntheses, microbiological and biochemical studies demonstrate that the known critical dependence of iron assimilation by microbes for growth and virulence can be exploited for the development of new approaches to antibiotic therapy. Iron recognition and active transport relies on the biosyntheses and use of microbe-selective iron-chelating compounds called siderophores. Our studies, and those of others, demonstrate that siderophores and analogs can be used for iron transport-mediated drug delivery (“Trojan Horse” antibiotics) and induction of iron limitation/starvation (Development of new agents to block iron assimilation). Recent extensions of the use of siderophores for the development of novel potent and selective anticancer agents are also described.

Antisense oligonucleotides with iron binding hydroxamate linkages are designed to act as sequence-selective cleaving agents of complementary nucleic acids through Fenton chemistry. Oligothymidylate analogs with hydroxamate linkages were efficiently synthesized from coupling of nucleoside intermediates, activated as p-nitrophenyl carbonates, with hydroxylamine derivatized nucleosides. Iron binding studies showed that hydroxamate linked oligonucleotides are effective iron chelators when there are three nonadjacent internucleosidic hydroxamate linkages available in the same oligonucleotide molecule. However, analysis of the CD spectra of an oligothymidylate 16mer, which contained complete substitution of all phosphates with hydroxamates, indicated that the hydroxamate linkage was too rigid to allow the analog to base pair with the complementary DNA d(A16). Syntheses of mix-linked thymidine oligomers with up to three hydroxamate linkages incorporated in the center of the sequence are also reported. Iron binding of the thymidine oligomer with hydroxamate linkages was confirmed by matrix assisted laser desorption mass spectrometry analysis. Nuclease stability assays showed that the modified oligonucleotides have enhanced resistance toward nuclease S1 (endonuclease) compared to natural oligonucleotides. A thymidine 16mer with three hydroxamate linkages incorporated in the center of the sequence was shown to be able to bind with both iron and its complementary polyA strand. A small destablizing effect was observed when the phosphodiester linkage was changed to the hydroxamate linkage. Under Fenton chemistry conditions, this novel iron binding oligothymidylate analog cleaved the complementary DNA strand sequence-selectively.

The recent rise in drug resistance found amongst community acquired infections has sparked renewed interest in developing antimicrobial agents that target resistant organisms and limit the natural selection of immune variants. Recent discoveries have shown that iron uptake systems in bacteria and fungi are suitable targets for developing such therapeutic agents. The use of siderophore-drug conjugates as “Trojan Horse” drug delivery agents has attracted particular interest in this area. This review will discuss efforts in our research group to study the salmycin class of “Trojan Horse” antibiotics. Inspired by the natural design of the salmycins, a series of desferridanoxamine-antibiotic conjugates were synthesized and tested in microbial growth inhibition assays. The results of these studies will be related to understanding the role of drug release in siderophore-mediated drug delivery with implications for future siderophore-drug conjugate design.

A siderophore conjugate was designed as a potential PSMA inhibitor and diagnostic agent for prostate cancer. A semi-rigid spacer was incorporated to avoid competitive participation of iron binding by the enzyme inhibitor relative to the siderophore component. Biological test results showed that, even with the extended scaffold, this compound is a potent PSMA inhibitor with an IC50 of 4 nM. This siderophore conjugate may be useful for detection of prostate-derived cancer cells by magnetic resonance imaging (MRI).

Norcarbovir (1) and norabacavir (2), the desmethylene derivatives of anti-HIV agents carbovir and abacavir, were efficiently synthesized from a common intermediate 4. Their antitumor and antiviral activities were evaluated and the results indicate norabacavir showed comparable anti-HIV activity to that of abacavir.

Inhibitors of NAALADase have shown promise for a variety of diseases associated with glutamate excitotoxicity, and could be useful for the diagnosis and therapy of prostate cancer. A series of novel enantiomerically pure 2-(phosphonomethyl)pentanedioic acid (2-PMPA) based NAALADase inhibitors were synthesized. These compounds were prepared from previously reported (S)-2-(hydroxyphosphinoylmethyl)pentanedioic acid benzyl ester 4. Biological test results showed that the new compounds are good to outstanding NAALADase inhibitors. Compounds 8b and 10b showed activity similar to the known potent inhibitor (S)-2-PMPA. Fluorescently labeled inhibitor 19b may potentially be used to study binding to prostate cancer cells by fluorescence microscopy, and siderophore-containing inhibitor 21b may be useful for detection of prostate-derived cancer cells by magnetic resonance imaging (MRI).

Syntheses of a series of new C-3′ hydroxamate-substituted cephalosporin derivatives with potent antibacterial and media-dependent anti-TB activity are described.

The structural similarity between β-lactam antibiotics, such as penicillin, and isoxazolidine-3,5-dicarboxylic acids led to the hypothesis that isoxazolidine-3,5-dicarboxylic acids could be effective analogs of β-lactam antibiotics. The syntheses of relevant isoxazolidine-3,5-dicarboxylic acids from acylnitroso Diels–Alder adducts and subsequent biological testing have shown that these first examples are inhibitors of Escherichia coli X580.

The syntheses and anti-tuberculosis activity of quinolone-cephalosporin conjugates (1 and 2) are described. Both showed broad-spectrum antibacterial activity and significant anti-TB activity. The carbamate-linked quinolone-cephem 2 showed better antimycobacterial activity, including anti-TB activity, than the direct amine-linked quinolone-cephem 1, while quinolone-cephem 1 was slightly more effective against some Gram-negative bacterial strains.

Inhibitors of NAALADase have shown promise for a variety of diseases associated with glutamate excitotoxicity, and could be useful for the diagnosis and therapy of prostate cancer. A series of novel enantiomerically pure 2-(phosphonomethyl)pentanedioic acid (2-PMPA) based NAALADase inhibitors were synthesized. These compounds were prepared from previously reported (S)-2-(hydroxyphosphinoylmethyl)pentanedioic acid benzyl ester 4. Biological test results showed that the new compounds are good to outstanding NAALADase inhibitors. Compounds 8b and 10b showed activity similar to the known potent inhibitor (S)-2-PMPA. Fluorescently labeled inhibitor 19b may potentially be used to study binding to prostate cancer cells by fluorescence microscopy, and siderophore-containing inhibitor 21b may be useful for detection of prostate-derived cancer cells by magnetic resonance imaging (MRI).

A series of 10,13-disubstituted 16-membered macrolides was synthesized using nitroso Diels–Alder reactions of leucomycin A7. Despite the extensive constituent functionalities in leucomycin, the hetero cycloaddition reactions proceeded in a highly regio- and stereoselective fashion. Subsequent chemical modifications of the nitroso cycloadducts, including N–O bond reduction, were also conducted. Most leucomycin derivatives retained antibiotic profiles similar to leucomycin A7, and, in contrast to leucomycin itself, several exhibited moderate antiproliferative and cytotoxic activity.

This review describes the use of nitroso Diels–Alder reactions for the functionalization of complex diene-containing natural products in order to generate libraries of compounds with potential biological activity. The application of this methodology to the structural modification of a series of natural products (thebaine, steroidal dienes, rapamycin, leucomycin, colchicine, isocolchicine and piperine) is discussed using relevant examples from the literature from 1973 onwards. The biological activity of the resulting compounds is also discussed. Additional comments are provided that evaluate the methodology as a useful tool in organic, bioorganic and medicinal chemistry.

Norcarbovir (1) and norabacavir (2), the desmethylene derivatives of anti-HIV agents carbovir and abacavir, were efficiently synthesized from a common intermediate 4. Their antitumor and antiviral activities were evaluated and the results indicate norabacavir showed comparable anti-HIV activity to that of abacavir.

Acylnitroso cycloadducts have proven to be valuable intermediates in the syntheses of a plethora of biologically active molecules. Recently, organometallic reagents were shown to open bicyclic acylnitroso cycloadducts and, more interestingly, the prospect of highly regioselective openings was raised. This transformation was employed in the synthesis of a compound with excellent inhibitory activity against 5-lipoxygenase ((±)-4a, IC50 51 nM), an important mediator of inflammation intimately involved in a number of disease states including asthma and cancer. Optimization of the copper-mediated organometallic ring opening reaction was accomplished allowing the further exploration of the biological activity. Synthesis of a number of derivatives with varying affinity for metal binding as well as pendant groups in a range of sizes was accomplished. Analogues were tested in a whole cell assay which revealed a subset of the compounds to be inhibitors of enzyme translocation, a mode of action not previously known and, potentially, extremely important for better understanding of the enzyme and inhibitor development. Additionally, the lead compound was tested in vivo in an established colon cancer model and showed very encouraging anti-tumorogenic properties.

Three analogs of mycobactin T, the siderophore secreted by Mycobacterium tuberculosis (Mtb) were synthesized and screened for their antibiotic activity against Mtb H37Rv and a broad panel of Gram-positive and Gram-negative bacteria. The synthetic mycobactins were potent (MIC90 0.02–0.88 μM in 7H12 media) and selective Mtb inhibitors, with no inhibitory activity observed against any other of the microorganisms tested. The maleimide-containing analog 40 represents a versatile platform for the development of mycobactin-drug conjugates, as well as other applications.