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ConspectusWithout question, natural products have provided the lion share of leads, if not drugs themselves, for the treatment of bacterial infections. The bacterial arms race, fueled by selection and survival pressures has delivered a natural arsenal of small molecules targeting the most essential of life processes. Antibiotics that target these critical intracellular processes face the formidable defense of both penetrating a bacterial cell membrane and avoiding efflux to exert their effect. These challenges are especially effective in Gram-negative (Gram-(-)) bacteria, which have a double membrane structure and efficient efflux systems from the combination of outer-membrane porins and inner membrane proton pumps. In this landscape of offense and defense, our clinically used antibiotics have only successfully targeted three intracellular processes for therapeutic intervention in Gram-(-) bacteria: dihydrofolate biosynthesis, transcription, and translation. Not surprisingly, such critical survival machinery is a popular target for bacterial warfare, and eight of our 14 classes of commonly used antibiotics target translation with the bacterial ribosome remaining one the most vetted targets for antimicrobial therapy. On the plus side, its anionic character attracts cationic inhibitors, which are generally more capable of penetrating the bacterial cell wall, and clinical resistance rates are usually manageable as mutation of such a highly evolved machine is difficult. On the down side, this highly evolved machine renders it difficult to inhibit selectively, and the inhibition of prokaryotic translation versus both eukaryotic cellular and mitochondrial translation is critical for clinical development and minimization of undesired toxicities.A class of natural products known as the "nucleoside antibiotics" have historically been recognized as universal inhibitors of the ribosome and can inhibit translation in prokaryotes, eukaryotes, and archaea. While they have served an essential role in dissecting the biochemical underpinnings of the enzymatic functions of the ribosome, they have not proven therapeutically useful as they target the highly conserved rRNA in the P-site and are toxic to mammalian cells. In this Account, we describe our studies on the natural product amicetin, a nucleoside antibiotic that we have demonstrated to break the rule of being a universal translation inhibitor. While the cytosine of amicetin mimics C75 of the 3'-CCA tail of the P-site tRNA akin to other nucleoside antibiotics, we advance a hypothesis that amicetin's unique interaction with the ribosomal protein uL16 exploits an untapped mechanism for selectively targeting the bacterial ribosome. A complex molecule comprised of a nucleoside, carbohydrates and amino acids, amicetin is also chemically unstable. Our initial attempts to stabilize and simplify this scaffold are presented with the ultimate goal of rebuilding the compound with improved penetrance to bacterial cells. If successful, this scaffold would demonstrate a path forward for a new class of antibiotics capable of selectively targeting the ribosomal P-site.

Prosthetic joint infection (PJI) is a well-known and persisting problem. Active release coatings have promise to provide early protection to an implant by eradicating small colony biofilm contaminants or planktonic bacteria that can form biofilm. Traditional antibiotics can be limited as active release agents in that they have limited effect against biofilms and develop resistance at sub-lethal concentrations. A unique first-in-class compound (CZ-01127) was assessed as the active release agent in a silicone (Si)-based coating to prevent PJI in a sheep model of joint space infection. Titanium (Ti) plugs contained a porous coated Ti (PCTi) region and polymer-coated region. Plugs were implanted into a femoral condyle of sheep to assess the effect of the Si polymer on cancellous bone ingrowth, the effect of CZ-01127 on bone ingrowth, and the ability of CZ-01127 to prevent PJI. Microbiological results showed that CZ-01127 was able to eradicate bacteria in the local region of the implanted plugs. Data further showed that Si did not adversely affect bone ingrowth. However, bacteria that reached the joint space (synovium) were not fully eradicated. Outcomes suggested that the CZ-01127 coating provided local protection to the implant system in a challenging model, the design of which could be beneficial for testing future antimicrobial therapies for PJI. STATEMENT OF SIGNIFICANCE: Periprosthetic joint infection (PJI) is now commonplace, and constitutes an underlying problem that patients and physicians face. Active release antibiotic coatings have potential to prevent these infections. Traditional antibiotics are limited in their ability to eradicate bacteria that reside in biofilms, and are more susceptible to resistance development. This study addressed these limitations by testing the efficacy of a unique antimicrobial compound in a coating that was tested in a challenging sheep model of PJI. The unique coating was able to eradicate bacteria and prevent infection in the environment adjacent to the implant. Bacteria that escaped into the joint space still caused infection, yet benchmark data can be used to optimize the coating and translate it toward clinical use.

In search of new anti-tuberculars compatible with anti-retroviral therapy we re-identified amicetin as a lead compound. Amicetin's binding to the 70S ribosomal subunit of Thermus thermophilus (Tth) has been unambiguously determined by crystallography and reveals it to occupy the peptidyl transferase center P-site of the ribosome. The amicetin binding site overlaps significantly with that of the well-known protein synthesis inhibitor balsticidin S. Amicetin, however, is the first compound structurally characterized to bind to the P-site with demonstrated selectivity for the inhibition of prokaryotic translation. The natural product-ribosome structure enabled the synthesis of simplified analogues that retained both potency and selectivity for the inhibition of prokaryotic translation.

We present data demonstrating the natural product mimic, zinaamidole A (ZNA), is a modulator of metal ion homeostasis causing cancer-selective cell death by specifically inducing cellular Zn(2+)-uptake in transformed cells. ZNA’s cancer selectivity was evaluated using metastatic, patient-derived breast cancer cells, established human breast cancer cell lines, and three-dimensional organoid models derived from normal and transformed mouse mammary glands. Structural analysis of ZNA demonstrated that the compound interacts with zinc through the N(2)-acyl-2-aminoimidazole core. Combination treatment with ZnSO(4) strongly potentiated ZNA’s cancer-specific cell death mechanism, an effect that was not observed with other transition metals. We show that Zn(2+)-dyshomeostasis induced by ZNA is unique and markedly more selective than other known Zn(2+)interacting compounds such as clioquinol. The in vivo bioactivity of ZNA was also assessed and revealed that tumor-bearing mice treated with ZNA had improved survival outcomes. Collectively, these data demonstrate that the N(2)-acyl-2aminoimidazole core of ZNA represents a powerful chemotype to induce cell death in cancer cells concurrently with a disruption in zinc homeostasis.

Biofilm-related infection is among the worst complication to prosthetic joint replacement procedures; once established on the implant surface, biofilms show strong recalcitrance to clinical antibiotic therapy, frequently requiring costly revision procedures and prolonged systemic antibiotics for their removal. A well-designed active release coating might assist host immunity in clearing bacterial contaminants within the narrow perioperative window and ultimately prevent microbial colonization of the joint prosthesis. A first-in-class compound (CZ-01127) was tested as the active release agent in a silicone (Si) coating using an in vitro dynamic flow model of surgical site contamination and compared with analogous coatings containing clinical gold-standard antibiotics vancomycin and gentamicin; the CZ-01127 coating outperformed both vancomycin and gentamicin coatings and was the only to decrease the methicillin-resistant Staphylococcus aureus (MRSA) inocula below detectable limits for the first 3 days. The antimicrobial activity of CZ-01127, and for comparison vancomycin and gentamicin, were characterized against both planktonic and biofilm MRSA using the minimum inhibitory concentration (MIC) assay, serial passages, and serial dilution tests against established biofilms grown with a CBR 90 CDC biofilm reactor. Despite a similar MIC (1 µg/ml) and behavior in a 25-day serial passage analysis, CZ-01127 displayed much greater bactericidal activity against established biofilms and was the only to decrease biofilm colony forming units (CFUs) below detectable limits at the highest concentration tested (500 µg/ml). Coating release profiles were characterized using ATR-FTIR and displayed burst release kinetics within the decisive period of the perioperative window suggesting the silicon carrier is broadly useful for screening antibiotic compound for local delivery applications. Statement of Significance With an aging population, an increasing number of people are undergoing total joint replacement procedures in which diseased joint tissues are replaced with permanent metallic implants. Some of these procedures are burdened by costly and debilitating infections. A promising approach to prevent infections is the use of an antimicrobial coating on the surface of the implant which releases antibiotics into the surgical site to prevent infection. In this study, we tested a new antibiotic compound formulated in a silicone coating. Data showed that this compound was more effective at killing pathogenic methicillin resistant Staphylococcus aureus (MRSA) bacteria than two clinical gold-standard antibiotics—vancomycin and gentamicin—and could be a promising agent for antimicrobial coating technologies.

Human d-3-phosphoglycerate dehydrogenase (PHGDH), the first enzyme in the serine biosynthetic pathway, is genomically amplified in tumors including breast cancer and melanoma. In PHGDH-amplified cancer cells, knockdown of PHGDH is not fully rescued by exogenous serine, suggesting possible additional growth-promoting roles for the enzyme. Here we show that, in addition to catalyzing oxidation of 3-phosphoglycerate, PHGDH catalyzes NADH-dependent reduction of α-ketoglutarate (AKG) to the oncometabolite d-2-hydroxyglutarate (d-2HG). Knockdown of PHGDH decreased cellular 2HG by approximately 50% in the PHGDH-amplified breast cancer cell lines MDA-MB-468 (normal concentration 93 μM) and BT-20 (normal concentration 35 μM) and overexpression of PHGDH increased cellular 2HG by over 2-fold in non-PHGDH-amplified MDA-MB-231 breast cancer cells, which normally display very low PHGDH expression. The reduced 2HG level in PHGDH knockdown cell lines can be rescued by PHGDH re-expression, but not by a catalytically inactive PHGDH mutant. The initial connection between cancer and d-2HG involved production of high levels of d-2HG by mutant isocitrate dehydrogenase. More recently, however, elevated d-2HG has been observed in breast cancer tumors without isocitrate dehydrogenase mutation. Our results suggest that PHGDH is one source of this d-2HG.

The yield of the regioselective and scalable reaction sequence marginally depends on the substitution pattern of the substrates.

It is demonstrated that dirhodium(II) is capable of catalyzing the chemo- and regioselective hydroamination of propargyl ureas.

A simple and efficient one-pot method for the synthesis of monoprotected guanidines is presented. Treatment of an acylcyanamide with chlorotrimethylsilane generates a reactive N-silylcarbodiimide capable of guanylating a variety of amines. Typically the reaction is complete in 15 min for primary and secondary aliphatic amines at rt. Hindered amines and anilines are also competent nucleophiles but require extended reaction times.

Cyclic and polycyclic guanidinium ion natural products have been shown to modulate a variety of important biological processes and their activities are often reliant on the unique hydrogen bond donor-acceptor topologies that these substructures display.1 While most synthetic methods to prepare guanidines rely on the addition of an amine (guanylation) of an activated thiourea or urea,2 alternative methodologies that generates peripheral C-N bonds from an intact guanidine nucleus have proven powerful for the preparation of polycyclic guanidine natural products.3a-e Our interest in the biological activity of guanidine natural products has prompted us to develop a synthetic platform that is capable of delivering cyclic guanidines having multiple ring sizes, substitution patterns, and oxidation states in short order. To this end we have been interested in the addition of guanidine N-H bonds across C-C π systems and recently reported a La(III) catalyzed tandem addition-hydroamination reaction of propargylcyanamides which required forcing conditions, and resulted in exclusive 5-exo-dig cyclization. 4 This led us to study the hydroamination of preformed di-Boc protected propargylguanidines of the type 1, in hopes of finding a 6-endo-dig selective process. Traditionally, metal catalyzed cyclizations on alkynes favor a 5-exo-dig pathway. This can be seen in Au(I) and Au(III) catalyzed cyclization of propargylcarbamates, propargylureas, and propargylamides, 5,6 as well as the Ti(IV)-amide hydroamination reactions of homopropargylamines. 7,8 Examples of cyclization of heteroatom nucleophiles onto alkynes leading to 6-endo-dig cyclization, although as synthetically significant as the 5-exo-dig products, are nevertheless much less common and usually observed with substrates in which the tether is largely sp2 hybridized. 9,10 During the preparation of this manuscript, Gin and co-workers described the AuCl3 catalyzed hydroamination of alkynes with 2-aminopyrimidines en route to the synthesis of crambedine.11 Their gold-catalyzed 6-exo-dig cyclization highlights the power of this approach in total synthesis. Achieving selectivity in these hydroaminations, particularly 5-exo-dig vs. 6-endo-dig, would present a valuable tool for the synthesis of complex guanidine containing natural products. This manuscript highlights the discovery of an unusual reactivity of dirhodium(II)-carboxylates as highly 6-endo-dig selective hydroamination catalysts in the cyclization of propargylguanidines, while Ag(I) is typically 5-exo-dig selective. We first examined the ability of traditional π-Lewis acids to catalyze the hydroamination of 1a (Table 1). Initial experiments highlighted that there were multiple reaction pathways available (entry 1). Cyclization of 1a in the presence of Cu(I) salts reliably gave a mixture of 6-endo-dig and 5-exo-dig products, 2a and 3a respectively, that are easily identified by the magnitude of either 3J or 4J coupling. From nOe experiments it was also confirmed that the 5-exo product 3a carried the Z-alkene configuration. The unanticipated product was the yne-guanidine 4, which may arise from a [1,3]-prototropic shift followed by isomerization as detailed by Gevorgyan for propargyl acetates.12 Table 1 Catalyst screen for propargylguanidine hydroamination. Ag(I) catalysis proved to be optimal for the generation of 3a with AgOAc giving a 1 : >20 ratio of 2a : 3a in 55% yield. It was further found that addition of AcOH to the AgOAc catalyzed reaction gave significant rate enhancements suggesting that protonolysis of the vinyl metal species was rate limiting, consistent with the known electrophilic hydroamination mechanism 13 Our first success in reversing the selectivity was found with gold (III) catalysts that favored the 6-endo product (entries 6-7). The sluggishness of the Au catalyzed reactions directed our attention to Rh(II). Dirhodium(II)-carboxylates have rarely been used to activate alkynes, however Murai has shown that Rh2(tfa)4 is a competent ene-ene-yne cycloisomerization catalyst and gives different product distributions than Pt(II) or Ru(II).14 Gratifyingly, Rh2(tfa)4 turned the selectivity over favoring the 6-endo product (entry 8). Quite to our surprise, given its decreased Lewis-acidity, Rh2(OAc)4 was a very selective catalyst favoring the 6-endo product 13 : 1 (entry 9). The reaction times were long and we suspected that this was due to catalyst solubility. The reaction was enhanced with Rh2(Oct)4 giving the 6-endo product in 81% isolated yield after 16 hours in > 20 : 1 (2a : 3a)selectivity as judged by 1H NMR (entry 10). Having identified a selective catalyst set, a substrate-selectivity profile was developed for the reaction (Table 2) and it was immediately apparent that the reactivity of the dirhodium(II)-carboxylates was quite unique. For example, cyclization of the p-methoxyphenyl alkyne (entry 1) with Ag(I) favored the 5-exo product, however with poor selectivity, consistent with the ability of this substituent to competitively stabilize a vinyl cation for 6-endo cyclization. In contrast, Rh(II) gave exclusively the 6-endo product 2b. Cyclization of electron poor aryl alkynes showed the opposite trend, with Ag(I) giving high 5-exo selectivity while Rh(II) was more modestly selective for the 6-endo product 2c (entry 2). Alkyl substituted alkynes gave the 6-endo products exclusively with Rh(II) (entries 3-7). This is in direct contrast to the Ag(I) catalyzed reactions, where modest selectivities for the 5-exo products are seen when the alkyne termini are electronically indistinguishable. Importantly for subsequent annulations and access to polycyclic skeletons, Rh2(Oct)4 tolerates useful functional groups (e.g. alcohols and alkyl chlorides, entries 4-7). These primary alkyl chlorides were also cleanly cyclized to the 5-exo-dig isomers 2g,h with Ag(I). Substitution is also permitted at the propargylic position wherein good selectivities are seen with Ag(I) for the 5-exo product and with Rh(II) for the 6-endo product (entry 8). Substituents on the guanidine nitrogen had modest effects on selectivity, with the 6-endo product always favored under Rh(II) catalysis (entries 9,10). The t-butyl alkyne 1l (entry 11) reacted well in the Ag(I) catalyzed process but was unreactive toward Rh(II), presumably due to the size of the Rh(II) catalyst. Table 2 Substrate scope for the Rh(II)-catalyzed hydroamination of propargylguanidines. The resultant cyclic eneguanidines contain a rich functional group handle for further manipulations (Scheme 1). For example bromination proceeds to give the 5-bromodihydropyrimidine 5 the structure of which was further confirmed by X-ray crystallography (Eq. 1). Oxidative cyclization of pendant nucleophiles gives the spirocyclic hemiaminal 6, reminiscent of the crambescidins (Eq. 2). Deprotection of the 5-membered ene-guanidine 2a under acidic conditions was accompanied by isomerization to give the 2-aminoimidazole 7 (Eq. 3). Reduction was also cleanly performed on 2b to give tetrahydropyrimidine 8 (Eq. 4). Scheme 1 Transformations of the resultant ene-guanidines. Examples of alkyne activation by dirhodium(II)-carboxylates are quite rare.8,15 Given the fact that reactions of internal alkynes have only been observed with Rh2(tfa)4 16 and that Rh2(Oct)4 is not able to activate alkynes independently,8 we were surprised that the dirhodium(II)-alkylcarboxylates were competent catalysts for this transformation especially at room temperature.17 This was especially true for the cyclization of electron deficient alkynes such as the pCF3Ph substituted alkyne 1c. We were even more surprised by the observation that 1c reacted ten times faster than the p-methoxy substituted alkyne 1b. Regardless of the poor π-Lewis aciditiy of these catalysts, this suggested that coordination of the alkyne was not rate limiting. Addition of 3 equivalents of acetic acid changed the selectivity from 5:1 to 1:1.7 (2c:3c, table 3 compare entries 1 and 2) suggesting that protonation of a vinyl-rhodium intermediate might be involved in the product determining step. The use of Rh2(tfa)4 as a catalyst also favored the 5-exo-dig product with 1c (entry 3). Taken together this may suggest a Curtin-Hammett scenario wherein the initial cyclization to form a 5- or 6-membered vinyl-rhodium anion is highly reversible and that perturbation of this pre-equilibrium by stabilizing the kinetically favored 5-exo-dig intermediate leads to poor selectivity.18 Table 3 Condition effects on selectivity. Mechanistically it is difficult to rationalize oxidation state changes in the dirhodium(II)-carboxylates upon arriving at a vinylrhodium intermediate (in this case the formation of Rh(I) in the dimer). This prompted us to examine better defined catalysts known to proceed through vinyl-rhodium intermediate.19 Although sluggish, Wilkinson’s catalyst promoted the cyclization and favored the 6-endo product in 10:1 selectivity (entry 4). Cationic Rh(I) however reversed the selectivity favoring the 5-exo product in 10:1 selectivity (entry 5). Again this may support the necessity for the initial cyclization event to be reversible in order to access the 6-endo product. More importantly this suggests access to more defined vinyl-rhodium intermediates that might be exploited for cascade reactivity. In conclusion we have demonstrated the unique ability of dirhodium(II)-carboxylates to catalyze the 6-endo-dig selective hydroamination of propargylguanidines. The resultant cyclic eneguanidines contain a rich latent functional group for the preparation of skeletally diverse cyclic guanidine natural product substructures. Studies to further understand the reactivity of dirhodium(II)-carboxylates and the application of these products to more complex targets are ongoing and will be reported in due course.

The total synthesis of (±)-heliannuol D and its epimer has been completed in 9 steps and 12% overall yield from 2-methylanisole. The benzoxepane moiety of the title compound, a common structural feature in the heliannuol family of natural products, is prepared by a biomimetic opening of an epoxide by a phenol.

A methodology to generate 2-thio- and 2-oxoimidazoles through an addition−cyclization−isomerization reaction of propargylcyanamides with thiol and alcohol nucleophiles is described. In general, the reaction sequence allows for the rapid formation of highly substituted 2-thio- and 2-oxoimidazoles in good to excellent yields.

This review will focus on the ability of the 2-aminoimidazole to occupy a unique subset of chemical space which makes it an ideal pharmacophore for the development of small molecule collections for discovery based research. These observations rely both on the use of 2-aminoimidazoles as building blocks in medicinal chemistry as well as the recent discovery of alkaloids from sponges of the genus Leucetta which exhibit a diverse range of biological activities around a relatively limited structural core. The preparation of these compounds will also be highlighted. In particular, marine natural products derived from sponges have provided valuable leads for therapeutic small molecules (3, 4). Surprisingly the large majority of these compounds have been isolated from organisms of the class Dermospongiae. In the mid 1980's chemists noted that the other major sponge class, Calcarea, had rarely been subject to chemical investigations. A flurry of efforts through the mid-1990's helped to establish biogenetic relationships among these sponges. Isolated to explore these inter- connections and not necessarily for specific biological responses the activities of these natural products have remained largely uncovered. Since these initial inves- tigations, an emerging structural class has recurrently been identified through bioassay guided isolation which contains the 2-aminoimidazole core. From the viewpoint of small molecule discovery this review will highlight alkaloids isolated from Leucetta sp. This small skeletal family has been shown to interrogate an incredibly diverse range of biological processes and thus represents an important discovery scaffold for both medicinal and discovery based research.

A valuable pharmacophore, the 2-aminoimidazole moiety, can be accessed with a variety of substitution patterns through an addition–hydroamination–isomerization sequence (see scheme; R1,R4,R5=alkyl; R3=alkyl, aryl; R2=H, alkyl, aryl). The synthesis of the propargyl cyanamide precursors through a three-component coupling enables the preparation of this important heterocyclic core structure in just three steps.

Aromatic bisabolene derivatives were prepared by two methods involving cross-coupling of organozinc reagents. The first synthesis of (+/-)-glandulone A (10), as well as syntheses of (+/-)-curcuhydroquinone (8) and (+/-)-curcuquinone (9), were accomplished via coupling of a secondary alkyl zinc reagent (1,5-dimethyl-4-hexenylzinc halide, 18) to protected bromohydroquinones using Pd(dppf)Cl(2) as catalyst. Coupling of arylzinc halides with alkenyl triflate 16 using Pd(PPh(3))(4) catalyst provided a number of bisabolene derivatives and led to syntheses of dehydro-alpha-curcumene (2), (+/-)-curcuphenol (3), and (+/-)-elvirol (13). A high-yield synthesis of the (+/-)-heliannuol D precursor 29 is also reported using this method.

The efficient synthesis of an A-ring synthon for the marine hepatatoxin cylindrospermopsin has been achieved. The key step features an intramolecular oxazinone N-oxide/alkene dipolar cycloaddition resulting in the establishment of the three contiguous stereogenic centers in the A-ring from one pre-existing stereogenic center in a single step.