General Information. 1 H and 13 C NMR spectra were recorded on a Bruker AMX-400, AMX-500 or AMX-6... more General Information. 1 H and 13 C NMR spectra were recorded on a Bruker AMX-400, AMX-500 or AMX-600 spectrometers in CDCl 3 with CHCl 3 (7.27 ppm for 1 H, 77ppm for 13 C) as a standard. Reagents and solvents were purchased from Aldrich, Acros or GFS and used without purification. Typical experimental procedure for the Cu(I)-catalyzed triazole synthesis (products 2, 4, 6, 8, 10, 12): Allylic azide (1 mmol) was mixed with phenylacetylene (1 mmol) in 1:1 mixture of tBuOH:H 2 O (2 ml) at room temperature (usually 23°C). To this mixture was added CuSO 4 •5H 2 O (0.05 mmol) and sodium ascorbate (0.10 mmol). The reaction was stirred overnight, then extracted with ethyl acetate, washed twice with dilute aqueous ammonium hydroxide and once with brine. The aqueous layer was back extracted once into ethyl acetate. The organic layers were combined, dried over MgSO 4 , and evaporated to dryness. Typical experimental procedure A for the synthesis of azidoepoxides 13, 15, 17: To an allylic azide (1 mmol) was added 0.3M aqueous solution of NaHCO 3 (7.5 ml) at room temperature. The inhomogeneous mixture was stirred vigorously and m-CPBA (70%, 1.1 mmol) was added slowly as a solid. The reaction was stirred for 2-8 hours; progress was monitored by TLC. Upon completion the reaction was extracted three times into diethyl ether, washed twice with 10% aqueous NaOH, dried over MgSO 4 and evaporated. Typical experimental procedure B for the synthesis of azidoepoxides 14, 16, 18: Allylic azide (1 mmol) was dissolved in dry CH 2 Cl 2 (3 ml). To the mixture was added dry m-CPBA (1.3 mmol) portionwise as a solid. The reaction was stirred overnight; progress was monitored by TLC. Upon completion the mixture was diluted with more CH 2 Cl 2 and washed twice with 10% aqueous NaOH, dried over MgSO 4 and evaporated.
Copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a widely utilized, reliable, and straightf... more Copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a widely utilized, reliable, and straightforward way for making covalent connections between building blocks containing various functional groups. It has been used in organic synthesis, medicinal chemistry, surface and polymer chemistry, and bioconjugation applications. Despite the apparent simplicity of the reaction, its mechanism involves multiple reversible steps involving coordination complexes of copper(I) acetylides of varying nuclearity. Understanding and controlling these equilibria is of paramount importance for channeling the reaction into the productive catalytic cycle. This tutorial review examines the history of the development of the CuAAC reaction, its key mechanistic aspects, and highlights the features that make it useful to practitioners in different fields of chemical science. † Part of a themed issue reviewing the latest applications of click chemistry.
Journal of the American Chemical Society, Dec 17, 2013
Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a ... more Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a facile, mild and convergent formal 1,3-insertion into N-H and O-H bonds of primary and secondary amides, various alcohols, and carboxylic acids to afford a wide range of vicinally bisfunctionalized Z-olefins with perfect regio-and stereoselectively. Utilizing the distinctive functionality installed through these reactions, a number of subsequent rearrangements and cyclizations expand the repertoire of valuable organic building blocks constructed by reactions of transition metal carbene complexes, including α-allenyl ketones and amino-substituted heterocycles.
Sulfamoyl azides are readily generated from secondary amines and a novel sulfonyl azide transfer ... more Sulfamoyl azides are readily generated from secondary amines and a novel sulfonyl azide transfer agent, 2,3-dimethyl-1H-imidazolium triflate. They react with alkynes in the presence of CuTC catalyst forming 1-sulfamoyl-1,2,3-triazoles. The latter are shelf-stable progenitors of rhodium azavinyl carbenes, versatile reactive intermediates that, among other reactions, readily and asymmetrically add to olefins. Sulfamoyl azides make only a fleeting appearance in organic synthesis. Their first preparation from sulfamoyl chlorides and sodium azide was reported in 1956 by American Cyanamid chemists. 1 However, this generally reliable route fails when preparation of arylsulfamoyl azides is attempted due to the undesired chlorination of the aromatic ring by sulfuryl chloride. Shozda and Vernon, 2 and later Griffiths, 3 developed an alternative synthesis of arylsulfamoyl azides using chlorosulfonyl azide. While this method circumvented the problem of ring chlorination, the chlorosulfonyl azide reagent was highly explosive and difficult to handle. Furthermore, the yields were modest at best, remaining generally in the 15-50% range. Recently, imidazole-1-sulfonyl azide hydrochloride was shown to be an efficient and convenient diazo transfer reagent. 4 During our investigations, we found that alkylation of the imidazole nitrogen eliminated its diazo transfer reactivity and instead led to the transfer of the sulfonyl azide group. In this study, we introduce the imidazolium salt 2 as a novel and efficient sulfonyl azide transfer reagent. The 2-methylated derivative 1 was prepared using a slight modification of the method developed by Goddard-Borger and Sticks for the synthesis of imidazole-1-sulfonyl azide hydrochloride. 4 The added methyl group at C-2 of the imidazole ring improves solubility of this derivative in organic solvents 5 and is also expected to increase its stability. The free base of the imidazole 1 was efficiently alkylated with methyl triflate. The product was isolated as a crystalline solid, yielding the imidazolium sulfonyl azide transfer reagent 2 in
The 1,3-dipolar cycloaddition reaction of azides and alkynes has been known for over 100 years an... more The 1,3-dipolar cycloaddition reaction of azides and alkynes has been known for over 100 years and was studied extensively by Huisgen and co-workers in the 1960s. 1 The resurgence of recent interest in the reaction has been stimulated by the discovery of the coppercatalyzed version of this process, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). 2 Copper catalysis increases the reaction rate by up to 10 7 and results in regioselective formation of 1,4-disubstituted triazoles. This reaction is generally recognized as the most striking example of click chemistry. 3 What makes a click reaction so appealing is its application to label molecules of interest in complex biological samples without interferences with any other chemical functionalities. 4 The most common catalyst systems for CuAAC employ water or alcohol solvents and use a Cu(II) salt in the presence of a reducing agent (often sodium ascorbate or metallic copper) to generate the required Cu(I) catalyst in situ. Cu(I) complexes can also be used directly, although the reaction often suffers from the formation of byproducts of red/ox processes catalyzed by copper and requires the addition of ligands to accelerate the cycloaddition. 2 Microwave (MW) irradiation was efficiently applied to accelerate the azide/alkyne click reaction. 5 Catalytic activity of metallic copper was established early on, 6 and although the reaction times were long, the final product was clean, and the workup consisted of a simple removal of the copper turnings. Copper clusters 7 have also been employed as precatalysts. Lipshutz et al. found that copper supported on charcoal (Cu(II)/C) was an efficient catalyst; 8 Cu(I)/C was also successfully employed by Cintas et al. in MW-assisted protocols. 9 Ultrasound (US) has been used to promote the CuAAC reaction as well. Sreedhar reported a sonochemical CuI-catalyzed synthesis of triazoles from terminal alkynes and alkyl/aryl azides, formed in situ. 10 Worthy of mention are the efficient applications of copper nanoparticles as substitutes of bulk copper metal although their preparation involves an additional step. 11 We studied this reaction under non-conventional conditions, 9,12 namely, power US and MW, alone or combined. The specific advantages of US 13 and MW 14 in organic synthesis have been widely described. Using both simultaneously may be beneficial to the rates, yields, and selectivity of the reactions, as recent examples show. 15 Here we describe a process in which metallic copper efficiently catalyzes azide-alkyne cycloadditions under US or simultaneous US/MW irradiation. Reactions involving metals
An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ... more An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ generated copper(I) acetylides and sulfonyl azides is described. Copper(I) thiophene-2-carboxylate (CuTC) catalyst produces the title compounds under both non-basic anhydrous and aqueous conditions in good yields.
Experimental procedures Reactions 1-7 were carried out under nitrogen atmosphere using standard S... more Experimental procedures Reactions 1-7 were carried out under nitrogen atmosphere using standard Schlenk techniques, unless otherwise stated. Reactions 8-11 were performed in tightly capped vials flushed with nitrogen. 1 H and 13 C NMR chemical shifts are relative to TMS. Mass Spectra were collected on a Finnigan TSQ 7000 Spectrometer and LC-MS data were obtained on an Agilent 1100 ESI-MS system. General procedure for Cp*RuCl(PPh 3) 2 catalyzed cycloadditions. A mixture of azide, alkyne and Cp*RuCl(PPh 3) 2 in organic solvent (8-20 mL, 0.07-0.2M in reactants) was stirred at a given temperature for a period of time indicated in the Table 1. The progress of the reaction was monitored by 1 H NMR or GC. In most cases the azide was completely consumed at the end of the reaction. The solvent was removed under vacuum and the product was purified by silica gel chromatography. The unreacted alkyne and traces of side products were first eluted out with hexane, followed by 1/1 hexane/ether. The pure 1,5-disubstututed triazole product was then obtained by elution with ether or chloroform. 1-benzyl-5-phenyl-1H-1,2,3-triazole (1a). N N N Ph (a) Benzylazide (0.400 g, 3.00 mmol), phenylacetylene (0.500 mL, 4.55 mmol), Cp*RuCl(PPh 3) 2 (25 mg, 0.031 mmol). Solvent, benzene; reaction temperature, 80 °C, reaction time, 2 h; yield, 0.56 g (80%). (b) Benzylazide (0.200 g, 1.50 mmol), phenylacetylene (0.300 mL, 2.73 mmol), Cp*RuCl(PPh 3) 2 (25 mg, 0.031 mmol). Solvent, tetrahydrofuran; reaction temperature, 65 °C; reaction time, 3 h; yield, 0.26 g (74%). (c) Benzylazide (0.100 g, 0.751 mmol), phenylacetylene (0.150 mL, 1.37 mmol), Cp*RuCl(PPh 3) 2 (30 mg, 0.038 mmol). Solvent, benzene; reaction temperature, r.t.; reaction time, 24 h; yield, 0.13 g (75%).
VIM-2 is an Ambler class B metallo-β-lactamase (MBL) capable of hydrolyzing a broad-spectrum of β... more VIM-2 is an Ambler class B metallo-β-lactamase (MBL) capable of hydrolyzing a broad-spectrum of β-lactam antibiotics. Although the discovery and development of MBL inhibitors continues to be an area of active research, an array of potent, small molecule inhibitors has yet to be fully characterized for VIM-2. In the presented research, a compound library screening approach was used to identify and characterize VIM-2 inhibitors from a library of pharmacologically active compounds as well as a focused "click" chemistry library. The four most potent VIM-2 inhibitors resulting from a VIM-2 screen were characterized by kinetic studies in order to determine K i and mechanism of enzyme inhibition. As a result, two previously described pharmacologic agents, mitoxantrone (1,4-Dihydroxy-5,8-bis([2-([2-hydroxyethyl]amino)ethyl]amino)-9,10-anthracenedione) and 4chloromercuribenzoic acid (pCMB) were found to be active, the former as a non-competitive inhibitor (K i = K′ i = 1.5 ± 0.2 μM) and the latter as a slowly reversible or irreversible inhibitor. Additionally, two novel sulfonyl-triazole analogs from the click library were identified as potent, competitive VIM-2 inhibitors: N-((4-((but-3-ynyloxy)methyl)-1H-1,2,3-triazol-5-yl)methyl)-4iodobenzenesulfonamide (1, K i = 0.41 ± 0.03 μM) and 4-iodo-N-((4-(methoxymethyl)-1H-1,2,3triazol-5-yl)methyl)benzenesulfonamide (2, K i = 1.4 ± 0.10 μM). Mitoxantrone and pCMB were also found to potentiate imipenem efficacy in MIC and synergy assays employing E. coli. Taken together, all four compounds represent useful chemical probes to further investigate mechanisms of VIM-2 inhibition in biochemical and microbiology-based assays.
Journal of the American Chemical Society, Dec 8, 2004
Huisgen's 1,3-dipolar cycloadditions become nonconcerted when copper(I) acetylides react with azi... more Huisgen's 1,3-dipolar cycloadditions become nonconcerted when copper(I) acetylides react with azides and nitrile oxides, providing ready access to 1,4-disubstituted 1,2,3-triazoles and 3,4-disubstituted isoxazoles, respectively. The process is highly reliable and exhibits an unusually wide scope with respect to both components. Computational studies revealed a stepwise mechanism involving unprecedented metallacycle intermediates, which appear to be common for a variety of dipoles.
Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, ... more Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, yet development of new antimicrobial agents with improved efficacy and ability to override increasingly common drug resistance remains a major challenge. Antimicrobial drug development typically proceeds by broad functional screens of large chemical libraries or hypothesis-driven exploration of single microbial targets, but both strategies have challenges that have limited the introduction of new antimicrobials. Here, we describe an alternative drug development strategy that identifies a sufficient but manageable number of promising targets, while reducing the risk of pursuing targets of unproven value. The strategy is based on defining and exploiting the incompletely understood adduction targets of 5-nitroimidazoles, which are proven antimicrobials against a wide range of anaerobic protozoan and bacterial pathogens. Comprehensive adductome analysis by modified click chemistry and multidimensional proteomics were applied to the model pathogen Giardia lamblia to identify dozens of adducted protein targets common to both 5'-nitroimidazole-sensitive and-resistant cells. The list was highly enriched for known targets in G. lamblia, including arginine deiminase, α-tubulin, carbamate kinase, and heat shock protein 90, demonstrating the utility of the approach. Importantly, over twenty potential novel drug targets were identified. Inhibitors of two representative new targets, NADP-specific glutamate dehydrogenase and peroxiredoxin, were found to have significant antigiardial activity. Furthermore, all the identified targets remained available in resistant cells, since giardicidal activity of the respective inhibitors was not impacted by resistance to 5'-nitroimidazoles. These results demonstrate that the combined use of click chemistry and proteomics has the potential to reveal alternative drug targets for overcoming antimicrobial drug resistance in protozoan parasites.
Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when cov... more Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when covalently inhibited by organophosphorus compounds (OPs), such as nerve agents and pesticides, can be reactivated by oximes. However, tabun remains among the most dangerous nerve agents due to the low reactivation efficacy of standard pyridinium aldoxime antidotes. Therefore, finding an optimal reactivator for prophylaxis against tabun toxicity and for post‐exposure treatment is a continued challenge. In this study, we analyzed the reactivation potency of 111 novel nucleophilic oximes mostly synthesized using the CuAAC triazole ligation between alkyne and azide building blocks. We identified several oximes with significantly improved in vitro reactivating potential for tabun‐inhibited human AChE, and in vivo antidotal efficacies in tabun‐exposed mice. Our findings offer a significantly improved platform for further development of antidotes and scavengers directed against tabun and related p...
Antimicrobial agents and chemotherapy, Jun 1, 2017
Giardia lamblia is an important and ubiquitous cause of diarrheal disease. The primary agents in ... more Giardia lamblia is an important and ubiquitous cause of diarrheal disease. The primary agents in the treatment of giardiasis are nitroheterocyclic drugs, particularly the imidazoles metronidazole and tinidazole and the thiazole nitazoxanide. Although these drugs are generally effective, treatment failures occur in up to 20% of cases, and resistance has been demonstrated in vivo and in vitro Prior work had suggested that side chain modifications of the imidazole core can lead to new effective 5-nitroimidazole drugs that can combat nitro drug resistance, but the full potential of nitroheterocycles other than imidazole to yield effective new antigiardial agents has not been explored. Here, we generated derivatives of two clinically utilized nitroheterocycles, nitrothiazole and nitrofuran, as well as a third heterocycle, nitropyrrole, which is related to nitroimidazole but has not been systematically investigated as an antimicrobial drug scaffold. Click chemistry was employed to synthes...
Copper(I)-catalyzed azide-alkyne cycloaddition has become a commonly employed method for the synt... more Copper(I)-catalyzed azide-alkyne cycloaddition has become a commonly employed method for the synthesis of complex molecular architectures under challenging conditions. Despite the widespread use of copper-catalyzed cycloaddition reactions, the mechanism of these processes has remained difficult to establish due to the involvement of multiple equilibria between several reactive intermediates. Real-time monitoring of a representative cycloaddition process via heat-flow reaction calorimetry revealed that monomeric copper acetylide complexes are not reactive toward organic azides unless an exogenous copper catalyst is added. Furthermore, crossover experiments with an isotopically enriched exogenous copper source illustrated the stepwise nature of the carbon-nitrogen bond-forming events and the equivalence of the two copper atoms within the cycloaddition steps.
The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are re... more The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are recognized as potential targets for cell directed therapies. CD33 and CD22 are prototypical members and are validated candidates for targeting acute myeloid leukaemia and non-Hodgkin's lymphomas due to their restricted expression on myeloid cells and B-cells, respectively. While nanoparticles decorated with high affinity siglec ligands represent an attractive platform for delivery of therapeutic agents to these cells, a lack of ligands with suitable affinity and/or selectivity has hampered progress. Herein we describe selective ligands for both of these siglecs, which when displayed on liposomal nanoparticles, can efficiently target the cells expressing them in peripheral human blood. Key to their identification was the development of a facile method for chemo-enzymatic synthesis of disubstituted sialic acid analogues, combined with iterative rounds of synthesis and rapid functional analysis using glycan microarrays.
An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ... more An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ generated copper(I) acetylides and sulfonyl azides is described. Copper(I) thiophene-2-carboxylate (CuTC) catalyst produces the title compounds under both non-basic anhydrous and aqueous conditions in good yields.
The acetylcholine-binding proteins (AChBPs), which serve as structural surrogates for the extrace... more The acetylcholine-binding proteins (AChBPs), which serve as structural surrogates for the extracellular domain of nicotinic acetylcholine receptors (nAChRs), were used as reaction templates for in situ click-chemistry reactions to generate a congeneric series of triazoles from azide and alkyne building blocks. The catalysis of in situ azide-alkyne cycloaddition reactions at a dynamic subunit interface facilitated the synthesis of potentially selective compounds for nAChRs. We investigated compound sets generated in situ with soluble AChBP templates through pharmacological characterization with ␣7 and ␣42 nAChRs and 5-hydroxytryptamine type 3A receptors. Analysis of activity differences between the triazole 1,5-synand 1,4-anti-isomers showed a preference for the 1,4-antitriazole regioisomers among nAChRs. To improve nAChR subtype selectivity, the highest-potency building block for ␣7 nAChRs, i.e., 3␣-azido-N-methylammonium tropane, was used for additional in situ reactions with a mutated Aplysia californica AChBP that was made to resemble the ligand-binding domain of the ␣7 nAChR. Fourteen of 50 possible triazole products were identified, and their corresponding tertiary analogs were synthesized. Pharmacological assays revealed that the mutated binding protein template provided enhanced selectivity of ligands through in situ reactions. Discrete trends in pharmacological profiles were evident, with most compounds emerging as ␣7 nAChR agonists and ␣42 nAChR antagonists. Triazoles bearing quaternary tropanes and aromatic groups were most potent for ␣7 nAChRs. Pharmacological characterization of the in situ reaction products established that click-chemistry synthesis with surrogate receptor templates offered novel extensions of fragment-based drug design that were applicable to multisubunit ion channels.
General Information. 1 H and 13 C NMR spectra were recorded on a Bruker AMX-400, AMX-500 or AMX-6... more General Information. 1 H and 13 C NMR spectra were recorded on a Bruker AMX-400, AMX-500 or AMX-600 spectrometers in CDCl 3 with CHCl 3 (7.27 ppm for 1 H, 77ppm for 13 C) as a standard. Reagents and solvents were purchased from Aldrich, Acros or GFS and used without purification. Typical experimental procedure for the Cu(I)-catalyzed triazole synthesis (products 2, 4, 6, 8, 10, 12): Allylic azide (1 mmol) was mixed with phenylacetylene (1 mmol) in 1:1 mixture of tBuOH:H 2 O (2 ml) at room temperature (usually 23°C). To this mixture was added CuSO 4 •5H 2 O (0.05 mmol) and sodium ascorbate (0.10 mmol). The reaction was stirred overnight, then extracted with ethyl acetate, washed twice with dilute aqueous ammonium hydroxide and once with brine. The aqueous layer was back extracted once into ethyl acetate. The organic layers were combined, dried over MgSO 4 , and evaporated to dryness. Typical experimental procedure A for the synthesis of azidoepoxides 13, 15, 17: To an allylic azide (1 mmol) was added 0.3M aqueous solution of NaHCO 3 (7.5 ml) at room temperature. The inhomogeneous mixture was stirred vigorously and m-CPBA (70%, 1.1 mmol) was added slowly as a solid. The reaction was stirred for 2-8 hours; progress was monitored by TLC. Upon completion the reaction was extracted three times into diethyl ether, washed twice with 10% aqueous NaOH, dried over MgSO 4 and evaporated. Typical experimental procedure B for the synthesis of azidoepoxides 14, 16, 18: Allylic azide (1 mmol) was dissolved in dry CH 2 Cl 2 (3 ml). To the mixture was added dry m-CPBA (1.3 mmol) portionwise as a solid. The reaction was stirred overnight; progress was monitored by TLC. Upon completion the mixture was diluted with more CH 2 Cl 2 and washed twice with 10% aqueous NaOH, dried over MgSO 4 and evaporated.
Copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a widely utilized, reliable, and straightf... more Copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a widely utilized, reliable, and straightforward way for making covalent connections between building blocks containing various functional groups. It has been used in organic synthesis, medicinal chemistry, surface and polymer chemistry, and bioconjugation applications. Despite the apparent simplicity of the reaction, its mechanism involves multiple reversible steps involving coordination complexes of copper(I) acetylides of varying nuclearity. Understanding and controlling these equilibria is of paramount importance for channeling the reaction into the productive catalytic cycle. This tutorial review examines the history of the development of the CuAAC reaction, its key mechanistic aspects, and highlights the features that make it useful to practitioners in different fields of chemical science. † Part of a themed issue reviewing the latest applications of click chemistry.
Journal of the American Chemical Society, Dec 17, 2013
Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a ... more Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a facile, mild and convergent formal 1,3-insertion into N-H and O-H bonds of primary and secondary amides, various alcohols, and carboxylic acids to afford a wide range of vicinally bisfunctionalized Z-olefins with perfect regio-and stereoselectively. Utilizing the distinctive functionality installed through these reactions, a number of subsequent rearrangements and cyclizations expand the repertoire of valuable organic building blocks constructed by reactions of transition metal carbene complexes, including α-allenyl ketones and amino-substituted heterocycles.
Sulfamoyl azides are readily generated from secondary amines and a novel sulfonyl azide transfer ... more Sulfamoyl azides are readily generated from secondary amines and a novel sulfonyl azide transfer agent, 2,3-dimethyl-1H-imidazolium triflate. They react with alkynes in the presence of CuTC catalyst forming 1-sulfamoyl-1,2,3-triazoles. The latter are shelf-stable progenitors of rhodium azavinyl carbenes, versatile reactive intermediates that, among other reactions, readily and asymmetrically add to olefins. Sulfamoyl azides make only a fleeting appearance in organic synthesis. Their first preparation from sulfamoyl chlorides and sodium azide was reported in 1956 by American Cyanamid chemists. 1 However, this generally reliable route fails when preparation of arylsulfamoyl azides is attempted due to the undesired chlorination of the aromatic ring by sulfuryl chloride. Shozda and Vernon, 2 and later Griffiths, 3 developed an alternative synthesis of arylsulfamoyl azides using chlorosulfonyl azide. While this method circumvented the problem of ring chlorination, the chlorosulfonyl azide reagent was highly explosive and difficult to handle. Furthermore, the yields were modest at best, remaining generally in the 15-50% range. Recently, imidazole-1-sulfonyl azide hydrochloride was shown to be an efficient and convenient diazo transfer reagent. 4 During our investigations, we found that alkylation of the imidazole nitrogen eliminated its diazo transfer reactivity and instead led to the transfer of the sulfonyl azide group. In this study, we introduce the imidazolium salt 2 as a novel and efficient sulfonyl azide transfer reagent. The 2-methylated derivative 1 was prepared using a slight modification of the method developed by Goddard-Borger and Sticks for the synthesis of imidazole-1-sulfonyl azide hydrochloride. 4 The added methyl group at C-2 of the imidazole ring improves solubility of this derivative in organic solvents 5 and is also expected to increase its stability. The free base of the imidazole 1 was efficiently alkylated with methyl triflate. The product was isolated as a crystalline solid, yielding the imidazolium sulfonyl azide transfer reagent 2 in
The 1,3-dipolar cycloaddition reaction of azides and alkynes has been known for over 100 years an... more The 1,3-dipolar cycloaddition reaction of azides and alkynes has been known for over 100 years and was studied extensively by Huisgen and co-workers in the 1960s. 1 The resurgence of recent interest in the reaction has been stimulated by the discovery of the coppercatalyzed version of this process, the copper-catalyzed azide-alkyne cycloaddition (CuAAC). 2 Copper catalysis increases the reaction rate by up to 10 7 and results in regioselective formation of 1,4-disubstituted triazoles. This reaction is generally recognized as the most striking example of click chemistry. 3 What makes a click reaction so appealing is its application to label molecules of interest in complex biological samples without interferences with any other chemical functionalities. 4 The most common catalyst systems for CuAAC employ water or alcohol solvents and use a Cu(II) salt in the presence of a reducing agent (often sodium ascorbate or metallic copper) to generate the required Cu(I) catalyst in situ. Cu(I) complexes can also be used directly, although the reaction often suffers from the formation of byproducts of red/ox processes catalyzed by copper and requires the addition of ligands to accelerate the cycloaddition. 2 Microwave (MW) irradiation was efficiently applied to accelerate the azide/alkyne click reaction. 5 Catalytic activity of metallic copper was established early on, 6 and although the reaction times were long, the final product was clean, and the workup consisted of a simple removal of the copper turnings. Copper clusters 7 have also been employed as precatalysts. Lipshutz et al. found that copper supported on charcoal (Cu(II)/C) was an efficient catalyst; 8 Cu(I)/C was also successfully employed by Cintas et al. in MW-assisted protocols. 9 Ultrasound (US) has been used to promote the CuAAC reaction as well. Sreedhar reported a sonochemical CuI-catalyzed synthesis of triazoles from terminal alkynes and alkyl/aryl azides, formed in situ. 10 Worthy of mention are the efficient applications of copper nanoparticles as substitutes of bulk copper metal although their preparation involves an additional step. 11 We studied this reaction under non-conventional conditions, 9,12 namely, power US and MW, alone or combined. The specific advantages of US 13 and MW 14 in organic synthesis have been widely described. Using both simultaneously may be beneficial to the rates, yields, and selectivity of the reactions, as recent examples show. 15 Here we describe a process in which metallic copper efficiently catalyzes azide-alkyne cycloadditions under US or simultaneous US/MW irradiation. Reactions involving metals
An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ... more An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ generated copper(I) acetylides and sulfonyl azides is described. Copper(I) thiophene-2-carboxylate (CuTC) catalyst produces the title compounds under both non-basic anhydrous and aqueous conditions in good yields.
Experimental procedures Reactions 1-7 were carried out under nitrogen atmosphere using standard S... more Experimental procedures Reactions 1-7 were carried out under nitrogen atmosphere using standard Schlenk techniques, unless otherwise stated. Reactions 8-11 were performed in tightly capped vials flushed with nitrogen. 1 H and 13 C NMR chemical shifts are relative to TMS. Mass Spectra were collected on a Finnigan TSQ 7000 Spectrometer and LC-MS data were obtained on an Agilent 1100 ESI-MS system. General procedure for Cp*RuCl(PPh 3) 2 catalyzed cycloadditions. A mixture of azide, alkyne and Cp*RuCl(PPh 3) 2 in organic solvent (8-20 mL, 0.07-0.2M in reactants) was stirred at a given temperature for a period of time indicated in the Table 1. The progress of the reaction was monitored by 1 H NMR or GC. In most cases the azide was completely consumed at the end of the reaction. The solvent was removed under vacuum and the product was purified by silica gel chromatography. The unreacted alkyne and traces of side products were first eluted out with hexane, followed by 1/1 hexane/ether. The pure 1,5-disubstututed triazole product was then obtained by elution with ether or chloroform. 1-benzyl-5-phenyl-1H-1,2,3-triazole (1a). N N N Ph (a) Benzylazide (0.400 g, 3.00 mmol), phenylacetylene (0.500 mL, 4.55 mmol), Cp*RuCl(PPh 3) 2 (25 mg, 0.031 mmol). Solvent, benzene; reaction temperature, 80 °C, reaction time, 2 h; yield, 0.56 g (80%). (b) Benzylazide (0.200 g, 1.50 mmol), phenylacetylene (0.300 mL, 2.73 mmol), Cp*RuCl(PPh 3) 2 (25 mg, 0.031 mmol). Solvent, tetrahydrofuran; reaction temperature, 65 °C; reaction time, 3 h; yield, 0.26 g (74%). (c) Benzylazide (0.100 g, 0.751 mmol), phenylacetylene (0.150 mL, 1.37 mmol), Cp*RuCl(PPh 3) 2 (30 mg, 0.038 mmol). Solvent, benzene; reaction temperature, r.t.; reaction time, 24 h; yield, 0.13 g (75%).
VIM-2 is an Ambler class B metallo-β-lactamase (MBL) capable of hydrolyzing a broad-spectrum of β... more VIM-2 is an Ambler class B metallo-β-lactamase (MBL) capable of hydrolyzing a broad-spectrum of β-lactam antibiotics. Although the discovery and development of MBL inhibitors continues to be an area of active research, an array of potent, small molecule inhibitors has yet to be fully characterized for VIM-2. In the presented research, a compound library screening approach was used to identify and characterize VIM-2 inhibitors from a library of pharmacologically active compounds as well as a focused "click" chemistry library. The four most potent VIM-2 inhibitors resulting from a VIM-2 screen were characterized by kinetic studies in order to determine K i and mechanism of enzyme inhibition. As a result, two previously described pharmacologic agents, mitoxantrone (1,4-Dihydroxy-5,8-bis([2-([2-hydroxyethyl]amino)ethyl]amino)-9,10-anthracenedione) and 4chloromercuribenzoic acid (pCMB) were found to be active, the former as a non-competitive inhibitor (K i = K′ i = 1.5 ± 0.2 μM) and the latter as a slowly reversible or irreversible inhibitor. Additionally, two novel sulfonyl-triazole analogs from the click library were identified as potent, competitive VIM-2 inhibitors: N-((4-((but-3-ynyloxy)methyl)-1H-1,2,3-triazol-5-yl)methyl)-4iodobenzenesulfonamide (1, K i = 0.41 ± 0.03 μM) and 4-iodo-N-((4-(methoxymethyl)-1H-1,2,3triazol-5-yl)methyl)benzenesulfonamide (2, K i = 1.4 ± 0.10 μM). Mitoxantrone and pCMB were also found to potentiate imipenem efficacy in MIC and synergy assays employing E. coli. Taken together, all four compounds represent useful chemical probes to further investigate mechanisms of VIM-2 inhibition in biochemical and microbiology-based assays.
Journal of the American Chemical Society, Dec 8, 2004
Huisgen's 1,3-dipolar cycloadditions become nonconcerted when copper(I) acetylides react with azi... more Huisgen's 1,3-dipolar cycloadditions become nonconcerted when copper(I) acetylides react with azides and nitrile oxides, providing ready access to 1,4-disubstituted 1,2,3-triazoles and 3,4-disubstituted isoxazoles, respectively. The process is highly reliable and exhibits an unusually wide scope with respect to both components. Computational studies revealed a stepwise mechanism involving unprecedented metallacycle intermediates, which appear to be common for a variety of dipoles.
Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, ... more Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, yet development of new antimicrobial agents with improved efficacy and ability to override increasingly common drug resistance remains a major challenge. Antimicrobial drug development typically proceeds by broad functional screens of large chemical libraries or hypothesis-driven exploration of single microbial targets, but both strategies have challenges that have limited the introduction of new antimicrobials. Here, we describe an alternative drug development strategy that identifies a sufficient but manageable number of promising targets, while reducing the risk of pursuing targets of unproven value. The strategy is based on defining and exploiting the incompletely understood adduction targets of 5-nitroimidazoles, which are proven antimicrobials against a wide range of anaerobic protozoan and bacterial pathogens. Comprehensive adductome analysis by modified click chemistry and multidimensional proteomics were applied to the model pathogen Giardia lamblia to identify dozens of adducted protein targets common to both 5'-nitroimidazole-sensitive and-resistant cells. The list was highly enriched for known targets in G. lamblia, including arginine deiminase, α-tubulin, carbamate kinase, and heat shock protein 90, demonstrating the utility of the approach. Importantly, over twenty potential novel drug targets were identified. Inhibitors of two representative new targets, NADP-specific glutamate dehydrogenase and peroxiredoxin, were found to have significant antigiardial activity. Furthermore, all the identified targets remained available in resistant cells, since giardicidal activity of the respective inhibitors was not impacted by resistance to 5'-nitroimidazoles. These results demonstrate that the combined use of click chemistry and proteomics has the potential to reveal alternative drug targets for overcoming antimicrobial drug resistance in protozoan parasites.
Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when cov... more Acetylcholinesterase (AChE), an enzyme that degrades the neurotransmitter acetylcholine, when covalently inhibited by organophosphorus compounds (OPs), such as nerve agents and pesticides, can be reactivated by oximes. However, tabun remains among the most dangerous nerve agents due to the low reactivation efficacy of standard pyridinium aldoxime antidotes. Therefore, finding an optimal reactivator for prophylaxis against tabun toxicity and for post‐exposure treatment is a continued challenge. In this study, we analyzed the reactivation potency of 111 novel nucleophilic oximes mostly synthesized using the CuAAC triazole ligation between alkyne and azide building blocks. We identified several oximes with significantly improved in vitro reactivating potential for tabun‐inhibited human AChE, and in vivo antidotal efficacies in tabun‐exposed mice. Our findings offer a significantly improved platform for further development of antidotes and scavengers directed against tabun and related p...
Antimicrobial agents and chemotherapy, Jun 1, 2017
Giardia lamblia is an important and ubiquitous cause of diarrheal disease. The primary agents in ... more Giardia lamblia is an important and ubiquitous cause of diarrheal disease. The primary agents in the treatment of giardiasis are nitroheterocyclic drugs, particularly the imidazoles metronidazole and tinidazole and the thiazole nitazoxanide. Although these drugs are generally effective, treatment failures occur in up to 20% of cases, and resistance has been demonstrated in vivo and in vitro Prior work had suggested that side chain modifications of the imidazole core can lead to new effective 5-nitroimidazole drugs that can combat nitro drug resistance, but the full potential of nitroheterocycles other than imidazole to yield effective new antigiardial agents has not been explored. Here, we generated derivatives of two clinically utilized nitroheterocycles, nitrothiazole and nitrofuran, as well as a third heterocycle, nitropyrrole, which is related to nitroimidazole but has not been systematically investigated as an antimicrobial drug scaffold. Click chemistry was employed to synthes...
Copper(I)-catalyzed azide-alkyne cycloaddition has become a commonly employed method for the synt... more Copper(I)-catalyzed azide-alkyne cycloaddition has become a commonly employed method for the synthesis of complex molecular architectures under challenging conditions. Despite the widespread use of copper-catalyzed cycloaddition reactions, the mechanism of these processes has remained difficult to establish due to the involvement of multiple equilibria between several reactive intermediates. Real-time monitoring of a representative cycloaddition process via heat-flow reaction calorimetry revealed that monomeric copper acetylide complexes are not reactive toward organic azides unless an exogenous copper catalyst is added. Furthermore, crossover experiments with an isotopically enriched exogenous copper source illustrated the stepwise nature of the carbon-nitrogen bond-forming events and the equivalence of the two copper atoms within the cycloaddition steps.
The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are re... more The siglec family of sialic acid-binding proteins are endocytic immune cell receptors that are recognized as potential targets for cell directed therapies. CD33 and CD22 are prototypical members and are validated candidates for targeting acute myeloid leukaemia and non-Hodgkin's lymphomas due to their restricted expression on myeloid cells and B-cells, respectively. While nanoparticles decorated with high affinity siglec ligands represent an attractive platform for delivery of therapeutic agents to these cells, a lack of ligands with suitable affinity and/or selectivity has hampered progress. Herein we describe selective ligands for both of these siglecs, which when displayed on liposomal nanoparticles, can efficiently target the cells expressing them in peripheral human blood. Key to their identification was the development of a facile method for chemo-enzymatic synthesis of disubstituted sialic acid analogues, combined with iterative rounds of synthesis and rapid functional analysis using glycan microarrays.
An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ... more An efficient room temperature method for the synthesis of 1-sulfonyl-1,2,3-triazoles from in situ generated copper(I) acetylides and sulfonyl azides is described. Copper(I) thiophene-2-carboxylate (CuTC) catalyst produces the title compounds under both non-basic anhydrous and aqueous conditions in good yields.
The acetylcholine-binding proteins (AChBPs), which serve as structural surrogates for the extrace... more The acetylcholine-binding proteins (AChBPs), which serve as structural surrogates for the extracellular domain of nicotinic acetylcholine receptors (nAChRs), were used as reaction templates for in situ click-chemistry reactions to generate a congeneric series of triazoles from azide and alkyne building blocks. The catalysis of in situ azide-alkyne cycloaddition reactions at a dynamic subunit interface facilitated the synthesis of potentially selective compounds for nAChRs. We investigated compound sets generated in situ with soluble AChBP templates through pharmacological characterization with ␣7 and ␣42 nAChRs and 5-hydroxytryptamine type 3A receptors. Analysis of activity differences between the triazole 1,5-synand 1,4-anti-isomers showed a preference for the 1,4-antitriazole regioisomers among nAChRs. To improve nAChR subtype selectivity, the highest-potency building block for ␣7 nAChRs, i.e., 3␣-azido-N-methylammonium tropane, was used for additional in situ reactions with a mutated Aplysia californica AChBP that was made to resemble the ligand-binding domain of the ␣7 nAChR. Fourteen of 50 possible triazole products were identified, and their corresponding tertiary analogs were synthesized. Pharmacological assays revealed that the mutated binding protein template provided enhanced selectivity of ligands through in situ reactions. Discrete trends in pharmacological profiles were evident, with most compounds emerging as ␣7 nAChR agonists and ␣42 nAChR antagonists. Triazoles bearing quaternary tropanes and aromatic groups were most potent for ␣7 nAChRs. Pharmacological characterization of the in situ reaction products established that click-chemistry synthesis with surrogate receptor templates offered novel extensions of fragment-based drug design that were applicable to multisubunit ion channels.
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Papers by Valery Fokin