Comparative Study of Batch and Continuous Flow Reactors in Selective Hydrogenation of Functional Groups in Organic Compounds: What Is More Effective?

Many research papers describe selective hydrogenation of functional groups, such as nitro groups, carbonyl groups, or unsaturated carbon bonds to obtain fine chemicals or precursors of pharmaceuticals. Quite often, the catalyst performance is investigated in batch or continuous flow reactors without finding advantages and disadvantages of this or that regime. At the same time, the transition from batch processes to continuous flow occurs on the industrial scale. However, the batch process can be preferable for some reactions, in spite of its drawbacks. This review article aims to identify all publications that consider selective hydrogenation of functional groups in organic compounds, both in batch and continuous flow reactors, at the same reaction conditions that allow making conclusions about the benefits of one of the regimes in a particular case.

Effect of Phosphorus Precursor, Reduction Temperature, and Support on the Catalytic Properties of Nickel Phosphide Catalysts in Continuous-Flow Reductive Amination of Ethyl Levulinate.

Levulinic acid and its esters (e.g., ethyl levulinate, EL) are platform chemicals derived from biomass feedstocks that can be converted to a variety of valuable compounds. Reductive amination of levulinates with primary amines and H2 over heterogeneous catalysts is an attractive method for the synthesis of N-alkyl-5-methyl-2-pyrrolidones, which are an environmentally friendly alternative to the common solvent N-methyl-2-pyrrolidone (NMP). In the present work, the catalytic properties of the different nickel phosphide catalysts supported on SiO2 and Al2O3 were studied in a reductive amination of EL with n-hexylamine to N-hexyl-5-methyl-2-pyrrolidone (HMP) in a flow reactor. The influence of the phosphorus precursor, reduction temperature, reactant ratio, and addition of acidic diluters on the catalyst performance was investigated. The Ni2P/SiO2 catalyst prepared using (NH4)2HPO4 and reduced at 600 °C provides the highest HMP yield, which reaches 98%. Although the presence of acid sites and a sufficient hydrogenating ability are important factors determining the pyrrolidone yield, the selectivity also depends on the specific features of EL adsorption on active catalytic sites.

Identification of potent inhibitors of the sortilin-progranulin interaction.

High-throughput screening methods have been used to identify two novel series of inhibitors that disrupt progranulin binding to sortilin. Exploration of structure-activity relationships (SAR) resulted in compounds with sufficient potency and physicochemical properties to enable co-crystallization with sortilin. These co-crystal structures supported observed SAR trends and provided guidance for additional avenues for designing compounds with additional interactions within the binding site.

Structural characterization of nonactive site, TrkA-selective kinase inhibitors.

Current therapies for chronic pain can have insufficient efficacy and lead to side effects, necessitating research of novel targets against pain. Although originally identified as an oncogene, Tropomyosin-related kinase A (TrkA) is linked to pain and elevated levels of NGF (the ligand for TrkA) are associated with chronic pain. Antibodies that block TrkA interaction with its ligand, NGF, are in clinical trials for pain relief. Here, we describe the identification of TrkA-specific inhibitors and the structural basis for their selectivity over other Trk family kinases. The X-ray structures reveal a binding site outside the kinase active site that uses residues from the kinase domain and the juxtamembrane region. Three modes of binding with the juxtamembrane region are characterized through a series of ligand-bound complexes. The structures indicate a critical pharmacophore on the compounds that leads to the distinct binding modes. The mode of interaction can allow TrkA selectivity over TrkB and TrkC or promiscuous, pan-Trk inhibition. This finding highlights the difficulty in characterizing the structure-activity relationship of a chemical series in the absence of structural information because of substantial differences in the interacting residues. These structures illustrate the flexibility of binding to sequences outside of-but adjacent to-the kinase domain of TrkA. This knowledge allows development of compounds with specificity for TrkA or the family of Trk proteins.

Two-Step One-Pot Reductive Amination of Furanic Aldehydes Using CuAlOx Catalyst in a Flow Reactor.

Aminomethylhydroxymethylfuran derivatives are well known compounds which are used in the pharmaceutical industry. Reductive amination of 5-hydroxymethylfurfural (HMF) derived from available non-edible lignocellulosic biomass is an attractive method for the synthesis of this class of compounds. In the present study, the synthesis of N-substituted 5-(hydroxymethyl)-2-furfuryl amines and 5-(acetoxymethyl)-2-furfuryl amines was performed by two-step process, which includes the condensation of furanic aldehydes (HMF and 5-acetoxymethylfurfural) with primary amines in methanol on the first step and the reduction of obtained imines with hydrogen in a flow reactor over CuAlOx catalyst derived from layered double hydroxide on the second step. This process does not require isolation and purification of intermediate imines and can be used to synthesize a number of aminomethylhydroxymethylfurans in good to excellent yield.

Cerastecins inhibit membrane lipooligosaccharide transport in drug-resistant Acinetobacter baumannii.

Carbapenem-resistant Acinetobacter baumannii infections have limited treatment options. Synthesis, transport and placement of lipopolysaccharide or lipooligosaccharide (LOS) in the outer membrane of Gram-negative bacteria are important for bacterial virulence and survival. Here we describe the cerastecins, inhibitors of the A. baumannii transporter MsbA, an LOS flippase. These molecules are potent and bactericidal against A. baumannii, including clinical carbapenem-resistant Acinetobacter baumannii isolates. Using cryo-electron microscopy and biochemical analysis, we show that the cerastecins adopt a serpentine configuration in the central vault of the MsbA dimer, stalling the enzyme and uncoupling ATP hydrolysis from substrate flipping. A derivative with optimized potency and pharmacokinetic properties showed efficacy in murine models of bloodstream or pulmonary A. baumannii infection. While resistance development is inevitable, targeting a clinically unexploited mechanism avoids existing antibiotic resistance mechanisms. Although clinical validation of LOS transport remains undetermined, the cerastecins may open a path to narrow-spectrum treatment modalities for important nosocomial infections.

Potent targeted activator of cell kill molecules eliminate cells expressing HIV-1.

Antiretroviral therapy inhibits HIV-1 replication but is not curative due to establishment of a persistent reservoir after virus integration into the host genome. Reservoir reduction is therefore an important HIV-1 cure strategy. Some HIV-1 nonnucleoside reverse transcriptase inhibitors induce HIV-1 selective cytotoxicity in vitro but require concentrations far exceeding approved dosages. Focusing on this secondary activity, we found bifunctional compounds with HIV-1-infected cell kill potency at clinically achievable concentrations. These targeted activator of cell kill (TACK) molecules bind the reverse transcriptase-p66 domain of monomeric Gag-Pol and act as allosteric modulators to accelerate dimerization, resulting in HIV-1+ cell death through premature intracellular viral protease activation. TACK molecules retain potent antiviral activity and selectively eliminate infected CD4+ T cells isolated from people living with HIV-1, supporting an immune-independent clearance strategy.

Discovery of a novel class of non-ATP site DFG-out state p38 inhibitors utilizing computationally assisted virtual fragment-based drug design (vFBDD).

Discovery of a new class of DFG-out p38α kinase inhibitors with no hinge interaction is described. A computationally assisted, virtual fragment-based drug design (vFBDD) platform was utilized to identify novel non-aromatic fragments which make productive hydrogen bond interactions with Arg 70 on the αC-helix. Molecules incorporating these fragments were found to be potent inhibitors of p38 kinase. X-ray co-crystal structures confirmed the predicted binding modes. A lead compound was identified as a potent (p38α IC(50)=22 nM) and highly selective (≥ 150-fold against 150 kinase panel) DFG-out p38 kinase inhibitor.

Discovery of an Anion-Dependent Farnesyltransferase Inhibitor from a Phenotypic Screen.

By employing a phenotypic screen, a set of compounds, exemplified by 1, were identified which potentiate the ability of histone deacetylase inhibitor vorinostat to reverse HIV latency. Proteome enrichment followed by quantitative mass spectrometric analysis employing a modified analogue of 1 as affinity bait identified farnesyl transferase (FTase) as the primary interacting protein in cell lysates. This ligand-FTase binding interaction was confirmed via X-ray crystallography and temperature dependent fluorescence studies, despite 1 lacking structural and binding similarity to known FTase inhibitors. Although multiple lines of evidence established the binding interaction, these ligands exhibited minimal inhibitory activity in a cell-free biochemical FTase inhibition assay. Subsequent modification of the biochemical assay by increasing anion concentration demonstrated FTase inhibitory activity in this novel class. We propose 1 binds together with the anion in the active site to inhibit farnesyl transferase. Implications for phenotypic screening deconvolution and HIV reactivation are discussed.

Analysis of imatinib and sorafenib binding to p38alpha compared with c-Abl and b-Raf provides structural insights for understanding the selectivity of inhibitors targeting the DFG-out form of protein kinases.

Protein kinases c-Abl, b-Raf, and p38alpha are recognized as important targets for therapeutic intervention. c-Abl and b-Raf are major targets of marketed oncology drugs Imatinib (Gleevec) and Sorafenib (Nexavar), respectively, and BIRB-796 is a p38alpha inhibitor that reached Phase II clinical trials. A shared feature of these drugs is the fact that they bind to the DFG-out forms of their kinase targets. Although the discovery of this class of kinase inhibitors has increased the level of emphasis on the design of DFG-out inhibitors, the structural determinants for their binding and stabilization of the DFG-out conformation remain unclear. To improve our understanding of these determinants, we determined cocrystal structures of Imatinib and Sorafenib with p38alpha. We also conducted a detailed analysis of Imatinib and Sorafenib binding to p38alpha in comparison with BIRB-796, including binding kinetics, binding interactions, the solvent accessible surface area (SASA) of the ligands, and stabilization of key structural elements of the protein upon ligand binding. Our results yield an improved understanding of the structural requirements for stabilizing the DFG-out form and a rationale for understanding the genesis of ligand selectivity among DFG-out inhibitors of protein kinases.

The rational design of a novel potent analogue of the 5'-AMP-activated protein kinase inhibitor compound C with improved selectivity and cellular activity.

We have designed and synthesized analogues of compound C, a non-specific inhibitor of 5'-AMP-activated protein kinase (AMPK), using a computational fragment-based drug design (FBDD) approach. Synthesizing only twenty-seven analogues yielded a compound that was equipotent to compound C in the inhibition of the human AMPK (hAMPK) α2 subunit in the heterotrimeric complex in vitro, exhibited significantly improved selectivity against a subset of relevant kinases, and demonstrated enhanced cellular inhibition of AMPK.

Discovery and SAR of a series of 4,6-diamino-1,3,5-triazin-2-ol as novel non-nucleoside reverse transcriptase inhibitors of HIV-1.

The discovery and SAR study of a series of 4,6-diamino-1,3,5-triazin-2-ol compounds as novel HIV-1 non-nucleoside reverse transcriptase inhibitors (NNRTIs) are reported. The lead compounds in this series showed excellent activity against wild-type and drug-resistant RT enzymes and viral strains. In addition, compounds from this series demonstrated favorable pharmacokinetic profile in rat. A preliminary modeling study was conducted to understand the binding mode of this series of compounds.

Structure-based design of substituted biphenyl ethylene ethers as ligands binding in the hydrophobic pocket of gp41 and blocking the helical bundle formation.

A series of substituted biphenyl ethylene ether compounds has been designed to target the gp41N-trimer in order to inhibit formation of the six-helical bundle that represents the end state of gp41-mediated viral fusion. A size exclusion HPLC based helical bundle formation (HBF) assay was developed to evaluate in vitro inhibitory affinity of the inhibitors. The most potent compound 1 had an IC(50) of 31microM. The binding of compound 1 to the proposed hydrophobic pocket of gp41 was further validated by site-directed peptide mutagenesis experiments.

Hepatitis C virus NS3 protease requires its NS4A cofactor peptide for optimal binding of a boronic acid inhibitor as shown by NMR.

NMR spectroscopy was used to characterize the hepatitis C virus (HCV) NS3 protease in a complex with the 24 residue peptide cofactor from NS4A and a boronic acid inhibitor, Ac-Asp-Glu-Val-Val-Pro-boroAlg-OH. Secondary-structure information, NOE constraints between protease and cofactor, and hydrogen-deuterium exchange rates revealed that the cofactor was an integral strand in the N-terminal beta-sheet of the complex as observed in X-ray crystal structures. Based upon chemical-shift perturbations, inhibitor-protein NOEs, and the protonation state of the catalytic histidine, the boronic acid inhibitor was bound in the substrate binding site as a transition state mimic. In the absence of cofactor, the inhibitor had a lower affinity for the protease. Although the inhibitor binds in the same location, differences were observed at the catalytic site of the protease.

Synthesis and pharmacological evaluation of N-(3-(1H-indol-4-yl)-5-(2-methoxyisonicotinoyl)phenyl)methanesulfonamide (LP-261), a potent antimitotic agent.

The synthesis and optimization of a series of orally bioavailable 1-(1H-indol-4-yl)-3,5-disubstituted benzene analogues as antimitotic agents are described. A functionalized dibromobenzene intermediate was used as a key scaffold, which when modified by sequential Suzuki coupling and Buchwald-Hartwig amination provided a flexible entry to 1,3,5-trisubstituted phenyl compounds. A 1H-indol-4-yl moiety at the 1-position was determined to be a critical feature for optimal potency. The compounds have been shown to induce cell cycle arrest at the G2/M phase and demonstrate efficacy in both cell viability and cell proliferation assays. The primary site of action for these agents is revealed by their colchicine competitive inhibition of tubulin polymerization, and a computational model has been developed for the association of these compounds to tubulin. An optimized lead LP-261 significantly inhibits growth of a human non-small-cell lung tumor (NCI-H522) in a mouse xenograft model.

Mutagenesis of p38alpha MAP kinase establishes key roles of Phe169 in function and structural dynamics and reveals a novel DFG-OUT state.

In order to study the role of Phe169 in p38alpha MAP kinase structure and function, wild-type p38alpha and five p38alpha DFG motif mutants were examined in vitro for phosphorylation by MKK6, kinase activity toward ATF2 substrate, thermal stability, and X-ray crystal structure. All six p38alpha variants were efficiently phosphorylated by MKK6. However, only one activated p38alpha mutant (F169Y) possessed measurable kinase activity (1% compared to wild-type). The loss of kinase activity among the DFG mutants may result from an inability to correctly position Asp168 in the activated form of p38alpha. Two mutations significantly increased the thermal stability of p38alpha (F169A DeltaTm = 1.3 degrees C and D168G DeltaTm = 3.8 degrees C), and two mutations significantly decreased the stability of p38alpha (F169R DeltaTm = -3.2 degrees C and F169G DeltaTm = -4.7 degrees C). Interestingly, X-ray crystal structures of two thermally destabilized p38alpha-F169R and p38alpha-F169G mutants revealed a DFG-OUT conformation in the absence of an inhibitor molecule. This DFG-OUT conformation, termed alpha-DFG-OUT, is different from the ones previously identified in p38alpha crystal structures with bound inhibitors and postulated from high-temperature molecular dynamics simulations. Taken together, these results indicate that Phe169 is optimized for p38alpha functional activity and structural dynamics, rather than for structural stability. The alpha-DFG-OUT conformation observed for p38alpha-F169R and p38alpha-F169G may represent a naturally occurring intermediate state of p38alpha that provides access for binding of allosteric inhibitors. A model of the local forces driving the DFG IN-OUT transition in p38alpha is proposed.

Two classes of p38alpha MAP kinase inhibitors having a common diphenylether core but exhibiting divergent binding modes.

Two new classes of diphenylether inhibitors of p38alpha MAP kinase are described. Both chemical classes are based on a common diphenylether core that is identified by simulated fragment annealing as one of the most favored chemotypes within a prominent hydrophobic pocket of the p38alpha ATP-binding site. In the fully elaborated molecules, the diphenylether moiety acts as an anchor occupying the deep pocket, while polar extensions make specific interactions with either the adenine binding site or the phosphate binding site of ATP. The synthesis, crystallographic analysis, and biological activity of these p38alpha inhibitors are discussed.

A Biacore biosensor method for detailed kinetic binding analysis of small molecule inhibitors of p38alpha mitogen-activated protein kinase.

Protein kinases are emerging as one of the most intensely studied classes of enzymes as their central roles in physiologically and clinically important cellular signaling events become more clearly understood. We report here the development of a real-time, label-free method to study protein kinase inhibitor binding kinetics using surface plasmon resonance-based biomolecular interaction analysis (Biacore). Utilizing p38alpha mitogen-activated protein kinase as a model system, we studied the binding properties of two known small molecule p38alpha inhibitors (SB-203580 and SKF-86002). Direct coupling of p38alpha to the biosensor surface in the presence of a reversible structure-stabilizing ligand (SB-203580) consistently produced greater than 90% active protein on the biosensor surface. The dissociation and kinetic constants derived using this Biacore method are in excellent agreement with values determined by other methods. Additionally, we extend the method to study the thermodynamics of small molecule binding to p38alpha and derive a detailed thermodynamic reaction pathway for SB-203580. The Biacore method reported here provides an efficient way to directly and reproducibly examine dissociation constants, kinetics, and thermodynamics for small molecules binding to p38alpha and possibly other protein kinases. Immobilization in the presence of a stabilizing ligand may further represent a broadly applicable paradigm for creation of highly active biosensor surfaces.

Improved expression, purification, and crystallization of p38alpha MAP kinase.

p38alpha mitogen-activated protein (MAP) kinase is widely expressed in many mammalian tissues and is activated as a part of signal transduction cascades that respond to inflammatory stimuli. The activation of p38 is known to trigger various biological effects, including cell death, differentiation, and proliferation. The central role played by p38alpha in cellular signaling events, including those that control a wide range of inflammatory and autoimmune diseases, makes it an attractive drug target. To develop optimized small molecule therapeutics targeting p38alpha, different techniques must be employed for the detailed biochemical, biophysical, and structural characterization of the interactions of p38alpha with lead compounds. These methods typically require large quantities of highly purified p38alpha protein. We describe here an improved expression and purification method for recombinant p38alpha production that reproducibly yields over 70 mg of highly purified protein per liter of shake flask bacterial culture. This yield is significantly higher than that previously reported for p38alpha production in Escherichia coli. We achieved a significant increase in soluble p38alpha protein expression by using the genetically modified E. coli strain BL21 DE3 Rosetta, which is optimized for expression of eukaryotic proteins with codons rarely used in E. coli. The p38alpha protein was purified to near homogeneity using a simple two-step procedure including nickel-chelating Sepharose chromatography followed by anion-exchange chromatography using MonoQ resin. Purified p38alpha was characterized using the standard commercially available small molecule inhibitor SB-203580. The binding association and dissociation rate constants determined by Biacore are in excellent agreement with previously reported values. The purified p38alpha protein was efficiently activated by MKK6 kinase to yield phosphorylated p38alpha. Purified p38alpha protein was also successfully crystallized, producing crystals diffracting to 1.9 angstroms, exceeding the highest resolution for p38alpha reported in the Protein DataBank. The simplicity and efficiency of this approach should prove useful for many laboratories that are interested in production of p38alpha for biochemical and biophysical studies and structure-based drug design.