Abstract
Laboratory Automation and High-Throughput Chemistry
Formation of 4-Aminopyrimidines via the Trimerization of Nitriles Using Focused Microwave Heating
A series of substituted aliphatic nitriles are trimerized to their corresponding pyrimidine structures by Ian R. Baxendale and Steven V. Ley under solvent-free conditions in the presence of catalytic quantities of potassium tert-butoxide using a focused microwave reactor. Multigram quantities of the corresponding 4-aminopyrimidines are prepared in high yields and purity following a simple and scaleable protocol (J. Comb. Chem.
A Highly Automated, Polymer-Assisted Strategy for the Preparation of 2-Alkylthiobenzimidazoles and N,N′-Dialkylbenzimidazolin-2-ones
A multistep, polymer-assisted solution phase strategy for the highly automated synthesis of 2- alkylthiobenzimidazole and N,N′-dialkylbenzimidazolin-2-one libraries is reported by E. Vickerstaffe et al. The approach incorporates in-line purification techniques to afford library products directly with high purities. The preparation of a 96-member 2-alkylthiobenzimidazoline library and a 72-member N,N′-dialkylbenzimidazolin-2-one library is achieved, using a single Sophas top-filtration robotic synthesizer that requires minimal manual intervention. Building block rehearsals are performed in both cases to establish that the monomers selected are likely to afford library compounds in acceptable yields and purities (J. Comb. Chem.
Discovery and High-Throughput Screening of Heteroleptic Iridium Complexes for Photoinduced Hydrogen Production
The catalytic process of photoinduced hydrogen generation via the reduction of water is explored by S. Bernhard et al. The use of parallel synthetic techniques facilitates the synthesis of a32-member library of heteroleptic iridium complexes that is screened, using high-throughput photophysical techniques, to identify six potential photosensitizers for use in catalytic photoinduced hydrogen production. A Pd/Ni thin film hydrogen selective sensor allows rapid quantification of hydrogen produced via illumination of aqueous systems of the photosensitizer, tris(2,2′-dipyridyl)dichlorocobalt ([Co(bpy)3]Cl2), and triethanolamine (a sacrificial reductant) with ultrabright light emitting diodes. The use of an eight-well parallel photoreactor expedites the investigation of the hydrogen evolution process and facilitates mechanistic studies. All six compounds produce considerably more hydrogen than commonly utilized photosensitizers and have relative quantum efficiencies of hydrogen production up to 37 times greater than that of Ru(bpy)3
2+ (J. Am. Chem. Soc.
Homologation of Monoterpenoids into New Sesquiterpenoids via Tandem Isomerisation/Claisen Rearrangement Reactions with Three-Component Ruthenium Catalysts, and Ru(methallyl)2(COD) Revealed by High-Throughput Screening Techniques
A catalytic system based on a ruthenium source Ru3(CO)12, a bulky imidazolinium salt, and Cs2CO3 is reported by C. Bruneau and P. H. Dixneuf et al. to be very efficient for the transformation of a 1,7-diene into a γ,δ-unsaturated aldehyde via tandem isomerisation/Claisen reactions. 1,6-Dienes arising from the terpenoids menthone and myrtenal are selectively transformed into the corresponding unsaturated aldehydes with this catalyst system. High-throughput experiments are undertaken to evaluate other multicomponent catalysts for these tandem reactions. An unexpected catalyst is found to be Ru(methallyl)2(COD), which can operate without additional ligand or reagent (Adv. Synth. Catal.
The Multisubstrate Screening of Asymmetric Catalysts
T. Satyanarayana and H. B. Kagan review the principle of the one-pot multisubstrate screening. This methodology is successfully applied to various types of catalyzed enantioselective reactions: borane reduction of ketones, addition of organozinc on aldehydes, conjugate addition of diethylzinc on cycloalkenones or nitroalkenes, hydroformylation of olefins, hetero-Diels-Alder reaction on a-keto esters, enzymatic hydrolysis of glycerol-type monoesters, as well as hydrogenation of 2-aryl-substituted terminal alkenes and enamides. The one-pot multicatalyst screening methodology is also discussed briefly (Adv. Synth. Catal.
Focused Combinatorial Library Design Based on Structural Diversity, Druglikeness, and Binding Affinity Score
X. Luo et al. reported an initial attempt to establish a new approach for generating a target-focused library using the combination of the scores of structural diversity and binding affinity with improved druglikeness scoring functions. A software package, named LD1.0, is developed on the basis of the new approach. One test on a cyclooxygenase (COX)2-focused library successfully reproduces the structures that are experimentally studied as COX2-selective inhibitors. Another test is performed on a peroxisome proliferator-activated receptors' γ-focused library design, which not only reproduces the key fragments in the approved thiazolidinedione drugs, but also generates some new structures that are more active than the approved drugs or published ligands. Both these tests require approximately 15% of the running time of the ordinary molecular docking method (J. Comb. Chem.
Design and Synthesis of Protein Superfamily-Targeted Chemical Libraries for Lead Identification and Optimization
S. J. Shuttleworth et al. reviewed original literature dating back to 1992 that outlines applications of parallel synthesis and combinatorial chemistry to the synthesis of compound libraries focused toward specific superfamilies of molecular targets. Target families that have received significant literature coverage include kinases, proteases, nuclear hormone receptors, and cell surface receptors, notably GPCRs (Curr. Med. Chem.
Reproducibility Across Microwave Instruments: Preparation of a Set of 24 Compounds on a Multiwell Plate under Temperature-Controlled Conditions
The application of microwave heating technology to the preparation of compound libraries has attracted the interest of the medicinal chemistry community as a powerful tool for rapid analogue synthesis while carrying out drug discovery. The blending of microwave heating technology and parallel synthesis is a logical consequence of the significant rate enhancements and often higher product yields associated with the use of microwave ovens and the increase in productivity afforded by combinatorial chemistry techniques. One of the key issues when carrying out microwave chemistry is reproducibility when going from one type of instrument to another. J. Alcazar reports the comparison of results between single-mode and multimode microwave ovens, when applying these instruments to preparing sets of compounds. The comparison of the results obtained from both systems shows no major yield differences. Therefore, the results of these experiments validate the use of multiwell plates under microwave irradiation as an alternative to speed up parallel synthesis (J. Comb. Chem.
Microwave-Assisted One-Pot Regioselective Synthesis of 2-Alkyl-3, 4-Dihydro-3-Oxo-2H-1, 4-Benzoxazines
A protocol for regioselective one-pot synthesis of 2-alkyl-3, 4-dihydro-3-oxo-2H-1, 4-benzoxazines under controlled microwave heating is reported by Wei-Min Dai, Xuan Wang, and Chen Ma (Tetrahedron
For the acyclic intermediate possessing an electron-withdrawing group, microwave heating is necessary for the annulation reaction. Use of a base such as DBU is critical for achieving the regioselectivity. The desired products possessing alkyl, aryl, halogen, nitro, sulfonyl group(s), and ring structures can be conveniently and efficiently prepared in synthetically useful chemical yields, typically in the range of 60-80%.
One-Pot Synthesis of N-Allylthioureas Using Supported Reagents
A simple and efficient method for the synthesis of N-allylthioureas from allylic bromides in one-pot is shared by Tadashi Aoyama et al. (Tetrahedron Lett. 2005, 29 (46), pp. 4875-4878). A supported reagent system, KSCN/SiO2-RNH3OAc/Al2O3, is used. Allyl bromide reacts first with KSCN/SiO2 and the product, allyl isothiocyanate, reacts with RNH3OAc/Al2O3 to give the final product, N-allylthiourea, in good yield.
This method may be applicable for laboratory scale and combinatorial synthesis of N-allylthioureas.
Directing-Protecting Groups for Carbohydrates: Design, Conformational Study, Synthesis, and Application to Regioselective Functionalization
A novel concept of regioselective transformation of secondary hydroxyl groups in carbohydrates is presented by Nicolas Moitessier, Pablo Englebienne, and Yves Chapleur (Tetrahedron
Microfluidic Chip Technology and MicroReactor Technology
Acid-Catalyzed Synthesis and Deprotection of Dimethyl Acetals in a Miniaturized Electroosmotic Flow Reactor
Through incorporating a series of polymer-supported acid catalysts into a miniaturized electroosmotic flow reactor, C. Wiles, P. Watts, and S. J. Haswell demonstrate a clean and efficient technique for the protection of aldehydes as their respective dimethyl acetal. In addition, they also report the acid catalyzed deacetalisation of 11 dimethyl acetals to their respective aldehyde. In all cases, the compounds described are obtained in high yield (>95%) and excellent purity (>99%) without the need for further product purification (Tetrahedron
Carbohydrates as the Next Frontier in Pharmaceutical Research
The synthesis, isolation, purification, and structure elucidation of carbohydrates has been a challenging goal for decades. D. B. Werz and P. H. Seeberger reviewed new methods to gain access to these complex molecules, including a fully automated oligosaccharide synthesizer. Easily accessible glycosyl phosphates and glycosyl trichloroacetimidates prove to be a powerful class of glycosylating agents for this purpose. High-yielding coupling steps on the solid support rely on the use of an excess amount of building blocks in the presence of a stoichiometric amount of TMSOTf. Suitable protection and deprotection strategies lead to the assembly of linear and even branched oligosaccharides that can now be performed in a fully automated manner (Chem. Eur. J.
Development of Microstructured Reactors to Enable Organic Synthesis Rather than Subduing Chemistry
More than ever, microreaction technology is entering into chemical process engineering and chemistry, complementing existing technologies. The benefits of microstructured reactors such as enhanced mass and heat transfer, defined residence time setting, and known, highly regular flow profiles give process intensification following standard chemical protocols. These unique features enable process chemistry with novel features. This includes handling of instable intermediates, safe processing in the explosive regime, and setting reaction temperature at unusually high levels. In these ways, the engineering benefits change the chemistry. V. Hessel, P. Löb, and H. Löwe describe many examples of organic reactions in microstructured reactors to underline the potential of novel chemistry by using micro-structured reactors (Curr. Org. Chem.
A Microreactor for Microwave-Assisted Capillary (Continuous Flow) Organic Synthesis
A capillary-based flow system for conducting microscale organic synthesis with the aid of microwave irradiation is presented by E. Corner and M. G. Organ. The capillary internal diameter investigated ranges from 200 to 1200 μm, while the flow rate varies between 2 and 40 μL/min, which corresponds to the sample being irradiated approximately 4 min. Other parameters include reaction concentration and power setting of the microwave. Excellent conversion is observed in a variety of cross coupling and ring-closing metathesis reactions employing metal catalysts and in nucleophilic aromatic substitution and Wittig reactions that do not employ metals. It is shown that capillaries coated internally with thin films of Pd metal show tremendous rate accelerations, and the thin films themselves are capable of catalyzing Suzuki—Miyaura reactions with no exogenous catalyst added. It is demonstrated that reagents in separate syringes can be coinjected into the capillary, mixed, and reacted with none of the laminar flow problems that plague microreactor (lab-on-a-chip) technology. These experiments pave the way for use of microwave-assisted, flow capillary synthesis as a powerful and efficient means for replacing “one-at-a-time” microwave synthesis to provide libraries of compounds in a scale suitable for biological screening purposes (J. Am. Chem. Soc.
Heat Transfer in a Membrane-Assisted Bubbling Fluidized Bed with Immersed Horizontal Tubes
Salim A. R. K. Deshmukh et al. described investigations into the heat transfer characteristics of a membrane-assisted bubbling fluidized bed (Int. J. Chem. React. Eng. 2005, 3). Fluidized beds, including fine powders, are finding increased application in the chemical and petrochemical industry because of their excellent mass and heat transfer characteristics. Conversion and product selectivity can be significantly decreased in such fluidized bed chemical reactors with axial gas back-mixing. When membranes are inserted in fluidized beds, large improvements in conversion and selectivity can be achieved. The first way is by optimizing axial concentration profiles via distributive feeding of one of the reactants or selective withdrawal of one of the products. The second way is to decrease the effective axial dispersion via compartmentalization of the fluidized bed. Insertion of membrane bundles in a suitable configuration impedes bubble growth, thereby reducing reactant bypass via rapidly rising large bubbles.
Many times, cooling or heating tubes are also submerged in the fluidized bed. The effective heat transfer coefficient between the tube surface and the fluidized bed is an important parameter in the design of these fluidized beds. The authors focus on the influence of the presence of membrane and heat transfer tube bundles; the effect of gas addition and removal via membrane tubes; and the spatial distribution of the time averaged tube-to-bed heat transfer coefficient.
The authors show that in a membrane-assisted fluidized bed, the product selectivity and/or operational safety can be enhanced, but care must be taken to include the effect of gas added through the membranes on the required heat transfer surface area.
For the future, a direct measurement of bubble size (distribution) and frequency using noninvasive electrical capacitance tomography techniques or optical/capacitance probes would be interesting to support the reported findings. Additionally, measurements with varying heat capacities in the emulsion phase with the addition or removal of gas via the membranes are recommended, as well as experiments with different membrane tube diameters and tube pitches, and the effect of fluidization conditions on the possible abrasion of submerged membrane tubes.
Nonlinear PI/PID Controllers for Free-Radical Polymerization Reactors
An article in the Journal of Chemical Engineering of Japan
With this background, the author begins the study. The work is carried out on the temperature control of a free-radical PCSTR system subject to physical input constraint and output multiplicity. With the I/O linearization method as a base of controller syntheses, the state-space formulation of PI- and PID-type state feedback control frameworks are applied for nonlinear relative-degree-one as well as for relative-degree-two systems. With the aid of the equilibrium- based design, the author developed modified PI/PID control frameworks with two tuning constants. The tuning procedures of the controller are simple and straightforward.
High-Throughput Analytics
Quality Control in Combinatorial Chemistry: Determinations of Amounts and Comparison of the Purity of LC-MS-Purified Samples by NMR, LC-UV and CLND
A new method for determining absolute purities of 20 purified samples from a combinatorial library is described by H.-J. Roth et al., using DMSO sidebands [1J{13C-1H}] as an internal standard for quantification. The absolute amounts obtained are compared with the amounts of samples obtained by weighing. Calculated weights are determined by chemiluminescent nitrogen detection chromatography, and relative purities are obtained by LC-UV chromatography (J. Comb. Chem.
Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient Conditions
R.B. Cody and J.A. Laramée present a new ion source for a rapid and non-contact analysis of different materials at ambient pressure and at ground potential (Anal. Chem.
The basic DART source consists of a tube with a needle, perforated disc, and grid electrodes located in several chambers through which a heated gas such as nitrogen or helium flows. The electrically initiated discharge can produce ions, electrons, and vibronically excited species. Different ionization mechanisms occur depending upon the nature of the carrier gas, analyte concentration, and polarity of ions. The outpouring gas can by aimed directly toward the mass spectrometer orifice, or reflected off a sample surface and into the mass spectrometer. Tested chemical classes include chemical warfare agents, pharmaceuticals, drugs, explosives, synthetic organics, and other toxic chemicals on surfaces like concrete, human skin, currency, plastics, clothing, vegetable skin, and different commodities of human civilization. Excellent quality mass spectra are obtained within 30 s without sample preparation.
Solvent-Free MALDI-MS for the Analysis of Biological Samples via a Mini-Ball Mill Approach
The use of solvent during preparation of samples for matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS) is often a detrimental problem for labile or low-quantity samples. M.L. Deinzer and S. Trimpin present an efficient, low-sample load mini-ball mill (MBM) sample preparation procedure for the solvent-free MALDI analysis of peptides and proteins (J. Am. Soc. Mass Spectrom.
The general protocol is based on the mixing of the analyte and the matrix powders using a mini-bead-beater, glass or zirconia/silica beads, and plastic PCR-tubes. Discussing the methodology comprises important parameters like bead size and materials, matrices, evaporation procedures and salts, sample amounts, grinding times, and a semi-solvent-free method. The main cause for the increased performance of solvent-free MALDI-MS is attributed to the grain. Being of a size that permits better contact between sample and matrix, it is obtained more easily through external grinding and shaking than when solvent is employed. The final evaluation of this MALDI method highlights its advantages as an improved matrix-assistance, no implication of the molecular weight of the analyte, the solubility, and the compatibility between the polarities of analyte and matrix.
Bioautomation and Screening
The CellCard System: A Novel Approach to Assessing Compound Selectivity for Lead Prioritization of G Protein-Coupled Receptors
Advances in high-throughput screening technologies have led to the identification of many small molecule “hits” with activities toward the target of interest. As the screening technologies become faster and more robust, the rate at which the molecules are identified continues to increase. This evolution of high-throughput screening technologies has put a serious strain on laboratories involved with downstream profiling of these hits using cell-based assays. The CellCard System, by enabling multiple targets and/or cell lines to be assayed simultaneously within a single well, provides a platform on which selectivity screening can be quickly and robustly performed. O. Beske, S. Goldbard, and P. Turpin describe two case studies using the β-lactamase and β-galactosidase reporter gene systems to characterize G protein-coupled receptor agonist activity. Using these examples, the authors demonstrate how the implementation of this technology enables assay miniaturization without microfluidic devices, and how the inclusion of intra-well controls can provide a means of data quality assessment within each well (Comb. Chem. High Throughput Screen.
G-Protein Coupled Receptor Assays: To Measure Affinity or Efficacy that is the Question
Cell-based assays have always played an important role in the pharmaceutical industry, providing information about the functional effects of compounds. These functional assays have traditionally accompanied facile biochemical high-throughput screening (HTS) programs, being applied as secondary assays in the later stages of lead development. Advances in genetic engineering, automation compatible functional assay technologies, and the introduction of more sophisticated robotic systems have facilitated the application of cell-based assays to primary screening. However, despite some apparent success to move these assays into the routine toolbox for HTS, certain preconceptions and concerns about cell-based assays persist and the subject remains a topic of much debate. C. Williams and A. Sewing use examples from the screening portfolio at Pfizer to discuss the practical and theoretical considerations of employing cell-based assays in HTS with a focus on G-protein coupled receptors (Comb. Chem. High Throughput Screen.
Development of a Microelectronic Chip Array for High-Throughput Genotyping of Helicobacter Species and Screening for Antimicrobial Resistance
Bacterial identification and typing, as well as antimicrobial resistance, are essential elements in epidemiological investigations and infectious disease control. For this reason, a technique that can perform multiple determinations in the same platform would be optimal. Xing et al. (J. Biomol. Screen.
Conventional antimicrobial susceptibility testing takes a long time because Helicobacter grows slowly. Simultaneous identification of Helicobacter species and the determination of antibiotic resistance with this technology will drastically reduce the time required for analysis. The determination of resistant mutations can potentially be used for studying nosocomial infections when applied in the context of an epidemiological investigation. In addition the assay is highly amenable to the development of other DNA-based assays.
Automation for Early ADME Assays
The clinical development of new drugs has often been stopped because of unfavorable pharmacokinetic effects. Therefore, it is desirable to analyze ADME properties earlier in the process of drug discovery. The transport of drugs across the intestinal epithelial cell barrier and hepatic clearance are the major determinant factors of in vivo bioavailability. Furthermore, the ability of different CYP isoforms to metabolize multiple substrates is responsible for the large number of well-documented drug interactions associated with CYP inhibition. The ADME Department of Fournier Pharma (Bioforum Eur.
For metabolic stability screening, liver microsomes in 96-well plates are used. For this screening purpose, the Staccato automation platform is combined with liquid chromatography/tandem mass spectrometry for the analysis of samples. In addition, the automation of a 96-well format inhibition assay for the three main human P450 isoforms (CYP2C9, CYP2D6, CYP3A4) is established on the Staccato automation platform to investigate drug—drug interactions associated with CYP inhibition.
The development of automated high-throughput assays to analyze ADME properties in the very early stages of drug development can significantly reduce costs in this area.