The effect of ligands and lithium chloride on the rates of the palladium catalyzed coupling between organic triflates and arylstannanes was studied. The dependence of the rate on the ligand is similar to the one previously reported for the coupling of vinylstannanes, but in the present case triphenylarsine is shown to be superior to both triphenylphosphine and tri(2-furyl)phosphine. The effect of added chloride is complex and varies depending on solvent and ligand used. Ortho-substituted arylstannanes tend to transfer alkyl moieties to a substantial extent, and therefore rates and efficiencies of aryl vs alkyl transfer were quantitated. When ortho substituents that are potentially coordinating to tin are used, no rate acceleration in the alkyl transfer process was observed, which is in contrast with two recently reported studies that suggest nucleophilic assistance at tin to be important in the transmetalation step. An important side reaction in the coupling of poorly reactive vinyltriflates and most aryltriflates is the Pd-induced homocoupling of the stannane to form biaryls. The experimental factors that control this process were evaluated.

The erythropoietin-producing hepatocellular (Eph) family of receptor tyrosine kinases regulates a multitude of physiological and pathological processes. Despite the numerous possible research and therapeutic applications of agents capable of modulating Eph receptor function, no small molecule inhibitors targeting the extracellular domain of these receptors have been identified. We have performed a high throughput screen to search for small molecules that inhibit ligand binding to the extracellular domain of the EphA4 receptor. This yielded a 2,5-dimethylpyrrolyl benzoic acid derivative able to inhibit the interaction of EphA4 with a peptide ligand as well as the natural ephrin ligands. Evaluation of a series of analogs identified an isomer with similar inhibitory properties and other less potent compounds. The two isomeric compounds act as competitive inhibitors, suggesting that they target the high affinity ligandbinding pocket of EphA4 and inhibit ephrin-A5 binding to EphA4 with K i values of 7 and 9 M in enzyme-linked immunosorbent assays. Interestingly, despite the ability of each ephrin ligand to promiscuously bind many Eph receptors, the two compounds selectively target EphA4 and the closely related EphA2 receptor. The compounds also inhibit ephrin-induced phosphorylation of EphA4 and EphA2 in cells, without affecting cell viability or the phosphorylation of other receptor tyrosine kinases. Furthermore, the compounds inhibit EphA4-mediated growth cone collapse in retinal explants and EphA2-dependent retraction of the cell periphery in prostate cancer cells. These data demonstrate that the Eph receptor-ephrin interface can be targeted by inhibitory small molecules and suggest that the two compounds identified will be useful to discriminate the activities of EphA4 and EphA2 from those of other co-expressed Eph receptors that are activated by the same ephrin ligands. Furthermore, the newly identified inhibitors represent possible leads for the development of therapies to treat pathologies in which EphA4 and EphA2 are involved, including nerve injuries and cancer.

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In vivo, a drug molecule undergoes its first chemical transformation within the liver via CYP450-catalyzed oxidation. The chemical outcome of the first pass hepatic oxidation is key information to any drug development process. Electrochemistry can be used to simulate CYP450 oxidation, yet it is often confined to the analytical scale, hampering product isolation and full characterization. In an effort to replicate hepatic oxidations, while retaining high throughput at the preparative scale, microfluidic technology and electrochemistry are combined in this study by using a microfluidic electrochemical cell. Several commercial drugs were subjected to continuous-flow electrolysis. They were chosen for their various chemical reactivity: their metabolites in vivo are generated via aromatic hydroxylation, alkyl oxidation, glutathione conjugation, or sulfoxidation. It is demonstrated that such metabolites can be synthesized by flow electrolysis at the 10 to 100 mg scale, and the purified products are fully characterized. KEYWORDS: Drug metabolites, electrochemistry, microfluidic synthesis, continuous-flow oxidation T he first step toward elimination of xenobiotic compounds in vivo occurs predominantly through first pass hepatic oxidation. In the liver, the oxidation takes place at the iron center of the porphyrin that constitutes the catalytic site of the CYP450 family of enzymes. 1,2 The oxidized metabolites (Phase I) may undergo subsequent transformations (Phase II) including, among others, conjugation with glutathione (GSH). This is of particular importance in the design of new drug candidates since their toxicity levels are regulated by such metabolic processes. 3,4 Significant research effort has been devoted to develop synthetic methodologies to simulate the metabolism of drugs. These methodologies fall into four distinct categories: microsomal incubation, porphyrin-catalyzed chemical oxidation, Fenton-type reactions, and electrochemical oxidations. 5,6 Electrosynthetic transformations offer the advantage of atom economy since the oxidation takes place at the surface of an electrode, which provides the electron from a current source. 7 These transformations are thus heterogeneous reactions by definition. To achieve high conversion, the substrate solution must be conductive, and the ratio of electrode surface-tosolution volume ought to be as high as possible. While electrosynthesis has been extensively performed in batch mode, 8,9 continuous-flow technology provides increased electrode surface ratios by design. 9 Additionally, at the microfluidic scale the distance between the working electrode and the counter electrode is shortened. This can help overcome conversion limitations linked to solution resistivity, and even enable electrolyte-free reactions. 10 In the past few years, the study of drug metabolites has benefited from the emergence of in-line electrochemical/mass spectrometry (EC-MS) systems used to generate phase I and phase II metabolites. 13,14 Unfortunately, full structure elucidation is limi...

A series of synthetic transformations were successfully and safely scaled up to multigram quantities using focused microwave irradiation with a continuous flow reaction cell that was developed in-house and which can be easily adapted to commercially available instrumentation. The representative reactions that were investigated included aromatic nucleophilic substitution (SNAr), esterification, and the Suzuki cross-coupling reaction. In general, the product yields were equivalent to or greater than those run under conventional thermal heating conditions.