Electrochemical Nickel-Catalyzed C(sp3)-C(sp3) Cross-Coupling of Alkyl Halides with Alkyl Tosylates.

Formation of new C(sp3)-C(sp3) bonds is a powerful synthetic tool to increase molecular diversity, which is highly sought after in medicinal chemistry. Traditional generation of carbon nucleophiles and more modern cross-electrophile-coupling methods typically lack sufficient selectivity when cross-coupling of analogous C(sp3)-containing reactants is attempted. Herein, we present a nickel-catalyzed, electrochemically driven method for the coupling of alkyl bromides with alkyl tosylates. Selective cross-coupling transformations were achieved even between C(sp3)-secondary bromides and tosylates. Key to achieve high selectivity was the combination of the tosylates with sodium bromide as the supporting electrolyte, gradually generating small amounts of the more reactive bromide by substitution and ensuring that one of the reaction partners in the nickel-catalyzed electroreductive process is maintained in excess during a large part of the process. The method has been demonstrated for a wide range of substrates (>30 compounds) in moderate to good yields. Further expanding the scope of electroorganic synthesis to C(sp3)-C(sp3) cross-coupling reactions is anticipated to facilitate the switch to green organic synthesis and encourage future innovative electrochemical transformations.

Self-Optimization of Continuous Flow Electrochemical Synthesis Using Fourier Transform Infrared Spectroscopy and Gas Chromatography.

A continuous-flow electrochemical synthesis platform has been developed to enable self-optimization of reaction conditions of organic electrochemical reactions using attenuated total reflection Fourier transform infrared spectroscopy (ATR FT-IR) and gas chromatography (GC) as online real-time monitoring techniques. We have overcome the challenges in using ATR FT-IR as the downstream analytical methods imposed when a large amount of hydrogen gas is produced from the counter electrode by designing two types of gas-liquid separators (GLS) for analysis of the product mixture flowing from the electrochemical reactor. In particular, we report an integrated GLS with an ATR FT-IR probe at the reactor outlet to give a facile and low-cost solution to determining the concentrations of products in gas-liquid two-phase flow. This approach provides a reliable method for quantifying low-volatile analytes, which can be problematic to be monitored by GC. Two electrochemical reactions the methoxylation of 1-formylpyrrolidine and the oxidation of 3-bromobenzyl alcohol were investigated to demonstrate that the optimal conditions can be located within the pre-defined multi-dimensional reaction parameter spaces without intervention of the operator by using the stable noisy optimization by branch and FIT (SNOBFIT) algorithm.

Rapid Optimization of Photoredox Reactions for Continuous-Flow Systems Using Microscale Batch Technology.

Photoredox catalysis has emerged as a powerful and versatile platform for the synthesis of complex molecules. While photocatalysis is already broadly used in small-scale batch chemistry across the pharmaceutical sector, recent efforts have focused on performing these transformations in process chemistry due to the inherent challenges of batch photocatalysis on scale. However, translating optimized batch conditions to flow setups is challenging, and a general approach that is rapid, convenient, and inexpensive remains largely elusive. Herein, we report the development of a new approach that uses a microscale high-throughput experimentation (HTE) platform to identify optimal reaction conditions that can be directly translated to flow systems. A key design point is to simulate the flow-vessel pathway within a microscale reaction plate, which enables the rapid identification of optimal flow reaction conditions using only a small number of simultaneous experiments. This approach has been validated against a range of widely used photoredox reactions and, importantly, was found to translate accurately to several commercial flow reactors. We expect that the generality and operational efficiency of this new HTE approach to photocatalysis will allow rapid identification of numerous flow protocols for scale.

Synthesis of Enantiopure Unnatural Amino Acids by Metallaphotoredox Catalysis.

We describe herein a two-step process for the conversion of serine to a wide array of optically pure unnatural amino acids. This method utilizes a photocatalytic cross-electrophile coupling between a bromoalkyl intermediate and a diverse set of aryl halides to produce artificial analogues of phenylalanine, tryptophan, and histidine. The reaction is tolerant of a broad range of functionalities and can be leveraged toward the scalable synthesis of valuable pharmaceutical scaffolds via flow technology.

Photochemical benzylic bromination in continuous flow using BrCCl3 and its application to telescoped p-methoxybenzyl protection.

BrCCl3 represents a rarely used benzylic brominating reagent with complementary reactivity to other reagents. Its reactivity has been revisited in continuous flow, revealing compatibility with electron-rich aromatic substrates. This has brought about the development of a p-methoxybenzyl bromide generator for PMB protection, which was successfully demonstrated on a pharmaceutically relevant intermediate on 11 g scale, giving 91% yield and a PMB-Br space-time-yield of 1.27 kg L-1 h-1.

Metallaphotoredox-Catalyzed Cross-Electrophile Csp3-Csp3 Coupling of Aliphatic Bromides.

A strategy for the installation of small alkyl fragments onto pharmaceutically relevant aliphatic structures has been established via metallaphotoredox catalysis. Herein, we report that tris(trimethylsilyl)silanol can be employed as an effective halogen abstraction reagent that, in combination with photoredox and nickel catalysis, allows a generic approach to Csp3-Csp3 cross-electrophile coupling. In this study, we demonstrate that a variety of aliphatic drug-like groups can be successfully coupled with a number of commercially available small alkyl electrophiles, including methyl tosylate and strained cyclic alkyl bromides. Moreover, the union of two secondary aliphatic carbon centers, a long-standing challenge for organic molecule construction, has been accomplished with a wide array of structural formats. Last, this technology can be selectively merged with Csp2-Csp3 aryl-alkyl couplings to build drug-like systems in a highly modular fashion.

Discovery of chiral dihydropyridopyrimidinones as potent, selective and orally bioavailable inhibitors of AKT.

During the course of our research efforts to develop potent and selective AKT inhibitors, we discovered enatiomerically pure substituted dihydropyridopyrimidinones (DHP) as potent inhibitors of protein kinase B/AKT with excellent selectivity against ROCK2. A key challenge in this program was the poor physicochemical properties of the initial lead compound 5. Integration of structure-based drug design and physical properties-based design resulted in replacement of a highly hydrophobic poly fluorinated aryl ring by a simple trifluoromethyl that led to identification of compound 6 with much improved physicochemical properties. Subsequent SAR studies led to the synthesis of new pyran analog 7 with improved cell potency. Further optimization of pharmacokintetics properties by increasing permeability with appropriate fluorinated alkyl led to compound 8 as a potent, selective AKT inhibitors that blocks the phosphorylation of GSK3ß in vivo and had robust, dose and concentration dependent efficacy in the U87MG tumor xenograft model.

Electrochemical Deprotection of para-Methoxybenzyl Ethers in a Flow Electrolysis Cell.

Electrochemical deprotection of p-methoxybenzyl (PMB) ethers was performed in an undivided electrochemical flow reactor in MeOH solution, leading to the unmasked alcohol and p-methoxybenzaldehyde dimethyl acetal as a byproduct. The electrochemical method removes the need for chemical oxidants, and added electrolyte (BF4NEt4) can be recovered and reused. The method was applied to 17 substrates with high conversions in a single pass, yields up to 92%, and up to 7.5 g h-1 productivity. The PMB protecting group was also selectively removed in the presence of some other common alcohol protecting groups.

Discovery of the First α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Antagonist Dependent upon Transmembrane AMPA Receptor Regulatory Protein (TARP) γ-8.

Transmembrane AMPA receptor regulatory proteins (TARPs) are a family of scaffolding proteins that regulate AMPA receptor trafficking and function. TARP γ-8 is one member of this family and is highly expressed within the hippocampus relative to the cerebellum. A selective TARP γ-8-dependent AMPA receptor antagonist (TDAA) is an innovative approach to modulate AMPA receptors in specific brain regions to potentially increase the therapeutic index relative to known non-TARP-dependent AMPA antagonists. We describe here, for the first time, the discovery of a noncompetitive AMPA receptor antagonist that is dependent on the presence of TARP γ-8. Three major iteration cycles were employed to improve upon potency, CYP1A2-dependent challenges, and in vivo clearance. An optimized molecule, compound (-)-25 (LY3130481), was fully protective against pentylenetetrazole-induced convulsions in rats without the motor impairment associated with non-TARP-dependent AMPA receptor antagonists. Compound (-)-25 could be utilized to provide proof of concept for antiepileptic efficacy with reduced motor side effects in patients.

Light-Induced C-H Arylation of (Hetero)arenes by In Situ Generated Diazo Anhydrides.

Diazo anhydrides (Ar-N=N-O-N=N-Ar) have been known since 1896 but have rarely been used in synthesis. This communication describes the development of a photochemical catalyst-free C-H arylation methodology for the preparation of bi(hetero)aryls by the one-pot reaction of anilines with tert-butyl nitrite and (hetero)arenes under neutral conditions. The key step in this procedure is the in situ formation and subsequent photochemical (>300 nm) homolytic cleavage of a transient diazo anhydride intermediate. The generated aryl radical then efficiently reacts with a (hetero)arene to form the desired bi(hetero)aryls producing only nitrogen, water, and tert-butanol as byproducts. The scope of the reaction for several substituted anilines and (hetero)arenes was investigated. A continuous-flow protocol increasing selectivity and safety has been developed enabling the experimentally straightforward preparation of a variety of substituted bi(hetero)aryls within 45 min of reaction time.

A continuous-flow protocol for light-induced benzylic fluorinations.

A continuous-flow protocol for the light-induced fluorination of benzylic compounds is presented. The procedure uses Selectfluor as the fluorine source and xanthone as an inexpensive and commercially available photoorganocatalyst. The flow photoreactor is based on transparent fluorinated ethylene propylene tubing and a household compact fluorescent lamp. The combination of xanthone with black-light irradiation results in very efficient fluorination. Good to excellent isolated yields were obtained for a variety of substrates bearing different functional groups applying residence times below 30 min.

Continuous flow α-trifluoromethylation of ketones by metal-free visible light photoredox catalysis.

A continuous-flow, two-step procedure for the preparation of α-CF3-substituted carbonyl compounds has been developed. The carbonyl substrates were converted in situ into the corresponding silyl enol ethers, mixed with the CF3 radical source, and then irradiated with visible light using a flow reactor based on transparent tubing and a household compact fluorescent lamp. The continuous protocol uses Eosin Y as an inexpensive photoredox catalyst and requires only 20 min to complete the two reaction steps.

A scalable procedure for light-induced benzylic brominations in continuous flow.

A continuous-flow protocol for the bromination of benzylic compounds with N-bromosuccinimide (NBS) is presented. The radical reactions were activated with a readily available household compact fluorescent lamp (CFL) using a simple flow reactor design based on transparent fluorinated ethylene polymer (FEP) tubing. All of the reactions were carried out using acetonitrile as the solvent, thus avoiding hazardous chlorinated solvents such as CCl4. For each substrate, only 1.05 equiv of NBS was necessary to fully transform the benzylic starting material into the corresponding bromide. The general character of the procedure was demonstrated by brominating a diverse set of 19 substrates containing different functional groups. Good to excellent isolated yields were obtained in all cases. The novel flow protocol can be readily scaled to multigram quantities by operating the reactor for longer time periods (throughput 30 mmol h(-1)), which is not easily possible in batch photochemical reactors. The bromination protocol can also be performed with equal efficiency in a larger flow reactor utilizing a more powerful lamp. For the bromination of phenylacetone as a model, a productivity of 180 mmol h(-1) for the desired bromide was achieved.

Discovery of a potent, dual serotonin and norepinephrine reuptake inhibitor.

The objective of the described research effort was to identify a novel serotonin and norepinephrine reuptake inhibitor (SNRI) with improved norepinephrine transporter activity and acceptable metabolic stability and exhibiting minimal drug-drug interaction. We describe herein the discovery of a series of 3-substituted pyrrolidines, exemplified by compound 1. Compound 1 is a selective SNRI in vitro and in vivo, has favorable ADME properties, and retains inhibitory activity in the formalin model of pain behavior. Compound 1 thus represents a potential new probe to explore utility of SNRIs in central nervous system disorders, including chronic pain conditions.

Privileged structure based ligands for melanocortin-4 receptors--aliphatic piperazine derivatives.

Aliphatic carbocyclic replacement of the benzyl group of compound 1 yielded compounds with high affinity for the melanocortin-4 receptor (MC4R). Compounds with a cyclohexyl group showed a consistent high affinity, while different polar groups with less basicity were good replacements for the original diethyl amines. Substitution of the polar group found in these privileged structures with an aliphatic moiety produced compounds with high affinity for MC4R.

Privileged structure based ligands for melanocortin receptors--substituted benzylic piperazine derivatives.

Replacement of the aryl piperazine moiety in compound 1 with a variety of substituted benzylic piperazines (6) yields compounds that afford melanocortin receptor 4 (MCR4) activity. Analogs with ortho substitution on the aromatic ring afforded the highest affinity. Resolution of the stereocenter of the benzylic piperazine based privileged structure revealed that the R-enantiomer was more active.

Synthesis of C3 benzo[1,2-e:3,4-e':5,6-e"]tribenzo[l]acephenanthrylenes ("crushed fullerene" derivatives) by intramolecular palladium-catalyzed arylation.

The C60 polyarenes 4, 5, 18 a, and 18 b have been synthesized from truxene by triple alkylation at C5, C10, and C15 followed by a palladium-catalyzed intramolecular arylation. The synthesis of "crushed fullerene" C60H30 (2) is the most efficient reported to date and proceeds in 33% overall yield.

Synthesis and self-association of syn-5,10,15-trialkylated truxenes.

syn-Trialkylated truxenes have been synthesized by alkylation of the truxene trianion followed by base-catalyzed isomerization of the anti isomers. 1H NMR studies, including a determination of diffusion coefficients, as well as molecular mechanics calculations, demonstrate that these derivatives self-associate in solution through arene-arene interactions. The benzyl derivatives show the strongest associations, which are a result of both benzyl-benzyl and truxene-truxene interactions.