Quinone-mediated hydrogen anode for non-aqueous reductive electrosynthesis.

Electrochemical synthesis can provide more sustainable routes to industrial chemicals1-3. Electrosynthetic oxidations may often be performed 'reagent-free', generating hydrogen (H2) derived from the substrate as the sole by-product at the counter electrode. Electrosynthetic reductions, however, require an external source of electrons. Sacrificial metal anodes are commonly used for small-scale applications4, but more sustainable options are needed at larger scale. Anodic water oxidation is an especially appealing option1,5,6, but many reductions require anhydrous, air-free reaction conditions. In such cases, H2 represents an ideal alternative, motivating the growing interest in the electrochemical hydrogen oxidation reaction (HOR) under non-aqueous conditions7-12. Here we report a mediated H2 anode that achieves indirect electrochemical oxidation of H2 by pairing thermal catalytic hydrogenation of an anthraquinone mediator with electrochemical oxidation of the anthrahydroquinone. This quinone-mediated H2 anode is used to support nickel-catalysed cross-electrophile coupling (XEC), a reaction class gaining widespread adoption in the pharmaceutical industry13-15. Initial validation of this method in small-scale batch reactions is followed by adaptation to a recirculating flow reactor that enables hectogram-scale synthesis of a pharmaceutical intermediate. The mediated H2 anode technology disclosed here offers a general strategy to support H2-driven electrosynthetic reductions.

Coupling-Condensation Strategy for the Convergent Synthesis of an Imidazole-Fused 2-Aminoquinoline NLRP3 Agonist.

The development of a convergent route to the NLRP3 (nucleotide-binding domain and leucine-rich repeat-containing protein 3) agonist BMS-986299 is reported. The synthesis relies on a key Miyaura borylation and a tandem Suzuki-Miyaura coupling between an iodoimidazole and an o-aminochloroarene, followed by acid-mediated cyclization to afford the aminoquinoline core. The subsequent Boc cleavage and regioselective acylation afford the target compound. Two routes to the iodoimidazole intermediate are presented, along with the synthesis of the o-aminochloroarene via Negishi coupling. The convergent six-step route leads to an 80% reduction in process mass intensity compared to the linear enabling synthesis.

Cesium Amide-Catalyzed Selective Deuteration of Benzylic C-H Bonds with D2 and Application for Tritiation of Pharmaceuticals.

Hydrogen isotope exchange (HIE) represents one of the most attractive labeling methods to synthesize deuterium- and tritium-labeled compounds. Catalytic HIE methods that enable site-selective C-H bond activation and exchange labeling with gaseous isotopes D2 and T2 are of vital importance, in particular for high-specific-activity tritiation of pharmaceuticals. As part of our interest in exploring s-block metals for catalytic transformations, we found CsN(SiMe3 )2 to be an efficient catalyst for selective HIE of benzylic C-H bonds with D2 gas. The reaction proceeds through a kinetic deprotonative equilibrium that establishes an exchange pathway between C-H bonds and D2 gas. By virtue of multiple C-H bonds activation and high activity (isotope enrichment up to 99 %), the simple cesium amide catalyst provided a very powerful and practically convenient labeling protocol for synthesis of highly deuterated compounds and high-specific-activity tritiation of pharmaceuticals.

Efficient Aliphatic Hydrogen-Isotope Exchange with Tritium Gas through the Merger of Photoredox and Hydrogenation Catalysts.

Employment of a combination of an organophotoredox catalyst with Wilkinson's catalyst (Rh(PPh3)3Cl) has given rise to an unprecedented method for hydrogen-isotope exchange (HIE) of aliphatic C(sp3)-H bonds of complex pharmaceuticals using T2 gas directly. Wilkinson's catalyst, commonly used for catalytic hydrogenations, was exploited as a precatalyst for activation of D2 or T2 and hydrogen atom transfer. In this combined methodology and mechanistic study, we demonstrate that by coupling photocatalysis with Rh catalysis, carbon-centered radicals generated via photoredox catalysis can be intercepted by Rh-hydride intermediates to deliver an effective hydrogen atom donor for hydrogen-isotope labeling of complex molecules in one step. By optimizing the ratio of the photocatalyst and Wilkinson's catalyst to balance the rate of the dual catalytic cycles, we can achieve efficient HIE and high recovery yield. This protocol was readily applied to direct HIE of C(sp3)-H bonds in 10 complex drug molecules, showing high isotope incorporation efficiency and exceptionally good functional group tolerance and demonstrating this approach as a practical and attractive labeling method for deuteration and tritiation.

Electrochemical Recycling of Adenosine Triphosphate in Biocatalytic Reaction Cascades.

Adenosine triphosphate (ATP) provides the driving force necessary for critical biological functions in all living organisms. In synthetic biocatalytic reactions, this cofactor is recycled in situ using high-energy stoichiometric reagents, an approach that generates waste and poses challenges with enzyme stability. On the other hand, an electrochemical recycling system would use electrons as a convenient source of energy. We report a method that uses electricity to turn over enzymes for ATP generation in a simplified cellular respiration mimic. The method is simple, robust, and scalable, as well as broadly applicable to complex enzymatic processes including a four-enzyme biocatalytic cascade in the synthesis of the antiviral molnupiravir.

Late-Stage Modification of Oligopeptides by Nickel-Catalyzed Stereoselective Radical Addition to Dehydroalanine.

Radical addition to dehydroalanine (Dha) represents an appealing, modular strategy to access non-canonical peptide analogues for drug discovery. Prior studies on radical addition to the Dha residue of peptides and proteins have demonstrated outstanding functional group compatibility, but the lack of stereoselectivity has limited the synthetic utility of this approach. Herein, we address this challenge by employing chiral nickel catalysts to control the stereoselectivity of radical addition to Dha on oligopeptides. The conditions accommodate a variety of primary and secondary electrophiles to introduce polyethylene glycol, biotin, halo-tag, and hydrophobic and hydrophilic side chains to the peptide. The reaction features catalyst control to largely override substrate-based control of stereochemical outcome for modification of short peptides. We anticipate that the discovery of chiral nickel complexes that confer catalyst control will allow rapid, late-stage modification of peptides featuring nonnatural sidechains.

Late-Stage Carbon Isotope Exchange of Aryl Nitriles through Ni-Catalyzed C-CN Bond Activation.

A facile one-pot strategy for 13CN and 14CN exchange with aryl, heteroaryl, and alkenyl nitriles using a Ni phosphine catalyst and BPh3 is described. This late-stage carbon isotope exchange (CIE) strategy employs labeled Zn(CN)2 to facilitate enrichment using the nonlabeled parent compound as the starting material, eliminating de novo synthesis for precursor development. A broad substrate scope encompassing multiple pharmaceuticals is disclosed, including the preparation of [14C] belzutifan to illustrate the exceptional functional group tolerance and utility of this labeling approach. Preliminary experimental and computational studies suggest the Lewis acid BPh3 is not critical for the oxidative addition step and instead plays a role in facilitating CN exchange on Ni. This CIE method dramatically reduces the synthetic steps and radioactive waste involved in preparation of 14C labeled tracers for clinical development.

Biocatalytic oxidation of alcohols using galactose oxidase and a manganese(III) activator for the synthesis of islatravir.

Galactose oxidase (GOase) is a Cu-dependent metalloenzyme that catalyzes the oxidation of alcohols to aldehydes. An evolved GOase variant was recently shown to catalyze a desymmetrizing oxidation as the first enzymatic step in the biocatalytic synthesis of islatravir. Horseradish peroxidase (HRP) is required to activate the GOase, introducing cost and protein burden to the process. Herein we describe that complexes of earth-abundant Mn(iii) (e.g. Mn(OAc)3) can be used at low loadings (2 mol%) as small molecule alternatives to HRP, providing similar yields and purity profiles. While an induction period is observed when using Mn(OAc)3 as the activator, employment of alternative Mn(iii) sources, such as Mn(acac)3 and K3[Mn(C2O4)3], eliminates the induction period and provides higher conversions to product. We demonstrate that use of the Mn(OAc)3 additive is also compatible with subsequent biocatalytic steps in the islatravir-forming cascade. Finally, to exhibit the wider utility of Mn(OAc)3, we show that Mn(OAc)3 functions as a suitable activator for several commercially available variants of GOase with a series of alcohol substrates.

Late-Stage 18 O Labeling of Primary Sulfonamides via a Degradation-Reconstruction Pathway.

A late-stage 18 O labeling approach of sulfonamides that employs the corresponding unlabeled molecule as the starting material was developed. Upon deamination of the sulfonamide, a sulfinate intermediate was isotopically enriched using eco-friendly reagents H2 18 O and 15 NH3 (aq) to afford a M+5 isotopologue of the parent compound. This degradation-reconstruction approach afforded isolated yields of up to 96 % for the stable isotope labeled (SIL) sulfonamides, and was compatible with multiple marketed therapeutics, including celecoxib, on a gram scale. The SIL products also exhibited no 18 O/16 O back exchange under extreme conditions, further validating the utility of this green strategy for drug labeling for both in vitro and in vivo use. This procedure was also adapted to include pharmaceutically relevant methyl sulfones by using 13 CH3 , affording M+5 isotopic enrichment, thereby illustrating the broad utility of this methodology.

Effects of Multiple Catalyst Deactivation Pathways and Continuous Ligand Recycling on the Kinetics of Pd-Catalyzed C-N Coupling Reactions.

Unusual Pd deactivation and inhibition pathways were observed in a C-N coupling system. Irreversible catalyst deactivation involved C-H insertion of Pd into BippyPhos leading to an off-cycle palladaphosphacyclobutene. Product inhibition led to deactivated Pd but released ligand in the process, allowing it to react with additional Pd precursor to re-enter the catalytic cycle. In situ recycling of the ligand allowed for an input L/Pd ratio of ≪1 with no impact on reaction kinetics.

C-H Arylation in the Formation of a Complex Pyrrolopyridine, the Commercial Synthesis of the Potent JAK2 Inhibitor, BMS-911543.

The development of an improved short and efficient commercial synthesis of the JAK2 inhibitor, a complex pyrrolopyridine, BMS-911543, is described. During the discovery and development of this synthesis, a Pd-catalyzed C-H functionalization was invented which enabled the rapid union of the key pyrrole and imidazole fragments. The synthesis of this complex, nitrogen-rich heterocycle was accomplished in only six steps (longest linear sequence) from readily available materials.

Enantioselective Synthesis of a γ-Secretase Modulator via Vinylogous Dynamic Kinetic Resolution.

Two efficient asymmetric routes to γ-secretase modulator BMS-932481, under investigation for Alzheimer's disease, have been developed. The key step for the first route involves a challenging enantioselective hydrogenation of an unfunctionalized trisubstituted alkene to establish the benzylic stereocenter, representing a very rare case of achieving high selectivity on a complex substrate. The second route demonstrates the first example of a vinylogous dynamic kinetic resolution (VDKR) ketone reduction, where the carbonyl and the racemizable stereocenter are not contiguous, but are conjugated through a pyrimidine ring. Not only did this transformation require both catalyst and substrate control to correctly establish the two stereocenters, but it also necessitated that the nonadjacent benzylic center of the ketone substrate be more acidic than that of the alcohol product to make the process dynamic. DFT computations aided the design of this novel VDKR pathway by reliably predicting the relative acidities of the intermediates involved.

Revisiting a Classic Transformation: A Lossen Rearrangement Initiated by Nitriles and "Pseudo-Catalytic" in Isocyanate.

The direct conversion of a hydroxamic acid to an amine has been accomplished in a single step in the synthesis of HIV drug candidate BMS-955176. This process utilizes catalytic base and proceeds under mild conditions (CH3CN, cat. DBU, 60 °C), without the need for strong electrophiles required for typical Lossen rearrangements, and can be applied to aliphatic and aromatic hydroxamic acids. Through investigation of the kinetics of this transformation, a mechanism was revealed involving a novel initiation pathway and a self-propagation cycle. The initiation pathway involves activation of hydroxamic acid by nitriles and subsequent Lossen rearrangement to generate the corresponding isocyanate. The isocyanate functions as a "pseudo-catalyst" for this system, leading to generation of product through a second Lossen rearrangement and regeneration of a new isocyanate molecule. Thorough mechanistic understanding allowed for this highly efficient process to be implemented on a 55 kg scale in 95.5% isolated yield.

A Mild, Functional Group Tolerant Addition of Organozinc Nucleophiles to N-Activated Quinolines and Isoquinolines.

An addition of organozinc nucleophiles to N-acyl activated quinolines and isoquinolines is described. Simple transmetalation with the corresponding Grignard reagents using ZnCl2 forms organozinc compounds which are functional group tolerant and stable to reactive acyl chloride reagents for extended periods. A wide variety of substrates which include reactive electron-withdrawing groups are well tolerated to form 2-substituted dihydroquinolines and dihydroisoquinolines. This methodology has been applied toward an improved synthetic route of uncialamycin and its analogs.

Synthesis of HIV-Maturation Inhibitor BMS-955176 from Betulin by an Enabling Oxidation Strategy.

A concise and scalable second generation synthesis of HIV maturation inhibitor BMS-955176 is described. The synthesis is framed by an oxidation strategy highlighted by a CuI mediated aerobic oxidation of betulin, a highly selective PIFA mediated dehydrogenation of an oxime, and a subsequent Lossen rearrangement which occurs through a unique reaction mechanism for the installation of the C17 amino functionality. The synthetic route proceeds in 7 steps with 47% overall yield and begins from the abundant and inexpensive natural product betulin.

Autoxidation Products of Betulonaldehyde.

Three major degradation products resulted from the exposure of betulonaldehyde (1) to air in solution at room temperature. From HRMS and NMR data, the products, which were isolated by preparative supercritical fluid chromatography (SFC), were identified as betulonic acid (2) and C-17 hydroperoxide epimers 3 (ß-OOH) and 4 (α-OOH). For 3 and 4, the H-18 multiplet pattern of the isolated products established the configuration at C-17.

High Bulk-Density Amorphous Dispersions to Enable Direct Compression of Reduced Tablet Size Amorphous Dosage Units.

Amorphous solid dispersions (ASDs) are an attractive option to improve the bioavailability of poorly water-soluble compounds. However, the material attributes of ASDs can present formulation and processability challenges, which are often mitigated by the addition of excipients albeit at the expense of tablet size. In this work, an ASD manufacturing train combining co-precipitation and thin film evaporation (TFE) was used to generate high bulk-density co-precipitated amorphous dispersion (cPAD). The cPAD/TFE material was directly compressed into tablets at amorphous solid dispersion loadings up to 89 wt%, representing a greater than 60% reduction in tablet size relative to formulated tablets containing spray dried intermediate (SDI). This high ASD loading was possible due to densification of the amorphous dispersion during drying by TFE. Pharmacokinetic performance of the TFE-isolated, co-precipitated dispersion was shown to be equivalent to an SDI formulation. These data highlight the downstream advantages of this novel ASD manufacturing pathway to facilitate reduced tablet size via high ASD loading in directly compressed tablets.

Reaction development and mechanistic study of a ruthenium catalyzed intramolecular asymmetric reductive amination en route to the dual Orexin inhibitor Suvorexant (MK-4305).

The first example of an intramolecular asymmetric reductive amination of a dialkyl ketone with an aliphatic amine has been developed for the synthesis of Suvorexant (MK-4305), a potent dual Orexin antagonist under development for the treatment of sleep disorders. This challenging transformation is mediated by a novel Ru-based transfer hydrogenation catalyst that provides the desired diazepane ring in 97% yield and 94.5% ee. Mechanistic studies have revealed that CO(2), produced as a necessary byproduct of this transfer hydrogenation reaction, has pronounced effects on the efficiency of the Ru catalyst, the form of the amine product, and the kinetics of the transformation. A simple kinetic model explains how product inhibition by CO(2) leads to overall first-order kinetics, but yields an apparent zero-order dependence on initial substrate concentration. The deleterious effects of CO(2) on reaction rates and product isolation can be overcome by purging CO(2) from the system. Moreover, the rate of ketone hydrogenation can be greatly accelerated by purging of CO(2) or trapping with nucleophilic secondary amines.

Unlocking the Potential of High-Throughput Experimentation for Electrochemistry with a Standardized Microscale Reactor.

Organic electrochemistry has emerged as an enabling and sustainable technology in modern organic synthesis. Despite the recent renaissance of electrosynthesis, the broad adoption of electrochemistry in the synthetic community, and especially in industrial settings, has been hindered by the lack of general, standardized platforms for high-throughput experimentation (HTE). Herein, we disclose the design of the HTe - Chem, a high-throughput microscale electrochemical reactor that is compatible with existing HTE infrastructure and enables the rapid evaluation of a broad array of electrochemical reaction parameters. Utilizing the HTe - Chem to accelerate reaction optimization, reaction discovery, and chemical library synthesis is illustrated using a suite of oxidative and reductive transformations under constant current, constant voltage, and electrophotochemical conditions.

Engineered Ribosyl-1-Kinase Enables Concise Synthesis of Molnupiravir, an Antiviral for COVID-19.

Molnupiravir (MK-4482) is an investigational antiviral agent that is under development for the treatment of COVID-19. Given the potential high demand and urgency for this compound, it was critical to develop a short and sustainable synthesis from simple raw materials that would minimize the time needed to manufacture and supply molnupiravir. The route reported here is enabled through the invention of a novel biocatalytic cascade featuring an engineered ribosyl-1-kinase and uridine phosphorylase. These engineered enzymes were deployed with a pyruvate-oxidase-enabled phosphate recycling strategy. Compared to the initial route, this synthesis of molnupiravir is 70% shorter and approximately 7-fold higher yielding. Looking forward, the biocatalytic approach to molnupiravir outlined here is anticipated to have broad applications for streamlining the synthesis of nucleosides in general.