Ni-electrocatalytic Csp3-Csp3 doubly decarboxylative coupling.

Cross-coupling between two similar or identical functional groups to form a new C-C bond is a powerful tool to rapidly assemble complex molecules from readily available building units, as seen with olefin cross-metathesis or various types of cross-electrophile coupling1,2. The Kolbe electrolysis involves the oxidative electrochemical decarboxylation of alkyl carboxylic acids to their corresponding radical species followed by recombination to generate a new C-C bond3-12. As one of the oldest known Csp3-Csp3 bond-forming reactions, it holds incredible promise for organic synthesis, yet its use has been almost non-existent. From the perspective of synthesis design, this transformation could allow one to agnostically execute syntheses without regard to polarity or neighbouring functionality just by coupling ubiquitous carboxylates13. In practice, this promise is undermined by the strongly oxidative electrolytic protocol used traditionally since the nineteenth century5, thereby severely limiting its scope. Here, we show how a mildly reductive Ni-electrocatalytic system can couple two different carboxylates by means of in situ generated redox-active esters, termed doubly decarboxylative cross-coupling. This operationally simple method can be used to heterocouple primary, secondary and even certain tertiary redox-active esters, thereby opening up a powerful new approach for synthesis. The reaction, which cannot be mimicked using stoichiometric metal reductants or photochemical conditions, tolerates a range of functional groups, is scalable and is used for the synthesis of 32 known compounds, reducing overall step counts by 73%.

Versatile Copper-Catalyzed γ-C(sp3)-H Lactonization of Aliphatic Acids.

Site-selective C(sp3)-H oxidation is of great importance in organic synthesis and drug discovery. γ-C(sp3)-H lactonization of free carboxylic acids provides the most straightforward means to prepare biologically important lactone scaffolds from abundant and inexpensive carboxylic acids; however, a versatile catalyst for this transformation with a broad substrate scope remains elusive. Herein, we report a simple yet broadly applicable and scalable γ-lactonization reaction of free aliphatic acids enabled by a copper catalyst in combination with inexpensive Selectfluor as the oxidant. This lactonization reaction exhibits compatibility with tertiary, benzylic, allylic, methylene, and primary γ-C-H bonds, affording access to a wide range of structurally diverse lactones such as spiro, fused, and bridged lactones. Notably, exclusive γ-methylene C-H lactonization of cycloalkane carboxylic acids and cycloalkane acetic acids was observed, giving either fused or bridged γ-lactones that are difficult to access by other methods. δ-C-H lactonization was only favored in the presence of tertiary δ-C-H bonds. The synthetic utility of this methodology was demonstrated by the late-stage functionalization of amino acids, drug molecules, and natural products, as well as a two-step total synthesis of (iso)mintlactones (the shortest synthesis reported to date).

Exploration of macrocyclic peptide binders to the extracellular CRD domain of human receptor tyrosine kinase-like orphan receptor 1 (ROR1).

Elevated levels of receptor tyrosine kinase-like orphan receptor 1 (RORl) expression are observed in multiple hematological and solid tumors, but not in most of the healthy adult tissues, identifying ROR1 as an attractive target for tumor-specific therapy. Herein we will describe the discovery of macrocyclic peptides as binders of the extracellular Cysteine-Rich Domain (CRD) of human ROR1 via mRNA in vitro selection technology using the PDPS platform, followed by exploration of sidechain SAR of parent macrocycle peptides, fluorescently labeled analogs, and a Peptide Drug Conjugate (PDC). The parent macrocyclic peptides represented by Compound 1 and Compound 14 displayed nanomolar cell-based binding to ROR1 and relatively good internalization in 786-O and MDA-MB-231 tumor cell lines. However, these peptides were not observed to induce apoptosis in Mia PaCa-2 cells, a model pancreatic tumor cell line with a relatively low level of cell surface expression of ROR1.

Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization.

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C-H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C-H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate-catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C-H cleavage step. For site-selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C-H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst-controlled enantioselective C-H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C-H activation reactions, setting the stage for broad industrial applications.

Synthesis of 1,3-Dienes via Ligand-Enabled Sequential Dehydrogenation of Aliphatic Acids.

1,3-Dienes are common scaffolds in biologically active natural products as well as building blocks for chemical synthesis. Developing efficient methods for the synthesis of diverse 1,3-dienes from simple starting materials is therefore highly desirable. Herein, we report a Pd(II)-catalyzed sequential dehydrogenation reaction of free aliphatic acids via ß-methylene C-H activation, which enables one-step synthesis of diverse E,E-1,3-dienes. Free aliphatic acids of varying complexities, including the antiasthmatic drug seratrodast, were found to be compatible with the reported protocol. Considering the high lability of 1,3-dienes and lack of protecting strategies, dehydrogenation of aliphatic acids to reveal 1,3-dienes at the late stage of synthesis offers an appealing strategy for the synthesis of complex molecules containing such motifs.

Ligand-accelerated non-directed C-H functionalization of arenes.

The directed activation of carbon-hydrogen bonds (C-H) is important in the development of synthetically useful reactions, owing to the proximity-induced reactivity and selectivity that is enabled by coordinating functional groups. Palladium-catalysed non-directed C-H activation could potentially enable further useful reactions, because it can reach more distant sites and be applied to substrates that do not contain appropriate directing groups; however, its development has faced substantial challenges associated with the lack of sufficiently active palladium catalysts. Currently used palladium catalysts are reactive only with electron-rich arenes, unless an excess of arene is used, which limits synthetic applications. Here we report a 2-pyridone ligand that binds to palladium and accelerates non-directed C-H functionalization with arene as the limiting reagent. This protocol is compatible with a broad range of aromatic substrates and we demonstrate direct functionalization of advanced synthetic intermediates, drug molecules and natural products that cannot be used in excessive quantities. We also developed C-H olefination and carboxylation protocols, demonstrating the applicability of our methodology to other transformations. The site selectivity in these transformations is governed by a combination of steric and electronic effects, with the pyridone ligand enhancing the influence of sterics on the selectivity, thus providing complementary selectivity to directed C-H functionalization.

Remote C-H Olefination of Heterocyclic Biaryls Enabled by Reversibly Bound Templates.

Remote C-H functionalization of heterocyclic biaryls will be of great importance in synthesis and medicinal chemistry. Through adjusting the geometric relationship of the directing atom and target C-H bonds, two new catalytic templates have been developed to enable the functionalization of the more hindered ortho-C-H bonds of heterobiaryls bearing directing heteroatom at the meta- or para-positions, affording unprecedented site-selectivity. The use of template chaperone also overcomes product inhibition and renders the directing templates catalytic. The utility of this protocol was demonstrated by olefination of heterocyclic biaryls with various substituents, overriding conventional steric and electronic effects. These ortho-C-H olefinated heterobiaryls are sterically hindered and can often be challenging to prepare through aryl-aryl coupling reactions.

One-Step Synthesis of ß-Alkylidene-γ-lactones via Ligand-Enabled ß,γ-Dehydrogenation of Aliphatic Acids.

Ligand-enabled Pd-catalyzed regioselective α,ß-dehydrogenation of carbonyl compounds via ß-methylene C-H activation has recently emerged as a promising transformation. Herein, we report the realization of ß,γ-dehydrogenation and subsequent vinyl C-H olefination reactions of free carboxylic acids, thus providing a unique method for the structural diversification of aliphatic acids containing α-quaternary centers through sequential functionalizations of two ß-C-H bonds and one γ-C-H bond. This tandem dehydrogenation-olefination-lactonization reaction offers a one-step preparation of ß-alkylidene-γ-lactones, which are often difficult to prepare through conventional methods, from inexpensive and abundant free aliphatic acids. A variety of free aliphatic acids, such as isosteviol and grandiflorolic acid natural products, and olefins are compatible with the reported protocol. The newly designed bidentate oxime ether-pyridone and morpholine-pyridone ligands are crucial for this tandem reaction to proceed. Notably, these ligands also enable preferential methylene C-H activation over the previously reported, competing process of methyl C-H bond olefination.

Ligand-Enabled C-H Hydroxylation with Aqueous H2O2 at Room Temperature.

With the large number of Pd(II)-catalyzed C-H activation reactions of native substrates developed in the past decade, the development of catalysts to enable the use of green oxidants under safe and practical conditions has become an increasingly important challenge. Notably, the compatibility of Pd(II) catalysts with sustainable aqueous H2O2 has been a long-standing challenge in catalysis including Wacker-type oxidations. We report herein a bifunctional bidentate carboxyl-pyridone (CarboxPyridone) ligand that enables room-temperature Pd-catalyzed C-H hydroxylation of a broad range of benzoic and phenylacetic acids with an industry-compatible oxidant, aqueous hydrogen peroxide (35% H2O2). The scalability of this methodology is demonstrated by a 1000 mmol scale reaction of ibuprofen (206 g) using only a 1 mol % Pd catalyst loading. The utility of this protocol is further illustrated through derivatization of the products and synthesis of polyfluorinated natural product coumestan and pterocarpene from phenol intermediates prepared using this methodology.

Ligand-Enabled ß-C(sp3 )-H Lactamization of Tosyl-Protected Aliphatic Amides Using a Practical Oxidant.

The development of C(sp3 )-H functionalization reactions that use common protecting groups and practical oxidants remains a significant challenge. Herein we report a monoprotected aminoethyl thioether (MPAThio) ligand-enabled ß-C(sp3 )-H lactamization of tosyl-protected aliphatic amides using tert-butyl hydrogen peroxide (TBHP) as the sole oxidant. This protocol features exceedingly mild reaction conditions, reliable scalability, and the use of practical oxidants and protecting groups. Further derivatization of the ß-lactam products enables the synthesis of a range of biologically important motifs including ß-amino acids, γ-amino alcohols, and azetidines.

Ligand Enabled Pd(II)-Catalyzed γ-C(sp3)-H Lactamization of Native Amides.

γ-Lactams form important structural cores of a range of medicinally relevant natural products and clinical drugs, principal examples being the new generation of immunomodulatory imide drugs (IMiDs) and the brivaracetam family. Compared to conventional multistep synthesis, an intramolecular γ-C-H amination of aliphatic amides would allow for the direct construction of valuable γ-lactam motifs from abundant amino acid precursors. Herein we report a novel 2-pyridone ligand enabled Pd(II)-catalyzed γ-C(sp3)-H lactamization of amino acid derived native amides, providing the convenient synthesis of γ-lactams, isoindolinones, and 2-imidazolidinones. C6-Substitution of the 2-pyridone ligand is crucial for the lactam formation. This protocol features the use of N-acyl amino acids, which serve as both the directing group and cyclization partner, practical and environmentally benign tert-butyl hydrogen peroxide (TBHP) as the sole bystanding oxidant, and a broad substrate scope. The utility of this protocol was demonstrated through the two-step syntheses of a lenalidomide analog and brivaracetam from readily available carboxylic acids and amino acids.

Iridium(I)-Catalyzed α-C(sp3)-H Alkylation of Saturated Azacycles.

Saturated azacycles are commonly encountered in bioactive compounds and approved therapeutic agents. The development of methods for functionalization of the α-methylene C-H bonds of these highly privileged building blocks is of great importance, especially in drug discovery. While much effort has been dedicated toward this goal by using a directed C-H activation approach, the development of directing groups that are both general as well as practical remains a significant challenge. Herein, the design and development of novel amidoxime directing groups is described for Ir(I)-catalyzed α-C(sp3)-H alkylation of saturated azacycles using readily available olefins as coupling partners. This protocol extends the scope of saturated azacycles to piperidines, azepane, and tetrahydroisoquinoline that are incompatible with our previously reported directing group. A variety of olefin coupling partners, including previously unreactive disubstituted terminal olefins and internal olefins, are compatible with this transformation. The selectivity for a branched α-C(sp3)-alkylation product is also observed for the first time when acrylate is used as the reaction partner. The development of practical, one-step installation and removal protocols further adds to the utility of amidoxime directing groups.

Site-Selective Functionalization of Methionine Residues via Photoredox Catalysis.

Bioconjugation technologies have revolutionized the practice of biology and medicine by allowing access to novel biomolecular scaffolds. New methods for residue-selective bioconjugation are highly sought to expand the toolbox for a variety of bioconjugation applications. Herein we report a site-selective methionine bioconjugation protocol that uses photoexcited lumiflavin to generate open-shell intermediates. This reduction-potential-gated strategy enables access to residues unavailable with traditional nucleophilicity-based conjugation methods. To demonstrate the versatility and robustness of this new protocol, we have modified various proteins and further utilized this functional handle to append diverse biological payloads.

Pd-Catalyzed γ-C(sp3)-H Fluorination of Free Amines.

The first example of free amine γ-C(sp3)-H fluorination is realized using 2-hydroxynicotinaldehyde as the transient directing group. A wide range of cyclohexyl and linear aliphatic amines could be fluorinated selectively at the γ-methyl and methylene positions. Electron withdrawing 3,5-disubstituted pyridone ligands were identified to facilitate this reaction. Computational studies suggest that the turnover determining step is likely the oxidative addition step for methylene fluorination, while it is likely the C-H activation step for methyl fluorination. The explicit participation of Ag results in a lower energetic span for methylene fluorination and a higher energetic span for methyl fluorination, which is consistent with the experimental observation that the addition of silver salt is desirable for methylene but not for methyl fluorination. Kinetic studies on methyl fluorination suggest that the substrate and PdL are involved in the rate-determining step, indicating that the C-H activation step may be partially rate-determining. Importantly, an energetically preferred pathway has identified an interesting pyridone-assisted bimetallic transition state for the oxidative addition step in methylene fluorination, thus uncovering a potential new role of the pyridone ligand.

Serine-Selective Bioconjugation.

This Communication reports the first general method for rapid, chemoselective, and modular functionalization of serine residues in native polypeptides, which uses a reagent platform based on the P(V) oxidation state. This redox-economical approach can be used to append nearly any kind of cargo onto serine, generating a stable, benign, and hydrophilic phosphorothioate linkage. The method tolerates all other known nucleophilic functional groups of naturally occurring proteinogenic amino acids. A variety of applications can be envisaged by this expansion of the toolbox of site-selective bioconjugation methods.

meta-Selective C-H Arylation of Fluoroarenes and Simple Arenes.

Fluorine is known to promote ortho-C-H metalation. Based upon this reactivity, we employed an activated norbornene that traps the ortho-palladation intermediate and is then relayed to the meta position, leading to meta-selective C-H arylation of fluoroarenes. Deuterium experiment suggests that this meta-arylation is initiated by ortho C-H activation and the catalytic cycle is terminated by C-2 protonation. A dual-ligand system is crucial for the observed high reactivity and site selectivity. Applying this approach to simple benzene or other arenes also affords arylation products with good yield and site selectivity.

meta C-H Arylation of Electron-Rich Arenes: Reversing the Conventional Site Selectivity.

Controlling site selectivity of C-H activation without using a directing group remains a significant challenge. While Pd(II) catalysts modulated by a mutually repulsive pyridine-type ligand have been shown to favor the relatively electron-rich carbon centers of arenes, reversing the selectivity to favor palladation at the relatively electron-deficient positions has not been possible. Herein we report the first catalytic system that effectively performs meta C-H arylation of a variety of alkoxy aromatics including 2,3-dihydrobenzofuran and chromane with exclusive meta site selectivity, thus reversing the conventional site selectivity governed by native electronic effects. The identification of an effective ligand and modified norbornene (NBE-CO2Me), as well as taking advantage of the statistics, are essential for achieving the exclusive meta selectivity.

Identification of substituted benzothiazole sulfones as potent and selective inhibitors of endothelial lipase.

A low level of high density lipoprotein (HDL) is an independent risk factor for cardiovascular disease. HDL reduces inflammation and plays a central role in reverse cholesterol transport, where cholesterol is removed from peripheral tissues and atherosclerotic plaque. One approach to increase plasma HDL is through inhibition of endothelial lipase (EL). EL hydrolyzes phospholipids in HDL resulting in reduction of plasma HDL. A series of benzothiazole sulfone amides was optimized for EL inhibition potency, lipase selectivity and improved pharmacokinetic profile leading to the identification of Compound 32. Compound 32 was evaluated in a mouse pharmacodynamic model and found to show no effect on HDL cholesterol level despite achieving targeted plasma exposure (Ctrough > 15 fold over mouse plasma EL IC50 over 4 days).

PdII -Catalyzed Enantioselective C(sp3 )-H Activation/Cross-Coupling Reactions of Free Carboxylic Acids.

PdII -catalyzed enantioselective C(sp3 )-H cross-coupling of free carboxylic acids with organoborons has been realized using either mono-protected amino acid (MPAA) ligands or mono-protected aminoethyl amine (MPAAM) ligands. A diverse range of aryl- and vinyl-boron reagents can be used as coupling partners to provide chiral carboxylic acids. This reaction provides an alternative approach to the enantioselective synthesis of cyclopropanecarboxylic acids and cyclobutanecarboxylic acids containing α-chiral tertiary and quaternary stereocenters. The utility of this reaction was further demonstrated by converting the carboxylic acid into cyclopropyl amine without loss of optical activity.

Overcoming the Limitations of γ- and δ-C-H Arylation of Amines through Ligand Development.

L,X-Type transient directing groups (TDGs) based on a reversible imine linkage have emerged as broadly useful tools for C-H activation of ketones and free amines. However, competitive binding interactions among multiple reaction components (TDG itself, substrate, and substrate-TDG adduct) with the palladium catalyst often lead to the formation of multiple unreactive complexes, rendering ligand development extremely challenging. Herein, we report the finding of versatile 2-pyridone ligands that addresses these problems and significantly improves the γ-methylene arylation of alkyl amines, extending the coupling partners to a wide range of medicinally important heteroaryl iodides and even previously unreactive heteroaryl bromides. The combination of an appropriate transient directing group and pyridone ligand has also enabled the δ-arylation of alkyl amines. Notably, our transient directing group design reveals the importance of matching the size of the Pd-chelation with different transient directing groups and the size of palladacycles generated from γ- and δ-C-H bonds: TDGs that coordinate with Pd(II) to form a six-membered chelate are selective toward γ-C-H bonds, whereas TDGs that coordinate with Pd(II) via a five-membered chelate tend to activate δ-C-H bonds. These findings provide an avenue for developing protecting group free and selective C-H functionalization using the transient directing group strategy.