Photoredox-mediated hydroalkylation and hydroarylation of functionalized olefins for DNA-encoded library synthesis

Hydroazidation of trifluoromethyl alkenes with trimethylsilyl azide enabled by organic photoredox catalysis

Herein, we report an organic photoredox-catalyzed hydroazidation of trifluoromethyl alkenes with user-friendly trimethylsilyl azide (TMSN3), enabling a direct access to a broad range of valuable β-CF3-azides with exclusive selectivity under mild reaction conditions. The synthetic utility of this reaction was demonstrated by the late-stage modification of complex drug derivatives, scale-up of the reaction and diverse further derivatizations of the β-CF3-azide product.

Diverse Synthesis of C-Glycosides by Stereoselective Ni-Catalyzed Carboboration of Glycals

C-Glycosides are important structures that are common to natural products and pharmaceutical agents. Established methods for their synthesis involve the reaction of an activated anomeric carbon. In this study, we report a conceptually new approach that involves the stereoselective Ni-catalyzed carboboration of glycals. In these reactions, not only is a C-C bond formed at the anomeric carbon, but a synthetically useful C-B bond is also installed. Upon C-B oxidation, differentially protected C-glycosides to be formed. In addition, stereospecific manipulation of the C-B bond leads to diverse C-glycosides. Finally, we report the application of this method in the synthesis of established C-glycosides, such as C-glycosyl amino acids, as well as a strategy to make all possible diastereomers at C1 and C2.

Hydrophobic Surfactant-DNA Complex (Surf-DNA) Enables DNA-Encoded-Library-Compatible Decarboxylative Arylation under Anhydrous Conditions

DNA-encoded libraries (DELs) are a key technology for identifying small-molecule hits in both the pharmaceutical industry and academia, but their chemical diversity is largely limited to water-compatible reactions to aid in the solubility and integrity of encoding DNA tags. To broaden the DEL chemical space, we present a workflow utilizing DNA-cationic surfactant complexation that enables dissolution and reactions on-DNA in anhydrous organic solvents. We demonstrate its utility by developing DEL-compatible photoredox decarboxylative C(sp2)-C(sp3) coupling under water-free conditions. The workflow is optimized for the 96-well format necessary for large-scale DEL productions, and it enables screening and optimization of DEL-compatible reactions in organic solvents.

Photoinduced Triphenylphosphine and Iodide Salt Promoted Reductive Decarboxylative Coupling

The transient electron donor-acceptor (EDA) complex has been an emerging area in the photoinduced organic synthesis field, generating radicals without exogenous transition-metal or organic dye-based photoredox catalysts. The catalytic platform to form suitable photoactive EDA complexes for photochemical reduction reactions remains underdeveloped. Herein, a general photoinduced reductive alkylation via the EDA complex strategy is described. A simple yet multifunctional system, triphenylphosphine and iodide salt, promotes the photoinduced decarboxylative hydroalkylation, and reductive defluorinative decarboxylative alkylation of trifluoromethyl alkenes, to access trifluoromethyl alkanes and gem-difluoroalkenes. Moreover, decarboxylative hydroalkylation can be applied to more kinds of electron-deficient alkenes as a general Giese addition reaction.

Evolution of chemistry and selection technology for DNA-encoded library

DNA-encoded chemical library (DEL) links the power of amplifiable genetics and the non-self-replicating chemical phenotypes, generating a diverse chemical world. In analogy with the biological world, the DEL world can evolve by using a chemical central dogma, wherein DNA replicates using the PCR reactions to amplify the genetic codes, DNA sequencing transcripts the genetic information, and DNA-compatible synthesis translates into chemical phenotypes. Importantly, DNA-compatible synthesis is the key to expanding the DEL chemical space. Besides, the evolution-driven selection system pushes the chemicals to evolve under the selective pressure, i.e., desired selection strategies. In this perspective, we summarized recent advances in expanding DEL synthetic toolbox and panning strategies, which will shed light on the drug discovery harnessing in vitro evolution of chemicals via DEL.

Solvent-Controllable C-F Bond Activation for Masked Formylation of α-Trifluoromethyl Alkenes via Organo-Photoredox Catalysis

A solvent-controllable organo-photoredox-catalyzed C-F bond activation for masked formylation of α-trifluoromethyl alkenes with low-priced 1,3-dioxolane as masked formyl radical equivalent has been described. Consequently, a diversity of masked formylated gem-difluoroalkenes and monofluoroalkenes are constructed in moderate to high yields. This approach merits readily available starting materials, mild reaction conditions, and broad substrate scope. The feasibility of this approach has been highlighted by the one-pot masked formylation/hydrolysis sequence to form γ,γ-difluoroallylic aldehydes and late-stage modification of pharmaceutical and natural product derivatives.

One-Pot Synthesis of Cereblon Proteolysis Targeting Chimeras via Photoinduced C(sp2)-C(sp3) Cross Coupling and Amide Formation for Proteolysis Targeting Chimera Library Synthesis

In this study, a one-pot synthesis via photoinduced C(sp2)-C(sp3) coupling followed by amide formation to access proteolysis targeting chimeras (PROTACs) was developed. The described protocol was studied on cereblon (CRBN)-based E3-ligase binders and (+)-JQ-1, a bromodomain inhibitor, to generate BET (bromodomain and extra terminal domain) targeting protein degraders. The generated PROTACs were profiled in vitro and tested for their degradation ability with several potent candidates identified. Upfront, the individual reactions of the one-pot transformation were carefully optimized for CRBN binder functionalization and multiple heterobifunctional linker moieties were designed and synthesized. Separate scopes detailing the C(sp2)-C(sp3) coupling and one-pot PROTAC synthesis are described in this report as well as a library miniaturization study showing the high-throughput compatibility. Overall, the developed protocol provides rapid access to PROTACs in a single process, thereby allowing efficient generation of CRBN-based PROTAC libraries.

On-DNA hydroalkylation of N-vinyl heterocycles via photoinduced EDA-complex activation

The emergence of DNA-encoded library (DEL) technology has provided a considerable advantage to the pharmaceutical industry in the pursuit of discovering novel therapeutic candidates for their drug development initiatives. This combinatorial technique not only offers a more economical, spatially efficient, and time-saving alternative to the existing ligand discovery methods, but also enables the exploration of additional chemical space by utilizing novel DNA-compatible synthetic transformations to leverage multifunctional building blocks from readily available substructures. In this report, a decarboxylative-based hydroalkylation of DNA-conjugated N-vinyl heterocycles enabled by single-electron transfer (SET) and subsequent hydrogen atom transfer through electron-donor/electron-acceptor (EDA) complex activation is detailed. The simplicity and robustness of this method permits inclusion of a broad array of alkyl radical precursors and DNA-tethered nitrogenous heterocyles to generate medicinally relevant substituted heterocycles with pendant functional groups. Moreover, a successful telescoped route provides the opportunity to access a broad range of intricate structural scaffolds by employing basic carboxylic acid feedstocks.

Diversifying chemical space of DNA-encoded libraries: synthesis of 2-oxa-1-azaspiro(bicyclo[3.2.0])heptanes on-DNA via visible light-mediated energy transfer catalysis

Azaspiro[3.3]heptanes are valuable synthetic targets for drug discovery programs. The challenges associated with the preparation and diversification of this moiety as compared to other small, saturated rings have led to limited applications of compounds containing this spirocycle. In this regard, important advances in the field of synthetic photochemistry have exploited the biradical nature of the triplet excited state of 2-isoxazoline-3-carboxylates, engaging these species in intermolecular coupling reactions under visible light irradiation. As a continuation of our program preparing F(sp3)-rich, structurally complex molecules for DNA-encoded library technology (DELT) applications via photocatalysis, we disclose herein the incorporation of unique and densely functionalized 2-oxa-1-azabicyclo[3.2.0]heptanes via [2+2] cycloaddition energy transfer sensitization, providing access to an unexplored library of azaspiro compounds, many of which include additional synthetic handles important for further functionalization of the DNA-conjugated products and for library production.

Ni-Catalyzed Reductive Dibenzylation of Trifluoromethylalkenes for CF3-Containing All-Carbon Quaternary Center Construction

A Ni-catalyzed reductive dicarbofunctionalization of α-CF3 styrenes with benzyl bromides has been accomplished. This transformation obviates the commonly facile β-F elimination effectively and enables the creation of CF3-substituted all-carbon quaternary centers of pharmaceutical interests. Preliminary mechanistic studies suggest a pathway consisting of benzyl radical addition and subsequent nickel-mediated benzylation of the resulting α-CF3-embedded tertiary C radical.

Dearomative intermolecular [2 + 2] photocycloaddition for construction of C(sp3)-rich heterospirocycles on-DNA

DNA-encoded library (DEL) screens have significantly impacted new lead compound identification efforts within drug discovery. An advantage of DELs compared to traditional screening methods is that an exponentially broader chemical space can be effectively screened using only nmol quantities of billions of DNA-tagged, drug-like molecules. The synthesis of DELs containing diverse, sp³-rich spirocycles, an important class of molecules in drug discovery, has not been previously reported. Herein, we demonstrate the synthesis of complex and novel spirocyclic cores via an on-DNA, visible light-mediated intermolecular [2 + 2] cycloaddition of olefins with heterocycles, including indoles, azaindoles, benzofurans, and coumarins. The DNA-tagged exo-methylenecyclobutane substrates were prepared from easily accessible alkyl iodides and styrene derivatives. Broad reactivity with many other DNA-conjugated alkene substrates was observed, including unactivated and activated alkenes, and the process is tolerant of various heterocycles. The cycloaddition was successfully scaled from 10 to 100 nmol without diminished yield, indicative of this reaction's suitability for DNA-encoded library production. Evaluation of DNA compatibility with the developed reaction in a mock-library format showed that the DNA barcode was maintained with high fidelity, with <1% mutated sequences and >99% amplifiable DNA from quantitative polymerase chain reaction (PCR) and next generation sequencing (NGS).

Photoredox-catalysed hydroaminoalkylation of on-DNA N-arylamines

An efficient approach to the photoredox-catalysed hydroaminoalkylation between on-DNA secondary N-substituted (hetero)arylamines and vinylarenes has been developed and explored. The methodology was examined with a broad scope of vinylarenes and secondary arylamines to establish a preferred building block profile for the process. Compatible substrates furnished the desired derivitised amine products in modest to excellent conversions and with minimal or no detectable by-products.

Photochemical Methods Applied to DNA Encoded Library (DEL) Synthesis

DNA-encoded library technology (DELT) is a new screening modality that allows efficient, cost-effective, and rapid identification of small molecules with potential biological activity. This emerging technique represents an enormous advancement that, in combination with other technologies such as high-throughput screening (HTS), fragment-based lead generation, and structure-based drug design, has the potential to transform how drug discovery is carried out. DELT is a hybrid technique in which chemically synthesized compounds are linked to unique genetic tags (or "barcodes") that contain readable information. In this way, millions to billions of building blocks (BBs) attached on-DNA via split-and-pool synthesis can be evaluated against a biological target in a single experiment. Polymerase chain reaction (PCR) amplification and next-generation sequencing (NGS) analysis of the unique sequence of oligonucleotides in the DNA tag are used to identify those ligands with high affinity for the target. This innovative fusion of genetic and chemical technologies was conceived in 1992 by Brenner and Lerner (Proc. Natl. Acad. Sci. 1992, 89, 5381-5383) and is under accelerated development with the implementation of new synthetic techniques and protocols that are compatible with DNA. In fact, reaction compatibility is a key parameter to increasing the chances of identification of a drug target ligand, and a central focus has been the development of new transformations and the transition to robust protocols for on-DNA synthesis. Because the sole use of the DNA tag is as an amplifiable identification barcode, its structural integrity during a new chemical process is mandatory. As such, the use of these sensitive, polyfunctional biological molecules as substrates typically requires aqueous solutions within defined pH and temperature ranges, which is considered a notable challenge in DEL synthesis.Using low-energy visible light as the driving force to promote chemical transformations represents an attractive alternative to classical synthetic methods, and it is an important and well-established synthetic tool for forging chemical bonds in a unique way via radical intermediates. Recent advances in the field of photocatalysis are extraordinary, and this powerful research arena is still under continuous development. Several applications taking advantage of the mild reaction conditions of photoinduced transformations have been directed toward DEL synthesis, allowing the expansion of chemical space available for the evaluation of new building blocks on-DNA. There are no doubts that visible-light-driven reactions have become one of the most powerful approaches for DELT, given the easy way they provide to construct new bonds and the challenges to achieve equal success via classical protocols.Key characteristics of photocatalytic synthesis include the short reaction times and efficiency, which translate into retention of DNA integrity. In this Account, we describe recent advances in the photoinduced diversification of building blocks prepared on-DNA, highlighting the amenability of the techniques employed for preserving the genetic structure of the molecules. We demonstrate with recent research from our group the applicability of photocatalysis to the field and include in the summary a table containing all the photoinduced methods reported to date for DELT, demonstrating their key aspects such as scope, applications, and DNA compatibilities. With this information, practitioners are provided with compelling reasons for developing/choosing photocatalytic methods for DELT applications.

Photoredox-Mediated Deoxygenative Alkylation of DNA-Tagged Alkenes with Activated Alcohols

DNA-encoded library (DEL) screens have become a key technology to find small molecule binders to biological targets for drug discovery applications. The development of new DNA-compatible chemistries to expand the accessible DEL chemical space is imperative to enhance screen success across broad target classes and modalities. Additionally, reactions that use commonly available building blocks as well as those that enable the fsp³ of library members to be increased would have high impact for accessing diverse drug-like structures. Herein, we report a DNA-compatible Giese-type addition of nonstabilized C-centered radicals generated by the deoxygenation of preactivated alcohols into on-DNA olefins. Although alcohols have been historically underused as a building block class within DEL synthesis, their activation to a xanthate enables Csp³-Csp³ coupling to furnish sp³-rich products. This reaction is compatible with multiple classes of functional groups, does not damage the DNA tag, and is suitable for use in DEL productions.

Synthesis of CF3-Substituted Alkylamines, 1,2-Bisazoles, and 1,4,5,6-Tetrahydro-1,2,4-triazines from Newly Designed Tetrazole-Activated Trifluoromethyl Alkenes

A new class of Michael acceptor, tetrazolyl-trifluoromethyl alkenes, has been discovered. They readily undergo Michael-type addition instead of addition-elimination reaction with aliphatic amines and azoles to furnish β-trifluoromethyl alkylamines and CF₃-substituted 1,2-bisazole derivatives, respectively. Additionally, some of the products are capable of engaging in microwave-assisted intramolecular denitrogenative annulation, leading to the formation of CF₃-substituted 1,4,5,6-tetrahydro-1,2,4-triazines that are otherwise difficult to access by other methodologies.

Asymmetric Hydroarylation Reactions Catalyzed by Transition Metals: Last 10 Years in a Mini Review

Hydroarylation reactions play a pivotal role in organic chemistry due to their versatility and efficiency. In the last 10 years, the scientific production around this reaction has been very high, but in its asymmetric version, the results are less. In this mini review, selected literature examples are considered to draw attention to directions of the asymmetric hydroarylation reaction mediated by transition metal catalysts. The selected works were grouped in two main sections. In the first, we reported examples relating the narrower definition of hydroarylation, namely the metal-catalyzed processes where inactivated aryl moiety undergoes a direct functionalization via insertion of an unsaturated compound. In the second part, hydroarylation reactions take place with the use of pre-activated aryl substrates, usually aryl-iodides or aryl-boronated.

Catalyst-Free C(sp2)-H Borylation through Aryl Radical Generation from Thiophenium Salts via Electron Donor-Acceptor Complex Formation

Aryl borates lie at the heart of carbon-carbon bond couplings, and they are widely applied to the synthesis of functional materials, pharmaceutical compounds, and natural products. Currently, synthetic methods for aryl borates are mostly limited to metal-catalyzed routes, and nonmetallic strategies remain comparatively underdeveloped. Herein, we report a mild, scalable, visible-light-induced cross-coupling between aryl dibenzothiophenium triflate salts and bis(catecholato)-diboron for the construction of C-B bonds in the absence of base, transition metal-ligand complex, or photoredox catalyst. Mechanistic studies reveal that this transformation is achieved through an electron donor-acceptor (EDA) complex activation in the absence of a catalyst. The mild reaction conditions allow the preparation of aromatic borates in good yields with excellent functional group tolerance. This photochemical protocol was also successfully applied to the late-stage modification of natural products and the synthesis of a drug intermediate, greatly demonstrating broadened utility.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

Biomacromolecule-Assisted Screening for Reaction Discovery and Catalyst Optimization

Reaction discovery and catalyst screening lie at the heart of synthetic organic chemistry. While there are efforts at de novo catalyst design using computation/artificial intelligence, at its core, synthetic chemistry is an experimental science. This review overviews biomacromolecule-assisted screening methods and the follow-on elaboration of chemistry so discovered. All three types of biomacromolecules discussed─enzymes, antibodies, and nucleic acids─have been used as "sensors" to provide a readout on product chirality exploiting their native chirality. Enzymatic sensing methods yield both UV-spectrophotometric and visible, colorimetric readouts. Antibody sensors provide direct fluorescent readout upon analyte binding in some cases or provide for cat-ELISA (Enzyme-Linked ImmunoSorbent Assay)-type readouts. DNA biomacromolecule-assisted screening allows for templation to facilitate reaction discovery, driving bimolecular reactions into a pseudo-unimolecular format. In addition, the ability to use DNA-encoded libraries permits the barcoding of reactants. All three types of biomacromolecule-based screens afford high sensitivity and selectivity. Among the chemical transformations discovered by enzymatic screening methods are the first Ni(0)-mediated asymmetric allylic amination and a new thiocyanopalladation/carbocyclization transformation in which both C-SCN and C-C bonds are fashioned sequentially. Cat-ELISA screening has identified new classes of sydnone-alkyne cycloadditions, and DNA-encoded screening has been exploited to uncover interesting oxidative Pd-mediated amido-alkyne/alkene coupling reactions.

On-DNA Hydroalkylation to Introduce Diverse Bicyclo[1.1.1]pentanes and Abundant Alkyls via Halogen Atom Transfer

DNA-encoded libraries have proven their tremendous value in the identification of new lead compounds for drug discovery. To access libraries in new chemical space, many methods have emerged to transpose traditional mol-scale reactivity to nmol-scale, on-DNA chemistry. However, procedures to access libraries with a greater fraction of C(sp3) content are still limited, and the need to "escape from flatland" more readily on-DNA remains. Herein, we report a Giese addition to install highly functionalized bicyclo[1.1.1]pentanes (BCPs) using tricyclo[1.1.1.01,3]pentane (TCP) as a radical linchpin, as well as other diverse alkyl groups, on-DNA from the corresponding organohalides as non-stabilized radical precursors. Telescoped procedures allow extension of the substrate pool by at least an order of magnitude to ubiquitous alcohols and carboxylic acids, allowing us to "upcycle" these abundant feedstocks to afford non-traditional libraries with different physicochemical properties for the small-molecule products (i.e., non-peptide libraries with acids). This approach is amenable to library production, as a DNA damage assessment revealed good PCR amplifiability and only 6% mutated sequences for a full-length DNA tag.

Sustainable Thioetherification via Electron Donor-Acceptor Photoactivation Using Thianthrenium Salts

The synthesis of sulfides has been widely studied because this functional subunit is prevalent in biomolecules and pharmaceuticals, as well as being a useful synthetic platform for further elaboration. Thus, various methods to build C-S bonds have been developed, but typically they require the use of precious metals or harsh conditions. Electron donor-acceptor (EDA) complex photoactivation strategies have emerged as versatile and sustainable ways to achieve C-S bond formation, avoiding challenges associated with previous methods. This work describes an open-to-air, photoinduced, site-selective C-H thioetherification from readily available reagents via EDA complex formation that tolerates a wide range of different functional groups. Moreover, C(sp² )-halogen bonds remain intact using this protocol, allowing late-stage installation of the sulfide motif in various bioactive scaffolds, while allowing yet further modification through more traditional C-X bond cleavage protocols. Additionally, various mechanistic investigations support the envisioned EDA complex scenario.

Tetrachlorophthalimides as Organocatalytic Acceptors for Electron Donor-Acceptor Complex Photoactivation

Excitation of photoactive electron donor-acceptor (EDA) complexes is an effective way to generate radicals. Applications in a catalytic regime typically use catalytic donors. Herein, we report that readily available electron-poor tetrachlorophthalimides can act as effective organocatalytic acceptors to trigger the formation of EDA complexes with a variety of radical precursors not amenable to previous catalytic methods. Excitation with visible light generates carbon radicals under mild conditions. The versatility of this EDA complex catalytic platform allowed us to develop mechanistically distinct radical reactions, including in combination with a cobalt-based catalytic system. Quantum yield measurements established that a closed catalytic cycle is operational, which hints at the ability of tetrachlorophthalimides to readily turn over and govern each catalytic cycle.

Nickel-Mediated Synthesis of Non-Anomeric C-Acyl Glycosides through Electron Donor-Acceptor Complex Photoactivation

The preparation of nonanomeric C-acyl-saccharides has been developed from two different carboxylic acid feedstocks. This transformation is driven by the synergistic interaction of an electron donor-acceptor complex and Ni catalysis. Primary-, secondary-, and tertiary redox-active esters are incorporated as coupling partners onto preactivated pyranosyl- and furanosyl acids, preserving their stereochemical integrity. The reaction occurs under mild conditions, without stoichiometric metal reductants or exogenous catalysts, using commercially available Hantzsch ester as the organic photoreductant.

DNA-Compatible Synthesis of α,β-Epoxyketones for DNA-Encoded Chemical Libraries

As a powerful platform in drug discovery, the DNA-encoded chemical library technique enables the generation of numerous chemical members with high structural diversity. Epoxides widely exist in a variety of approved drugs and clinical candidates, eliciting multiple pharmaceutical activities. Herein, we report a non-oxidative DNA-compatible synthesis of di-/trisubstituted α,β-epoxyketones by implementing aldehydes and α-chlorinated ketones as abundant building blocks. This methodology was demonstrated to cover a broad substrate scope with medium-to-excellent conversions. Further structural diversification and transformation were also successfully explored to fully leverage α,β-epoxyketone moiety.

Metal-free deoxygenative coupling of alcohol-derived benzoates and pyridines for small molecules and DNA-encoded libraries synthesis

A visible-light-mediated metal-free method for the deoxygenative coupling of alcohol-derived benzoates and pyridines. Given the mild and water-compatible conditions, small molecules and DNA headpieces are functionalized with a wide range of alcohols.

Additive-Free, Visible-Light-Enabled Decarboxylative Alkylation of Enamides

Enamides are versatile precursors for synthesizing bioactive compounds. As their alkylations often require perstoichiometric amounts of oxidants, transition metals, or photocatalysts, we herein report a simple alternative for their alkylations by just using visible light to irradiate the mixture of the readily available N-hydroxyphthalimide esters and enamides without an additive. The reaction involves the photoactivation of a π-π stacking EDA complex between the substrates.