Functionalization of DNA-Tagged Alkenes Enabled by Visible-Light-Induced C–H Activation of N-Aryl Tertiary Amines

Iron-Catalyzed Multicomponent C-H Alkylation of in Situ Generated Imines via Photoinduced Ligand-to-Metal Charge Transfer

Herein, we describe a novel photoinduced iron-catalyzed strategy for multicomponent C-H alkylation of in situ generated imines. By utilizing the alkyl radicals generated through iron-mediated photocatalytic C-H activation, the imines formed in situ are further subjected to addition reactions, resulting in the synthesis of various secondary and tertiary amine products. This method is simple to operate and does not require additional oxidants. It is applicable to inert alkane substrates such as cyclic alkanes, cyclic ethers, toluene, and ketones. The reaction is also compatible with various aromatic amines, alkyl amines, halogenated aromatic amines, as well as aromatic aldehydes, alkyl aldehydes, and cinnamaldehyde, among other different types of aldehydes.

DNA-Compatible Cyclization Reaction to Access 1,3,4-Oxadiazoles and 1,2,4-Triazoles

DNA-encoded chemical library (DECL) technology is a commonly employed screening platform in both the pharmaceutical industry and academia. To expand the chemical space of DECLs, new and robust DNA-compatible reactions are sought after. In particular, DNA-compatible cyclization reactions are highly valued, as these reactions tend to be atom economical and thus may provide lead- and drug-like molecules. Herein, we report two new methodologies employing DNA-conjugated thiosemicarbazides as a common precursor, yielding highly substituted 1,3,4-oxadiazoles and 1,2,4-triazoles. These two novel DNA-compatible reactions feature a high conversion efficiency and broad substrate scope under mild conditions that do not observably degrade DNA.

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.

Unexpected Cyclization Product Discovery from the Photoinduced Bioconjugation Chemistry between Tetrazole and Amine

Bioconjugation chemistry has emerged as a powerful tool for the modification of diverse biomolecules under mild conditions. Tetrazole, initially proposed as a bioorthogonal photoclick handle for 1,3-dipolar cyclization with alkenes, was later demonstrated to possess broader photoreactivity with carboxylic acids, serving as a versatile bioconjugation and photoaffinity labeling probe. In this study, we unexpectedly discovered and validated the photoreactivity between tetrazole and primary amine to afford a new 1,2,4-triazole cyclization product. Given the significance of functionalized N-heterocycles in medicinal chemistry, we successfully harnessed the serendipitously discovered reaction to synthesize both pharmacologically relevant DNA-encoded chemical libraries (DELs) and small molecule compounds bearing 1,2,4-triazole scaffolds. Furthermore, the mild reaction conditions and stable 1,2,4-triazole linkage found broad application in photoinduced bioconjugation scenarios, spanning from intramolecular peptide macrocyclization and templated DNA reaction cross-linking to intermolecular photoaffinity labeling of proteins. Triazole cross-linking products on lysine side chains were identified in tetrazole-labeled proteins, refining the comprehensive understanding of the photo-cross-linking profiles of tetrazole-based probes. Altogether, this tetrazole-amine bioconjugation expands the current bioconjugation toolbox and creates new possibilities at the interface of medicinal chemistry and chemical biology.

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.

DNA-Compatible Synthesis of Thiazolidione Derivatives via Three-Component Annulation and Knoevenagel Condensation

Thiazolidione, conferring drug-like properties, is an important heterocycle that widely exists in medicinally relevant molecules. In this work, by efficiently assembling various DNA-tagged primary amines, abundant aryl isothiocyanates, and ethyl bromoacetate, we present a DNA-compatible three-component annulation to generate a 2-iminothiazolidin-4-one scaffold, which was further decorated via Knoevenagel condensation by employing (hetero)aryl and alkyl aldehydes. These thiazolidione derivatives should find broad use in focused DNA-encoded library construction.

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).

Privileged heterocycles for DNA-encoded library design and hit-to-lead optimization

It is well known that heterocyclic compounds play a key role in improving drug activity, target selectivity, physicochemical properties as well as reducing toxicity. In this review, we summarized the representative heterocyclic structures involved in hit compounds which were obtained from DNA-encoded library from 2013 to 2021. In some examples, the state of the art in heterocycle-based DEL synthesis and hit-to-lead optimization are highlighted. We hope that more and more novel heterocycle-based DEL toolboxes and in-depth pharmaceutical research on these lead compounds can be developed to accelerate the discovery of new drugs.

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.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

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.

Aryl diazonium intermediates enable mild DNA-compatible C-C bond formation for medicinally relevant combinatorial library synthesis

Forging carbon-carbon (C-C) linkage in DNA-encoded combinatorial library synthesis represents a fundamental task for drug discovery, especially with broad substrate scope and exquisite functional group tolerance. Here we reported the palladium-catalyzed Suzuki-Miyaura, Heck and Hiyama type cross-coupling via DNA-conjugated aryl diazonium intermediates for DNA-encoded chemical library (DEL) synthesis. Starting from commodity arylamines, this synthetic route facilely delivers vast chemical diversity at a mild temperature and pH, thus circumventing damage to fragile functional groups. Given its orthogonality with traditional aryl halide-based cross-coupling, the aryl diazonium-centered strategy expands the compatible synthesis of complex C-C bond-connected scaffolds. In addition, DNA-tethered pharmaceutical compounds (e.g., HDAC inhibitor) are constructed without decomposition of susceptible bioactive warheads (e.g., hydroxamic acid), emphasizing the superiority of the aryl diazonium-based approach. Together with the convenient transformation into an aryl azide photo-crosslinker, aryl diazonium's DNA-compatible diversification synergistically demonstrated its competence to create medicinally relevant combinatorial libraries and investigate protein-ligand interactions in pharmaceutical research.

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.

Visible Light-Promoted Divergent Benzoheterocyclization from Aldehydes for DNA-Encoded Chemical Libraries

Benzoheterocyclics have been widely adopted as drug-like core scaffolds that can be incorporated into DNA-encoded chemical library technology for high-throughput hit discovery. Here, we present a visible light-promoted divergent synthesis of on-DNA benzoheterocycles from aldehydes. Four types of DNA-conjugated benzoheterocyclics were obtained under mild conditions with a broad substrate scope. A cross substrate scope study, together with enzymatic ligation and subsequent chemical diversifications, were conducted, demonstrating the feasibility of this approach in DNA-encoded chemical library construction.

Photoredox Deaminative Alkylation in DNA-Encoded Library Synthesis

Herein, we report an on-DNA photoredox-mediated deaminative alkylation method for diversifying DNA-tagged acrylamide substrate with amine-derived radicals. The radicals can be conveniently generated from sterically hindered primary amines, and the deaminative alkylation can tolerate a broad array of radical precursors. Furthermore, the methodology is applicable to Boc-protected diamines, free amino acids, and aryl halides, which bear functional groups enabling additional rounds of diversification. The method is believed to offer a high potential for constructing DNA-encoded libraries, as was demonstrated by the production of a mock library in a 2 × 3 matrix format and confirmation of DNA stability by UPLC-MS and qPCR experiments.

Functionalization of DNA-Tagged Alkenes with Diazo Compounds via Photocatalysis

To explore potential chemical space using DNA-encoded library (DEL) technology, the development of various types of robust DNA-compatible reactions is urgently needed. Diazo compounds, which serve as valuable building blocks and important synthons in synthetic chemistry, have been rarely applied in DEL synthesis, probably because of their potential modifications of the bases and phosphate backbone of DNA. Herein we report two cases of DNA-compatible reactions with alkenes and diazo compounds, providing corresponding hydroalkylation and cyclopropanation products in moderate to excellent yields. Notably, these transformations not only provide new access to C(sp³)-C(sp³) bond formation in DELs with excellent functional group tolerance but also represent practical ligation methods to introduce functionalized molecules into DNA.

Reversible Covalent Headpiece Enables Interconversion between Double- and Single-Stranded DNA-Encoded Chemical Libraries

The use of a proper encoding methodology is one of the most important aspects when practicing DEL technology. A "headpiece"-based double-stranded DEL encoding method is currently the most widely used for productive DEL. However, the robustness of double-stranded DEL construction conflicts with the versatility presented by single-stranded DEL applications. We here report a novel encoding method, which is based on a "reversible covalent headpiece (RCHP)". The RCHP allows reversible interconversion between double- and single-stranded DNA formats, providing an avenue to robust synthesis and allowing for the applications in distinct setups. We have validated the versatility of this encoding method with encoded self-assembled chemical library and DNA-encoded dynamic library technology. Notably, based on the RCHP-settled library construction, a unique "ternary covalent complex" mediating ligand isolation methodology against non-immobilized targets was developed.

Understanding Data Noise and Uncertainty through Analysis of Replicate Samples in DNA-Encoded Library Selection

By analyzing data sets of replicate DNA-Encoded Library (DEL) selections, an approach for estimating the noise level of the experiment has been developed. Using a logarithm transformation of the number of counts associated with each compound and a subset of compounds with the highest number of counts, it is possible to assess the quality of the data through normalizing the replicates and use this same data to estimate the noise in the experiment. The noise level is seen to be dependent on sequencing depth as well as specific selection conditions. The noise estimation is independent of any cutoff used to remove low frequency compounds from the data analysis. The removal of compounds with only 1-5 read counts greatly reduces some of the challenges encountered in DEL data analysis as it can reduce the data set by greater than 100-fold without impacting the interpretation of the results.

The expanding reaction toolkit for DNA-encoded libraries

Over the past decade, DNA-encoded libraries (DELs) have emerged as a leading platform for small molecule drug discovery among pharmaceutical companies, biotech companies and academic drug hunters alike. This revolutionary technology has tremendous potential that is yet to be fully realized, as the exploration of therapeutically relevant chemical space is fueled by the ever-expanding repertoire of DNA-compatible reactions used to construct the libraries. Advances in direct coupling reactions, like photo-catalytic cross couplings, unique cyclizations such as the formation of 1,2,4-oxadiazoles, and new functional group transformations are valuable contributions to the DEL reaction toolkit, and indicate where future reaction development efforts should focus in order to maximize the productivity of DELs.

On the design of lead-like DNA-encoded chemical libraries

DNA-encoded libraries (DELs) are becoming an established technology for finding ligands for protein targets. We have abstracted and analysed libraries from the literature to assess the synthesis strategy, selections of reactions and monomers and their propensity to reveal hits. DELs have led to hit compounds across a range of diverse protein classes. The range of reactions and monomers utilised has been relatively limited and the hits are often higher in molecular weight than might be considered ideal. Considerations for future library designs with reference to chemical diversity and lead-like properties are discussed.