Sequencing [4 + 1]-Cycloaddition and Aza-Michael Addition Reactions: A Diastereoselective Cascade for the Rapid Access of Pyrido[2′,1′:2,3]/Thiazolo[2′,3′:2,3]imidazo[1,5-a]quinolone Scaffolds as Potential Antibacterial and Anticancer Motifs

The Groebke-Blackburn-Bienaymé (GBB) reaction: A powerful tool for generating diverse heterocyclic scaffold libraries in anticancer drug discovery

The Groebke-Blackburn-Bienaymé (GBB) reaction is a versatile multi-component (MCR) synthetic methodology that has transformed anticancer drug discovery by enabling the rapid and efficient generation of diverse heterocyclic scaffolds. These scaffolds, such as imidazo[1,2-a]pyridines, imidazo[1,2-a]pyrimidines, and their derivatives, are highly valuable moieties for targeting critical cancer pathways. The modular nature of the GBB reaction, coupled with post-reaction modifications, allows the design of compounds with tailored structures and enhanced pharmacological properties. GBB-derived compounds exhibit broad anticancer potential by modulating diverse molecular targets. These include protein kinases (e.g. Rock2, Gsk3β, B-Raf), microtubule dynamics via tubulin inhibition, and G-quadruplex DNA stabilization in oncogene promoters (e.g., c-MYC, BCL2), disrupting key mechanisms of tumour progression. Moreover, they target epigenetic regulators such as HDACs, CBP/P300 bromodomains, and BET bromodomains, affecting transcriptional regulation and chromatin remodeling. Immune checkpoints (e.g., PD-1/PD-L1), enzymes such as autotaxin, TDP1, and Hsp90, as well as apoptotic regulators (e.g., Bcl-2, BAG3), are also effectively inhibited. More importantly, these compounds address challenging targets, including KRAS G12C mutations and the menin-MLL interaction, offering solutions to previously "undruggable" pathways. The unparalleled efficiency of GBB reaction and its ability to generate structurally diverse, bioactive compounds spanning multiple oncogenic mechanisms highlights its central role in advancing anticancer drug discovery and its transformative impact on therapeutic innovation.

Hypervalent Sulfur Derivatives as Sulfenylating Reagents: Visible-Light-Mediated Direct Thiolation of Activated C(sp2)-H Bonds with Dihalosulfuranes

In contrast to hypervalent iodine compounds, the chemistry of their sulfur analogues has been considerably less explored. Herein, we report the direct C-H bond thiolation of electron-rich heterocycles, arenes, and 1,3-dicarbonyls by dichlorosulfuranes under mild conditions. Mechanistic studies and density functional theory calculations suggest the radical chain mechanism of the disclosed transformation. The key to success is attributed to a strikingly low S-Cl bond dissociation energy, which enables the generation of radical species upon exposure to daylight.

Design, synthesis and antimycobacterial activity of imidazo[1,5-a]quinolines and their zinc-complexes

Tuberculosis has remained one of the world's deadliest infectious diseases. The complexity and numerous adverse effects of current treatment options as well as the emergence of multi-drug resistant M. tuberculosis (Mtb) demand research and innovation efforts to yield new anti-mycobacterial agents. In this study, we synthesized a series of imidazo[1,5-a]quinolines, including 4 new analogs, and evaluated their activity against Mtb. Inspired by previous studies, we also designed 8 compounds featuring a coordinated metal ion, determined their absolute configuration by single-crystal X-ray diffraction and included them in the bioactivity study. Remarkably, the metal complexation of 5c with either Zn2+ or Fe2+ increased the Mtb inhibitory activity of the compound 12.5-fold and reduced its cytotoxicity. Ultimately, out of the 21 analyzed imidazo[1,5-a]quinoline analogs, two zinc complexes (C1 and C7) showed the strongest, specific activity against Mtb H37Rv in vitro (IC90 = 7.7 and 17.7 μM).

Palladium-catalyzed ligand-regulated divergent synthesis of pyrrole[2,3-b]indoles and ureas from 2-ethynylanilines and isocyanides

Herein, a palladium-catalyzed and ligand-controlled protocol for the divergent synthesis of pyrrole[2,3-b]indole and urea derivatives has been described. Pyrrole[2,3-b]indoles ("cyclization on" products) via tandem cyclization of o-alkynylanilines with isocyanides in the absence of a ligand and ureas ("cyclization off" products) via oxidative amination of anilines with isocyanides in the presence of a ligand were obtained both in moderate to good yields with high selectivity. In this chemistry, cyclic and acyclic products were easily accessed with the same starting materials under the regulation of the ligand.

Discovery of novel papain-like protease inhibitors for potential treatment of COVID-19

The recent emergence of different SARS-CoV-2 variants creates an urgent need to develop more effective therapeutic agents to prevent COVID-19 outbreaks. Among SARS-CoV-2 essential proteases is papain-like protease (SARS-CoV-2 PLpro), which plays multiple roles in regulating SARS-CoV-2 viral spread and innate immunity such as deubiquitinating and deISG15ylating (interferon-induced gene 15) activities. Many studies are currently focused on targeting this protease to tackle SARS-CoV-2 infection. In this context, we performed a phenotypic screening using an in-house pilot compounds collection possessing a diverse skeleta against SARS-CoV-2 PLpro. This screen identified SIMR3030 as a potent inhibitor of SARS-CoV-2. SIMR3030 has been shown to exhibit deubiquitinating activity and inhibition of SARS-CoV-2 specific gene expression (ORF1b and Spike) in infected host cells and possessing virucidal activity. Moreover, SIMR3030 was demonstrated to inhibit the expression of inflammatory markers, including IFN-α, IL-6, and OAS1, which are reported to mediate the development of cytokine storms and aggressive immune responses. In vitro absorption, distribution, metabolism, and excretion (ADME) assessment of the drug-likeness properties of SIMR3030 demonstrated good microsomal stability in liver microsomes. Furthermore, SIMR3030 demonstrated very low potency as an inhibitor of CYP450, CYP3A4, CYP2D6 and CYP2C9 which rules out any potential drug-drug interactions. In addition, SIMR3030 showed moderate permeability in Caco2-cells. Critically, SIMR3030 has maintained a high in vivo safety profile at different concentrations. Molecular modeling studies of SIMR3030 in the active sites of SARS-CoV-2 and MERS-CoV PLpro were performed to shed light on the binding modes of this inhibitor. This study demonstrates that SIMR3030 is a potent inhibitor of SARS-CoV-2 PLpro that forms the foundation for developing new drugs to tackle the COVID-19 pandemic and may pave the way for the development of novel therapeutics for a possible future outbreak of new SARS-CoV-2 variants or other Coronavirus species.

Stereodivergent Desymmetrization of Phenols En Route to Modular Access to Densely Functionalized Quinazoline and Oxazine Scaffolds

The de novo assembly of stereochemically and skeletally diverse scaffolds is a powerful tool for the discovery of novel chemotypes. Hence, the development of modular, step- and atom-economic synthetic methods to access stereochemically and skeletally diverse compound collection is particularly important. Herein, we show a metal-free, stereodivergent build/couple/pair strategy that allows access to a unique collection of benzo[5,6][1,4]oxazino[4,3-a]quinazoline, quinolino[1,2-a]quinazoline and benzo[b]benzo [4,5]imidazo[1,2-d][1,4]oxazine scaffolds with complete diastereocontrol and wide distribution of molecular architectures. This metal-free process proceeds via desymmetrization of phenol derivatives. The cascade unites Mannich with aza-Michael addition reactions, providing expeditious entries to diverse classes of molecular shapes in a single operation.

Diaza-1,3-butadienes as Useful Intermediate in Heterocycles Synthesis

Many heterocyclic compounds can be synthetized using diaza-1,3-butadienes (DADs) as key structural precursors. Isolated and in situ diaza-1,3-butadienes, produced from their respective precursors (typically imines and hydrazones) under a variety of conditions, can both react with a wide range of substrates in many kinds of reactions. Most of these reactions discussed here include nucleophilic additions, Michael-type reactions, cycloadditions, Diels–Alder, inverse electron demand Diels–Alder, and aza-Diels–Alder reactions. This review focuses on the reports during the last 10 years employing 1,2-diaza-, 1,3-diaza-, 2,3-diaza-, and 1,4-diaza-1,3-butadienes as intermediates to synthesize heterocycles such as indole, pyrazole, 1,2,3-triazole, imidazoline, pyrimidinone, pyrazoline, -lactam, and imidazolidine, among others. Fused heterocycles, such as quinazoline, isoquinoline, and dihydroquinoxaline derivatives, are also included in the review.

Cs2CO3 catalyzed direct aza-Michael addition of azoles to α,β-unsaturated malonates

A highly efficient method for the synthesis of azole derivatives via a direct aza-Michael addition of azoles to α,β-unsaturated malonates using Cs₂CO₃ as a catalyst, has been successfully developed. A series of azole derivatives have been obtained in up to 94% yield and the reaction could be amplified to gram scale in excellent yield in the presence of 10 mol% of Cs₂CO₃.

A highly efficient method for the synthesis of azole derivatives via a direct aza-Michael addition of azoles to α,β-unsaturated malonates has been successfully developed. A series of azole derivatives have been obtained in up to 94% yield.

Stereodivergent Complexity-to-Diversity Strategy en Route to the Synthesis of Nature-Inspired Skeleta

The complexity-to-diversity (CtD) strategy has become one of the most powerful tools used to transform complex natural products into diverse skeleta. However, the reactions utilized in this process are often limited by their compatibility with existing functional groups, which in turn restricts access to the desired skeletal diversity. In the course of employing a CtD strategy en route to the synthesis of natural product-inspired compounds, our group has developed several stereodivergent strategies employing indoloquinolizine natural product analogues as starting materials. These transformations led to the rapid and diastereoselective synthesis of diverse classes of natural product-like architectures, including camptothecin-inspired analogues, azecane medium-sized ring systems, arborescidine-inspired systems, etc. This manifestation required a drastic modification of the synthetic design that ultimately led to modular and diastereoselective access to a diverse collection of various classes of biologically significant natural product analogues. The reported strategies provide a unique platform that will be broadly applicable to other late-stage natural product transformation approaches.

Stereoselective Late-Stage Transformations of Indolo[2,3-a]quinolizines Skeleta to Nature-Inspired Scaffolds

The indolo[2,3-a]quinolizines, canthines, and arborescidines natural products exhibit a wide range of bioactivities including anticancer, antiviral, antibacterial, and anti-inflammatory, among others. Therefore, the development of modular and efficient strategies to access the core scaffolds of these classes of natural products is a remarkable achievement. The Complexity-to-Diversity (CtD) strategy has become a powerful tool that transforms natural products into skeletal and stereochemical diversity. However, many of the reactions that could be utilized in this process are limited by the type of functional groups present in the starting material, which restrict transformations into a variety of products to achieve the desired diversity. In the course of employing a (CtD) strategy en route to the synthesis of nature-inspired compounds, unexpected stereoelectronic-driven rearrangement reactions have been discovered. These reactions provided a rapid access to indolo[2,3-a]quinolizines-, canthines-, and arborescidines-inspired alkaloids in a modular and diastereoselective manner. The disclosed strategies will be widely applicable to other late-stage natural product transformation programs and drug discovery initiatives.

Discovery of Novel Small-Molecule Inhibitors of SARS-CoV-2 Main Protease as Potential Leads for COVID-19 Treatment

The main protease of SARS-CoV-2 virus, M^pro, is an essential element for viral replication, and inhibitors targeting M^pro are currently being investigated in many drug development programs as a possible treatment for COVID-19. An in vitro pilot screen of a highly focused collection of compounds was initiated to identify new lead scaffolds for M^pro. These efforts identified a number of hits. The most effective of these was compound SIMR-2418 having an inhibitory IC₅₀ value of 20.7 μM. Molecular modeling studies were performed to understand the binding characteristics of the identified compounds. The presence of a cyclohexenone warhead group facilitated covalent binding with the Cys¹⁴⁵ residue of M^pro. Our results highlight the challenges of targeting M^pro protease and pave the way toward the discovery of potent lead molecules.

Divergent Strategy for Diastereocontrolled Synthesis of Small- and Medium-Ring Architectures

Nitrogen and oxygen medium rings, in particular nine-membered rings, epitomize a unique area of chemical space that occurs in many natural products and biologically appealing compounds. The scarcity of 8- to 12-membered rings among clinically approved drugs is indicative of the difficulties associated with their synthesis, principally owing to the unfavorable entropy and transannular strain. We report here a scandium triflate-catalyzed reaction that allows for a modular access to a diverse collection of nine-membered ring heterocycles in a one-pot cascade and with complete diastereocontrol. This cascade features an intramolecular addition of an acyl group-derived enol to a α,β-unsaturated carbonyl moiety, leading to N- and O-derived medium-ring systems. Computational studies using the density functional theory support the proposed mechanism. Additionally, a one-pot cascade leading to hexacyclic chromeno[3',4':2,3]indolizino[8,7-b]indole architectures, with six fused rings and four contiguous chiral centers, is reported. This novel cascade features many concerted events, including the formation of two azomethine ylides, [3 + 2]-cycloaddition, 1,3-sigmatropic rearrangement, Michael addition, and Pictet-Spengler reaction among others. Phenotypic screening of the resulting oxazonine collection identified chemical probes that regulate mitochondrial membrane potential, adenosine 5'-triphosphate contents, and reactive oxygen species levels in hepatoma cells (Hepa1-6), a promising approach for targeting cancer and metabolic disorders.

Hydrogen-Bond-Guided Reaction of Cyclohexadienone-aldehydes with Amines: Synthesis of an Aminal Group Containing a Fused Tetracyclic Framework

A modular approach has been developed for an efficient synthesis of an aminal group containing a new tetracyclic framework. The strategy has been devised based on selective hydrogen-bond-guided aza-Michael addition of heteroaromatic amines to cyclohexadienone-aldehydes. The reaction is highly atom economic and practical and involves stereoselective construction of four new C-N bonds and four rings. The synthetic utility of the tetracyclic product was explored. The role of a H-bond was explained with the help of experimental and density functional theory (DFT) computation studies.

Remarkable Potential of Zerumbone to Generate a Library with Six Natural Product-like Skeletons by Natural Material-Related Diversity-Oriented Synthesis

Diversity-oriented synthesis (DOS) is an effective strategy for the quick creation of diverse and high three-dimensional compounds from simple starting materials. The selection of a starting material is the key to constructing useful, chemically diverse compound libraries for the development of new drugs. Here, we report a novel, general, and facile strategy for the creation of diverse compounds with high structural diversity from readily available natural products, such as zerumbone, as the synthetic starting material. Zerumbone is the major component of the essential oil from wild ginger, Zingiber zerumbet Smith. It is noteworthy that zerumbone has a powerful latent reactivity, partly because of its three double bonds, two conjugated and one isolated, and a double conjugated carbonyl group in an 11-membered ring structure. In fact, zerumbone has been shown to be a successful example of natural material-related DOS (NMRDOS). We will report that zerumbone can be converted in one chemical step from four zerumbone derivatives into rare and markedly different scaffolds by transannulation.

Stereocontrolled transformations of cyclohexadienone derivatives to access stereochemically rich and natural product-inspired architectures

The last two decades or so have witnessed an upsurge in defining the art of designing complex natural products and nature-inspired molecules. Throughout these decades, fundamental insights into stereocontrolled, step-economic and atom-economical synthesis principles were achieved by the numerous synthetic accomplishments particularly in diversity-oriented synthesis (DOS). This has empowered the visualization of the third dimension in synthetic design and thus has resulted in a dramatic increase with today's diversity-oriented synthesis (DOS) at the forefront enabling access to diverse scaffolds with a high degree of stereochemical and skeletal complexity. To this end, a starting material-based approach is one of the powerful tools utilized in DOS that allows rapid access to molecular architectures with a high sp3 content. Skeletal and stereochemical diversity is often paramount for the selective modulation of the biological function of a complementary protein in the biological space. In this context, stereocontrolled transformation of cyclohexadienone scaffolds has positioned itself as a powerful platform for the rapid generation of stereochemically enriched and natural product-inspired compound collections. In this review, we cover multidirectional synthetic strategies that utilized cyclohexadienone derivatives as pluripotent building blocks en route for the construction of novel chemical space.