Protein-Templated Ugi Reactions versus In-Situ Ligation Screening: Two Roads to the Identification of SARS-CoV-2 Main Protease Inhibitors

Protein-templated fragment ligation was established as a method for the rapid identification of high affinity ligands, and multicomponent reactions (MCR) such as the Ugi four-component reaction (Ugi 4CR) have been efficient in the synthesis of drug candidates. Thus, the combination of both strategies should provide a powerful approach to drug discovery. Here, we investigate protein-templated Ugi 4CR quantitatively using a fluorescence-based enzyme assay, HPLC-QTOF mass spectrometry (MS), and native protein MS with SARS-CoV-2 main protease as template. Ugi reactions were analyzed in aqueous buffer at varying pH and fragment concentration. Potent inhibitors of the protease were formed in presence of the protein via Ugi 4CR together with Ugi three-component reaction (Ugi 3CR) products. Binding of inhibitors to the protease was confirmed by native MS and resulted in the dimerization of the protein target. Formation of Ugi products was, however, more efficient in the non-templated reaction, apparently due to interactions of the protein with the isocyanide and imine fragments. Consequently, in-situ ligation screening of Ugi 4CR products was identified as a superior approach to the discovery of SARS-CoV-2 protease inhibitors.

Modular Assembly of 2-Aminoaniline Derivatives by Merging Hydroxylamine-Passerini and Hetero-Cope Rearrangement

A metal-free three-component protocol that combines a hydroxylamine-Passerini reaction and hetero-Cope rearrangement was realized, which enables the modular assembly of a wide range of structurally new and interesting 2-aminoanilines bearing an α-hydroxyamide substructure.

Tsuji-Trost Based Cascades: From Passerini Adducts of Cinnamaldehyde to Allylated Oxazolones

The Passerini coupling of cinnamaldehyde derivatives affords allylic esters that may behave as both electrophiles and nucleophiles in Tsuji-Trost reactions. We present herein the interaction of the latter with methylallylcarbonate, leading to the formation of oxazolidine-diones. The efficiency of the process relies on the building up of a CO2 overpressure in the medium. A reaction mechanism highlighting the reversibility of the Tsuji-Trost reaction is proposed for the process.

New oxacycles on the block: benzodioxepinones via a Passerini reaction

Oxacycles and benzoxepanes are privileged motifs present in a variety of natural products and functional molecules. However, their synthetic access is limited. Here, we demonstrate a rapid synthesis of unprecedented benzoxepanes from readily available starting materials in one step via a Passerini multicomponent reaction. The reaction proceeds smoothly under mild reaction conditions. We have obtained a single-crystal X-ray structure, revealing a butterfly conformation, combined with useful structural features. In addition, we have performed both a full interaction map on the X-ray structure and a profile analysis of a virtual library based on the proposed scaffold with a special focus on certain physicochemical parameters to demonstrate their potential usage in drug discovery.

Synthesis of Functional Isosorbide-Based Polyesters and Polyamides by Passerini Three-Component Polymerization

Environmental issues are becoming more and more prominent, and bio-based polymers are essential to alleviate environmental degradation by replacing traditional polymers. With this context, a new family of functional isosorbide-based polyesters and polyamides with high glass transition temperature are prepared via Passerini-Three component polymerization (P-3CP). To optimize the P-3CP conditions, the influence of the polymerization solvent, temperature, feed ratio on the molar mass of final polymers are investigated. The higher molar mass (up to 10100 g/mol) and yield (>70 %) are achieved under very mild conditions (30 °C, standard atmosphere). Functional side groups, such as alkenyl, alkynyl and methyl ester, were introduced into polymer structure via P-3CP by using functional isocyanides. The obtained polyesters and polyamides are characterized by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies, differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). All polymers are thermal stable and amorphous with variable glass transition temperatures (Tg ). The obtained polyester has Tg up to 87.5 °C, while the Tg of polyamides (ISPA-2) is detected to be 97.5 °C depending on the amide bonds in the polymer backbone and the benzene ring side groups. The cytotoxicity is investigated by the CCK-8 assay against mBMSC cells to confirm the biological safety. Overall, this novel strategy provides an efficient approach to produce functional isosorbide-based polyesters and polyamides, which are promising prospect for being applied to biodegradable materials.

Multicomponent synthesis and photophysical study of novel α,β-unsaturated carbonyl depsipeptides and peptoids

Multicomponent reactions were performed to develop novel α,β-unsaturated carbonyl depsipeptides and peptoids incorporating various chromophores such as cinnamic, coumarin, and quinolines. Thus, through the Passerini and Ugi multicomponent reactions (P-3CR and U-4CR), we obtained thirteen depsipeptides and peptoids in moderate to high yield following the established protocol and fundamentally varying the electron-rich carboxylic acid as reactants. UV/Vis spectroscopy was utilized to study the photophysical properties of the newly synthesized compounds. Differences between the carbonyl-substituted chromophores cause differences in electron delocalization that can be captured in the spectra. The near UV regions of all the compounds exhibited strong absorption bands. Compounds P2, P5, U2, U5, and U7 displayed absorption bands in the range of 250-350 nm, absorbing radiation in this broad region of the electromagnetic spectrum. A photostability study for U5 showed that its molecular structure does not change after exposure to UV radiation. Fluorescence analysis showed an incipient emission of U5, while U6 showed blue fluorescence under UV radiation. The photophysical properties and electronic structure were also determined by TD-DFT theoretical study.

Ni-Catalyzed Mild Synthesis of Sulfonylurea via Tandem Coupling of Sulfonyl Azide, Isocyanide, and Water

An efficient, mild, and novel route is developed to synthesize sulfonylurea via the nickel-catalyzed tandem coupling of sulfonyl azide, isocyanide, and water in aqueous media. The sulfonyl azide is expected to act as a nitrene precursor, which upon reaction with isocyanide generates carbodiimide. Herein, water acts as a nucleophile and reacts with carbodiimide to deliver the product. The protocol uses an inexpensive nickel catalyst, environmentally friendly water (as the nucleophile), and room temperature and provides products in moderate to good yields.

Multicomponent Reaction-Assisted Drug Discovery: A Time- and Cost-Effective Green Approach Speeding Up Identification and Optimization of Anticancer Drugs

Multicomponent reactions (MCRs) have emerged as a powerful strategy in synthetic organic chemistry due to their widespread applications in drug discovery and development. MCRs are flexible transformations in which three or more substrates react to form structurally complex products with high atomic efficiency. They are being increasingly appreciated as a highly exploratory and evolutionary tool by the medicinal chemistry community, opening the door to more sustainable, cost-effective and rapid synthesis of biologically active molecules. In recent years, MCR-based synthetic strategies have found extensive application in the field of drug discovery, and several anticancer drugs have been synthesized through MCRs. In this review, we present an overview of representative and recent literature examples documenting different approaches and applications of MCRs in the development of new anticancer drugs.

Synthesis, Characterization and Chemistry of Tetrakis(Propargylisocyanide) Copper(I) Complex

The kinetically unstable propargylisocyanide was reacted with the tetrakis(acetonitrile) copper(I) hexafluorophosphate and the formed complex was then involved in a copper-catalyzed alkyne-azide cycloaddition reaction (CuAAC). After the decomplexation of the adduct, the isocyanide was engaged in a Ugi reaction. By such a complexation, reactions can be carried out on the CC triple bond without the constraint of the instability of the free compound or the competitive reactivity of the isocyanide group.

4-(N-Phthalimido)phenyl Isonitrile as a Novel Convertible Isocyanide Analogue with the Odorless Property as an Extra Bonus

We explored a new isonitrile, namely 4-(N-phthalimido)phenyl isonitrile, with extraordinary features. The novel isocyanide has a pharmacophore, the phthalimido (Pht) group, that possesses promising pharmaceutical activities. We found that the novel isonitrile is unexpectedly odorless as an extra bonus which makes its handling easy in organic synthesis to serve as a scaffold for building several new amide derivatives through multicomponent reactions, overcoming the stink of common aromatic isonitriles such as phenyl isonitrile, benzyl isonitrile, p-nitrophenyl isonitrile, and ethyl 4-isocyano benzoate. The novel isonitrile 9 serves as a source of N-protected isonitrile with a Pht group, where the Pht group can be easily removed via hydrazinolysis, affording the corresponding primary amine/alcohol scaffold which could be used as a precursor to synthesize Passerini products via acylation directly to afford Passerini adducts 14 and 15 without carrying out the traditional Passerini three-component reaction; this new isonitrile is considered as a novel convertible isocyanide analogue.

Structure optimization of new tumor-selective Passerini α-acyloxy carboxamides as Caspase-3/7 activators

Selective elimination of tumors has always been the mainstay of oncology research. The on-going research underlying the cellular apoptotic mechanisms reveal caspases activation, especially the key effector caspase-3, as a personalized tumor-selective therapeutic strategy. Our continued research protocol has exploited new optimized Passerini α-acyloxy carboxamides as efficient apoptotic inducers via caspase-3/7 dependent mechanism with highly selective anticancer profiles. The adopted design rationale relied on excluding structural alerts of previous leads, while merging various pharmacophoric motifs of natural and synthetic caspase activators via optimized one-pot Passerini reaction conditions. The prepared compounds resulting from Passerini reaction were screened for their cytotoxic activities against colorectal Caco-2 and liver HepG-2 cancer cells compared to normal fibroblasts utilizing MTT assay. Notably, all compounds exhibited promising low-range submicromolar IC₅₀ against the studied cancer cell lines, with outstanding tumor selectivity (SI values up to 266). Hence, they were superior to 5-fluorouracil. Notably, 7a, 7g, and 7j conferred the highest potencies against Caco-2 and HepG-2 cells and were selected for further mechanistic studies. Caspas-3/7 activation assay of the hit compounds and flow cytometric analysis of the treated apoptotic cancer cells demonstrated their significant caspase activation potential (up to 4.2 folds) and apoptotic induction capacities (up to 58.7%). Further assessment of Bcl2 expression was performed being a physiological caspase-3 substrate. Herein, the three studied Passerini adducts were able to downregulate Bcl2 in the treated Caco-2 cells. Importantly, the mechanistic studies results of the three hits echoed their preliminary MTT antiproliferative potencies data highlighting their caspase-3 dependent apoptotic induction. Finally, the in silico predicted physicochemical and pharmacokinetic profiles, as well as ligand efficiency metrics were drug-like.