Catalytic Friedel‐Crafts Reactions on Saturated Heterocycles and Small Rings for sp 3 ‐sp 2 Coupling of Medicinally Relevant Fragments

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

Chemical insights into the synthetic chemistry of five-membered saturated heterocycles-a transition metal-catalyzed approach

Drug design and delivery is primarily based on the hunt for new potent drug candidates and novel synthetic techniques. Recently, saturated heterocycles have gained enormous attention in medicinal chemistry as evidenced by the medicinal drugs listed in the FDA Orange Book. Therefore, the demand for novel saturated heterocyclic syntheses has increased tremendously. Transition metal (TM)-catalyzed reactions have remained the prime priority in heterocyclic syntheses for the last three decades. Nowadays, TM catalysis is well adorned by combining it with other techniques such as bio- and/or enzyme-catalyzed reactions, organocatalysis, or using two different metals in a single catalysis. This review highlights the recent developments of the transition metal-catalyzed synthesis of five-membered saturated heterocycles.

Synthesis, Characterization, and Antimicrobial Activity Screening of Some Novel 3-(2-(3-(Substituted benzyloxy)oxetan-3-yl)-3-fluorophenoxy)-8-fluoro-2-methylquinoline Derivatives as Potential Antimycobacterial Agents

Microbial infections remain a grave threat to global health security due to increasing antibiotic resistance. The coronavirus pandemic has increased the risk of microbial infection. To combat these infections, the search for new therapeutic agents is in high demand. A series of new 3-(2-(3-(substituted benzyloxy)oxetan-3-yl)-3-fluorophenoxy)-8-fluoro-2-methylquinoline (9a-i) derivatives have been synthesized. The structure of synthesized compounds was analyzed by spectroscopic methods. The newly synthesized oxetanyl-quinoline derivatives were evaluated for in vitro antibacterial activity against Escherichia coli (NCIM 2574), Proteus mirabilis (NCIM 2388), Bacillus subtilis (NCIM 2063), Staphylococcus albus (NCIM 2178), and in vitro antifungal activity against Aspergillus niger (ATCC 504) and Candida albicans (NCIM 3100). Six oxetanyl-quinoline derivatives 9a, 9b, 9c, 9d, 9e, and 9h have shown good antibacterial activity against P. mirabilis with MIC 31.25-62.5 μM, 3-(((3-(2-fluoro-6-((8-fluoro-2-methylquinolin-3-yl)oxy)phenyl)oxetan-3-yl)oxy)methyl)benzonitrile (9f) reporting comparable activity against P. mirabilis with respect to the standard drug streptomycin. Compound 9a also showed good activity against B. subtilis with MIC 31.25 μM. The eight compounds 9a, 9b, 9d, 9e, 9f, 9g, 9h, and 9i have shown good antifungal activity against A. niger. The synthesized compounds were also screened for antimycobacterial activity against Mycobacterium tuberculosis H37Rv by MTT assay. Among the nine derivatives, compounds 9b, 9c, 9d, 9f, 9g, 9h, and 9i showed excellent antimycobacterial activity with MIC 3.41-12.23 μM, and two derivatives showed good activity with MIC 27.29-57.73 μM. All the derivatives were further evaluated for cytotoxicity against the Vero cell line and were found to be nontoxic. The in silico study of compounds 9a-i was performed against ATP synthase (PDB ID: 4V1F) and most of the compounds showed the stable and significant binding to ATP synthase, confirming their plausible mode of action as ATP synthase inhibitors. Thus, the significant antimycobacterial activity of 3-(2-(3-(substituted benzyloxy)oxetan-3-yl)-3-fluorophenoxy)-8-fluoro-2-methylquinoline derivatives has suggested that the oxatenyl-quinoline compounds could assist in the development of lead compounds to treat mycobacterial infections.

Oxetan-3-ols as 1,2-bis-Electrophiles in a Brønsted-Acid-Catalyzed Synthesis of 1,4-Dioxanes

Annulations that combine diacceptors with bis-nucleophiles are uncommon. Here, we report the synthesis of 1,4-dioxanes from 3-aryloxetan-3-ols, as 1,2-bis-electrophiles and 1,2-diols. Brønsted acid Tf₂NH catalyzes both the selective activation of the oxetanol, to form an oxetane carbocation that reacts with the diol, and intramolecular ring opening of the oxetane. High regio- and diastereoselectivity are achieved with unsymmetrical diols. The substituted dioxanes and fused bicyclic products present interesting motifs for drug discovery and can be further functionalized.

Computationally driven design of an artificial metalloenzyme using supramolecular anchoring strategies of iridium complexes to alcohol dehydrogenase

Artificial metalloenzymes (ArMs) confer non-biological reactivities to biomolecules, whilst taking advantage of the biomolecular architecture in terms of their selectivity and renewable origin. In particular, the design of ArMs by the supramolecular anchoring of metal catalysts to protein hosts provides flexible and easy to optimise systems. The use of cofactor dependent enzymes as hosts gives the advantage of both a (hydrophobic) binding site for the substrate and a cofactor pocket to accommodate the catalyst. Here, we present a computationally driven design approach of ArMs for the transfer hydrogenation reaction of cyclic imines, starting from the NADP⁺-dependent alcohol dehydrogenase from Thermoanaerobacter brockii (TbADH). We tested and developed a molecular docking workflow to define and optimize iridium catalysts with high affinity for the cofactor binding site of TbADH. The workflow uses high throughput docking of compound libraries to identify key structural motifs for high affinity, followed by higher accuracy docking methods on smaller, focused ligand and catalyst libraries. Iridium sulfonamide catalysts were selected and synthesised, containing either a triol, a furane, or a carboxylic acid to provide the interaction with the cofactor binding pocket. IC₅₀ values of the resulting complexes during TbADH-catalysed alcohol oxidation were determined by competition experiments and were between 4.410 mM and 0.052 mM, demonstrating the affinity of the iridium complexes for either the substrate or the cofactor binding pocket of TbADH. The catalytic activity of the free iridium complexes in solution showed a maximal turnover number (TON) of 90 for the reduction of salsolidine by the triol-functionalised iridium catalyst, whilst in the presence of TbADH, only the iridium catalyst with the triol anchoring functionality showed activity for the same reaction (TON of 36 after 24 h). The observation that the artificial metalloenzymes developed here lacked stereoselectivity demonstrates the need for the further investigation and optimisation of the ArM. Our results serve as a starting point for the design of robust artificial metalloenzymes, exploiting supramolecular anchoring to natural NAD(P)H binding pockets.

Investigating 3,3-diaryloxetanes as potential bioisosteres through matched molecular pair analysis

Oxetanes have received increasing interest in medicinal chemistry as attractive polar and low molecular weight motifs. The application of oxetanes as replacements for methylene, methyl, gem-dimethyl and carbonyl groups has been demonstrated to often improve chemical properties of target molecules for drug discovery purposes. The investigation of the properties of 3,3-diaryloxetanes, particularly of interest as a benzophenone replacement, remains largely unexplored. With recent synthetic advances in accessing this motif we studied the effects of 3,3-diaryloxetanes on the physicochemical properties of 'drug-like' molecules. Here, we describe our efforts in the design and synthesis of a range of drug-like compounds for matched molecular pair analysis to investigate the viability of the 3,3-diaryloxetane motif as a replacement group in drug discovery. We conclude that the properties of the diaryloxetanes and ketones are similar, and generally superior to related alkyl linkers, and that diaryloxetanes provide a potentially useful new design element.

Boron insertion into alkyl ether bonds via zinc/nickel tandem catalysis

Mild methods to cleave the carbon-oxygen (C-O) bond in alkyl ethers could simplify chemical syntheses through the elaboration of these robust, readily available precursors. Here we report that dibromoboranes react with alkyl ethers in the presence of a nickel catalyst and zinc reductant to insert boron into the C-O bond. Subsequent reactivity can effect oxygen-to-nitrogen substitution or one-carbon homologation of cyclic ethers and more broadly streamline preparation of bioactive compounds. Mechanistic studies reveal a cleavage-then-rebound pathway via zinc/nickel tandem catalysis.

Short Synthesis of Oxetane and Azetidine 3-Aryl-3-carboxylic Acid Derivatives by Selective Furan Oxidative Cleavage

Four-membered rings remain underexplored motifs despite offering attractive physicochemical properties for medicinal chemistry. Arylacetic acids bearing oxetanes, azetidines, and cyclobutanes are prepared in two steps: a catalytic Friedel-Crafts reaction from four-membered ring alcohol substrates, followed by mild oxidative cleavage. The suitability of the products as building blocks is reflected in their facile purification and amenability to derivatization. Examples include heteroaromatics and aryltriflates, as well as oxetane-derived profen drug analogues and a new endomorphin derivative containing an azetidine amino acid residue.

C(sp3)-H arylation to construct all-syn cyclobutane-based heterobicyclic systems: a novel fragment collection

All-syn fused cyclobutanes remain an elusive chemotype and thus present an interesting synthetic challenge. Herein, we report the successful application of Pd-catalysed C(sp3)-H arylation of cyclobutane compounds to generate all-syn heterobicyclic fragments using an innovative 'inside-out' approach. Through this strategy we generate a virtual collection of 90 fragments, which we demonstrate to have enhanced three-dimensionality and superior fragment-like properties compared to existing collections.

Synthesis of Simple 3,3-Diarylazetidines from N-Boc-3-arylazetidinols Using Friedel-Crafts Arylation Conditions

A synthesis of 3,3-diarylazetidines from N-Boc-3-aryl-3-azetidinols using Friedel-Crafts arylation conditions with AlCl₃ is described. A series of substituted diarylazetidines were readily prepared and isolated as the oxalate salts in high yield and high purity. The 3,3-diarylazetidine oxalates were then easily converted into N-alkyl and N-acyl analogues (RX, NaHCO₃/DMF/100 °C) in high overall yields.