Library Generation via Postcondensation Modifications of Isocyanide-Based Multicomponent Reactions

Investigation of tetrasubstituted heterocycles reveals hydantoins as a promising scaffold for development of novel antimicrobials with membranolytic properties

Mimics of antimicrobial peptides (AMPs) have been proposed as a promising class of antimicrobial agents. We report the analysis of five tetrasubstituted, cationic, amphipathic heterocycles as potential AMP mimics. The analysis showed that the heterocyclic scaffold had a strong influence on the haemolytic activity of the compounds, and the hydantoin scaffold was identified as a promising template for drug lead development. Subsequently, a total of 20 hydantoin derivatives were studied for their antimicrobial potency and haemolytic activity. We found 19 of these derivatives to have very low haemolytic toxicity and identified three lead structures, 2dA, 6cG, and 6dG with very promising broad-spectrum antimicrobial activity. Lead structure 6dG displayed minimum inhibitory concentration (MIC) values as low as 1 μg/mL against Gram-positive bacteria and 4-16 μg/mL against Gram-negative bacteria. Initial mode of action (MoA) studies performed on the amine derivative 6cG, utilizing a luciferase-based biosensor assay, suggested a strong membrane disrupting effect on the outer and inner membrane of Escherichia coli. Our findings show that the physical properties and structural arrangement induced by the heterocyclic scaffolds are important factors in the design of AMP mimics.

Sequential Knoevenagel [4+1] Cycloaddition-Condensation-Aza-Friedel-Crafts Intramolecular Cyclization: A 4-Center-3-Component Reaction Toward Tunable Fluorescent Indolizine Tetracycles

A two-step multicomponent reaction oxidation protocol is reported herein, which affords novel tunable fluorescent tetracyclic indolizines. The procedure involves a novel 4-center-3-component reaction, which proceeds via a sequential Knoevenagel condensation, [4+1] cycloaddition, and imine condensation to afford imino-indolizines. Products then undergo cyclization and are oxidized in situ to afford fluorescent tetracycles, which are readily tunable through modification of diversity elements.

Synthesis of Spiro[chromene-imidazo[1,2-a]pyridin]-3'-imines via 6-exo-dig Cyclization Reaction

A transition-metal-free postmodification of the Groebke-Blackburn-Bienaymé (GBB) reaction for the synthesis of spiro[chromene-imidazo[1,2-a]pyridin]-3'-imine was discovered. The unusual transformation represents the first example of activation and the reaction of the imidazole carbon atom. In this postcondensational modification, KOt-Bu acts as a base, which, after the isomerization of an alkyne moiety to allene, causes the next unique nucleophilic reaction of the imidazole carbon atom that results in spirocyclic structures. The proposed reaction mechanism was confirmed based on the DFT calculations.

Tetrazoles via Multicomponent Reactions

Tetrazole derivatives are a prime class of heterocycles, very important to medicinal chemistry and drug design due to not only their bioisosterism to carboxylic acid and amide moieties but also to their metabolic stability and other beneficial physicochemical properties. Although more than 20 FDA-approved drugs contain 1 H- or 2 H-tetrazole substituents, their exact binding mode, structural biology, 3D conformations, and in general their chemical behavior is not fully understood. Importantly, multicomponent reaction (MCR) chemistry offers convergent access to multiple tetrazole scaffolds providing the three important elements of novelty, diversity, and complexity, yet MCR pathways to tetrazoles are far from completely explored. Here, we review the use of multicomponent reactions for the preparation of substituted tetrazole derivatives. We highlight specific applications and general trends holding therein and discuss synthetic approaches and their value by analyzing scope and limitations, and also enlighten their receptor binding mode. Finally, we estimated the prospects of further research in this field.

Methyl isocyanide as a convertible functional group for the synthesis of spirocyclic oxindole γ-lactams via post-Ugi-4CR/transamidation/cyclization in a one-pot, three-step sequence

The synthesis of spiro[indoline-3,2'-pyrrole]-2,5'(1'H)-diones and spiro[indoline-3,2'-pyrrolidine]-2,5'-diones, via a post-Ugi-domino transamidation/cyclization sequential process, has been achieved in three sequential steps utilizing a one-pot reaction protocol. The variation in carboxylic acid substrates allows for the generation of new chiral racemic quaternary carbon centers under basic conditions providing molecular diversity and a small library of spirocyclic oxindoles.

Aza-Riley Oxidation of Ugi-Azide and Ugi-3CR Products toward Vicinal Tricarbonyl Amides: Two-Step MCR-Oxidation Methodology Accessing Functionalized α,β-Diketoamides and α,β-Diketotetrazoles

Direct oxidative deamination of glyoxal-derived Ugi-azide and Ugi three-component reaction products readily affords vicinal tricarbonyls (α,β-diketoamides) and α,β-diketotetrazoles with two diversity elements. This significant extension of our previously described multicomponent reaction-oxidative deamination methodology is proposed to proceed through a mechanistically distinct SeO₂-mediated C-N oxidation derived from an active enol of α-amino-β-ketone systems, effectively an aza-Riley oxidation. This methodology accesses diverse VTC systems from prototypical amines, glyoxaldehydes, and isocyanide building blocks in a mere two steps.

Recent Advances in the Synthesis of Hydantoins: The State of the Art of a Valuable Scaffold

The review highlights the hydantoin syntheses presented from the point of view of the preparation methods. Novel synthetic routes to various hydantoin structures, the advances brought to the classical methods in the aim of producing more sustainable and environmentally friendly procedures for the preparation of these biomolecules, and a critical comparison of the different synthetic approaches developed in the last twelve years are also described. The review is composed of 95 schemes, 8 figures and 528 references for the last 12 years and includes the description of the hydantoin-based marketed drugs and clinical candidates.

Copper(i) catalyzed oxidative hydrolysis of Ugi 3-component and Ugi-azide reaction products towards 2° α-ketoamides and α-ketotetrazoles

Catalytic Cu(i) mediated C–N oxidation of Ugi-3-component and Ugi-azide reaction products affords 2° α-ketoamides 1 and α-ketotetrazoles 2.

One-pot synthesis of tetracyclic fused imidazo[1,2-a]pyridines via a three-component reaction

A novel three-component reaction has been developed to assemble biologically and pharmaceutically important tetracyclic fused imidazo[1,2-a]pyridines in a one-pot fashion utilizing readily available 2-aminopyridines, isatins and isocyanides. The three-component coupling proceeds through the Groebke–Blackburn–Bienaymé reaction followed by a retro-aza-ene reaction and subsequent nucleophilic reaction of the in-situ generated imidazo[1,2-a]pyridines bearing an isocyanate functional group.

Versatile Protecting-Group Free Tetrazolomethane Amine Synthesis by Ugi Reaction

The use of ammonia in the Ugi reaction is often problematic due to low yields and multiple side reactions. Here, we report the use of ammonia in the tetrazole Ugi variation providing a clean, good-to-high yielding reaction, especially with ketones as oxo components. The scope and limitations of this reaction and a structure-reactivity relationship are provided by performing >85 reactions. The primary amine component of the α-amino tetrazole is a versatile starting material for further reactions.

Synthesis of Tetrazolo-Fused Benzodiazepines and Benzodiazepinones by a Two-Step Protocol Using an Ugi-Azide Reaction for Initial Diversity Generation

A two-step strategy for the synthesis of arrays of tricyclic tetrazolo-fused benzodiazepines and benzodiazepinones has been investigated. The protocol uses ortho-N-Boc phenylisocyanides and phenylglyoxaldehydes or ethyl glyoxylate in the 4-component Ugi-Azide reaction to afford MCR (Multi Component Reactions) derived adducts equipped with the desired diversity inputs. A subsequent acidic treatment (TFA/DCE) allows a simultaneous deprotection-cyclization leading to the final products.

A novel route to synthesize libraries of quinoxalines via Petasis methodology in two synthetic operations

This communication reveals an innovative and facile procedure to prepare quinoxalines in two synthetic steps. The microwave assisted Petasis reaction is followed by the acid mediated unmasking of an internal amino nucleophile, cyclodehydration and oxidation to give collections of quinoxalines in good to excellent yields.

Use of cyclohexylisocyanide and methyl 2-isocyanoacetate as convertible isocyanides for microwave-assisted fluorous synthesis of 1,4-benzodiazepine-2,5-dione library

A new protocol in which cyclohexylisocyanide and methyl 2-isocyanoacetate are used as convertible isocyanides for Ugi/de-Boc/cyclization/Suzuki synthesis of biaryl-substituted 1,4-benzodiazepine-2,5-diones has been developed. Ugi reactions of Boc-protected anthranilic acids, fluorous benzaldehydes, amines, and cyclohexylisocyanide or methyl 2-isocyanoacetate were carried out at room temperature. Microwave-promoted de-Boc/cyclization reactions afforded 1,4-benzodiazepine-2,5-diones (BZDs). Suzuki coupling reactions further derivatized the BZD ring by removing the fluorous tag and introducing the biaryl group. A thirty three-member biaryl-substituted BZD library containing four points of diversity was prepared by microwave-assisted solution-phase fluorous parallel synthesis.

Emerging molecular diversity from the intra-molecular Ugi reaction: iterative efficiency in medicinal chemistry

This review details a now established area within the isonitrile multi-component reaction (IMCR) field of study, namely employing bi-functional reagents in the Ugi reaction for the construction of screening sets with the additional element or even possibly 'metric' of enhanced 'iterative efficiency potential'. The concept of 'iterative efficiency' will be briefly introduced, coupled with discussion on new synthetic routes to select bi-functional IMCR precursors and their use in the generation of pharmacologically relevant 'molecular diversity'.

Emerging approaches for the syntheses of bicyclic imidazo[1,2-x]-heterocycles

Imidazo-[1,2-x]heterocycles are versatile building blocks for use in both a 'drug hunters' quest to discover new leads and a chemical biologists search for effective molecular tools in 'cell perturbation' studies. At the front end of the drug discovery flow chart, the last 5-10 years have witnessed the discovery of new high-throughput methodologies which very quickly have enabled access to virtual libraries of these chemo-types in the realm of 10(7) derivatives. Interestingly, these often neglected cores in patent cooperation treaty (PCT) applications appear in several highly effective marketed drugs, completing the medicinal chemists search for clinical success. Such rigid chemo-types, all containing a bridgehead nitrogen atom, are thus poised for an ever increasing impact on the discovery and development of new molecular therapeutics. The following mini-review will briefly cover therapeutic utility, chemical methodologies and automation developed to enable preparation of arrays of these chemo-types in a high-throughput manner. Synthetic emphasis is placed on a 3-component-3-center isocyanide based multi-component reaction (IMCR), which spans solution, solid phase, flourous and microwave assisted organic synthesis.