Synthesis of fused imidazole rings by sequential van Leusen/C–H bond activation

Synthesis of Imidazole-Based Medicinal Molecules Utilizing the van Leusen Imidazole Synthesis

Imidazole and its derivatives are one of the most vital and universal heterocycles in medicinal chemistry. Owing to their special structural features, these compounds exhibit a widespread spectrum of significant pharmacological or biological activities, and are widely researched and applied by pharmaceutical companies for drug discovery. The van Leusen reaction based on tosylmethylisocyanides (TosMICs) is one of the most appropriate strategies to synthetize imidazole-based medicinal molecules, which has been increasingly developed on account of its advantages. In this review, we summarize the recent developments of the chemical synthesis and bioactivity of imidazole-containing medicinal small molecules, utilizing the van Leusen imidazole synthesis from 1977.

One-Pot Synthesis of 2,4,5-Trisubstituted Imidazoles via [2 + 2 + 1] Cycloannulation of 1,3-Bishet(aryl)-monothio-1,3-diketones, α-Substituted Methylamines and Sodium Nitrite through α-Nitrosation of Enaminones

An efficient one-pot synthesis of a series of diversely functionalized trisubstituted 4(5)het(aroyl)-2,5(4)-het(aryl)/alkylimidazoles from readily available 1,3-bishet(aryl)monothio-1,3-diketones has been reported. This novel sequential one-pot, three step protocol, wherein three new carbon nitrogen bonds are formed in contiguous fashion, involves in situ generation of enaminones by reaction of monothio-1,3-diketones with α-substituted methylamines, followed by their α-nitrosation with sodium nitrite and subsequent base mediated intramolecular heterocyclization of the resulting α-hydroxyiminoimines to trisubstituted imidazoles in high yields under mild conditions. These newly prepared 4(5)-het(aroyl)-5(4)-het(aryl)/alkylimidazoles are shown to exist as tautomeric mixture, however, their subsequent alkylation with methyl iodide in the presence of potassium carbonate affords 1-N-methy-2,5-bishet(aryl)-4-het(aroyl)imidazoles in highly regioselective fashion in most of the cases. Synthesis of few 4(5)-(2-hydroxyphenyl)-2,5(4)-substituted imidazoles, which are known to be good coordinating ligands, has also been reported. A probable mechanism for the formation of these imidazoles from hydroxyiminoimine intermediates has also been suggested.

A Van Leusen deprotection-cyclization strategy as a fast entry into two imidazoquinoxaline families

A concise synthesis of two pharmacologically relevant classes of molecules possessing the imidazoquinoxaline core is reported. The protocol involves use of 1,2-phenylenediamines and glyoxylic acid derivatives, namely ethyl glyoxylate or benzylglyoxamide, along with tosylmethylisocyanides in a microwave-assisted Van Leusen three-component condensation. Subsequent unmasking (Boc removal) of an internal amino-nucleophile promotes deprotection and cyclization that take place either spontaneously in a one-pot fashion to give 8 or upon acidic treatment under microwave irradiation after isolation of the imidazole intermediate to give 11. Of note, a tricyclic framework is hence assembled by means of a rapid and straightforward method with a high bond-forming efficiency.

Straightforward assembly of phenylimidazoquinoxalines via a one-pot two-step MCR process

An efficient multicomponent-based methodology providing a new entry into a medicinally important complex heterocyclic core is presented. The strategy is very general and able to endow target compounds with the highest possible number of diversity points. Notably, four new chemical bonds and two aromatic rings are formed in a one-pot fashion.

Synthesis of Diverse Heterocyclic Scaffolds via Tandem Additions to Imine Derivatives and Ring-Forming Reactions

A novel strategy has been developed for the efficient syntheses of diverse arrays of heterocyclic compounds. The key elements of the approach comprise a Mannich-type, multicomponent coupling reaction in which functionalized amines, aromatic aldehydes, acylating agents, and π- and organometallic nucleophiles are combined to generate intermediates that are then further transformed into diverse heterocyclic scaffolds via a variety of cyclization manifolds. Significantly, many of these scaffolds bear functionality that may be exploited by further manipulation to create diverse collections of compounds having substructures found in biologically active natural products and clinically useful drugs. The practical utility of this strategy was exemplified by its application to the first, and extraordinarily concise synthesis of the isopavine alkaloid roelactamine.

Applications of multicomponent reactions to the synthesis of diverse heterocyclic scaffolds

The sequencing of multicomponent reactions (MCRs) and subsequent cyclization reactions is a powerful stratagem for the rapid synthesis of diverse heterocyclic scaffolds. The optimal MCR is sufficiently flexible that it can be employed to generate adducts bearing a variety of functional groups that may then be selectively paired to enable different cyclization manifolds, thereby leading to a diverse collection of products. The growing interest in diversity-oriented synthesis has led to increased attention to this paradigm for library synthesis, which has inspired many advances in the design and implementation of MCRs for the construction of diverse heterocyclic scaffolds.

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.