Multicomponent Ugi Reaction of Indole- N-carboxylic Acids: Expeditious Access to Indole Carboxamide Amino Amides

Recent advances in Ugi reaction and its post-transformation strategies for crafting diverse indole frameworks

Indoles and their fused analogues are ubiquitous structural scaffolds present in a plethora of bioactive natural products and pharmaceutical compounds, showcasing their importance in drug design and delivery. Since the discovery of isocyanide-based multicomponent reactions (IMCRs), especially the Ugi reaction, there has been exponential growth in the development of new methodologies based on the post-modification of the Ugi reaction. This has led to the creation of diverse heterocyclic scaffolds, macromolecules, and natural products. This review summarizes the progression of Ugi-multicomponent reactions (MCRs) and their post-modification methods from 2012 to mid-2024. It highlights the documented protocols for constructing various indoles and hetero-fused indole derivatives in an enantioselective, regioselective, and chemoselective manner, emphasising their significance for medicinal chemists.

Efficient synthesis of amides from secondary alcohols and CH3CN promoted by Fe(NO3)3·9H2O

The Ritter reaction is the most attractive method for synthesizing amides, and various acids have been used to promote this reaction. Compared to these acids, Fe(NO3)3·9H2O is less toxic and costly, and it shows relatively high Lewis acidity and great catalytic activity. In this study, a simple and efficient protocol involving Fe(NO3)3·9H2O as an additive for the synthesis of amides was developed. Various secondary alcohols could be reacted with CH3CN to obtain their corresponding products, with CH3CN being used as a reactant and solvent. This protocol was found to be applicable to a wide range of alcohols and nitrile substrates. In general, it was found that substrates containing electron-donating-groups offered the corresponding amides in good to excellent yields, while those with electron-withdrawing groups offered low to moderate yields. Meanwhile, this approach was scalable to the gram level, offering an attractive opportunity for further application in organic synthesis.

Multicomponent Reactions for Expeditious Construction of β-Indole Carboxamide Amino Amides

A multicomponent reaction of N-indole carboxylic acids, aldehydes, amines, and C2 building blocks can be transformed to structurally diverse β-indole carboxamide amino amides. In this multicomponent reaction, the ynamides and triazenyl alkynes act as the C2 building block, and this protocol features readily available starting materials, high atom economy, and mild reaction conditions. Besides, the acyl triazene group in the product can be easily transformed to differential groups to expand the structural diversity.

Ugi Four-Component Reactions Using Alternative Reactants

The Ugi four-component reaction (Ugi-4CR) undoubtedly is the most prominent multicomponent reaction (MCRs) that has sparked organic chemists' interest in the field. It has been widely used in the synthesis of diverse heterocycle molecules such as potential drugs, natural product analogs, pseudo peptides, macrocycles, and functional materials. The Ugi-4CRs involve the use of an amine, an aldehyde or ketone, an isocyanide, and a carboxylic acid to produce an α-acetamido carboxamide derivative, which has significantly advanced the field of isocyanide-based MCRs. The so-called intermediate nitrilium ion could be trapped by a nucleophile such as azide, N-hydroxyphthalimide, thiol, saccharin, phenol, water, and hydrogen sulfide instead of the original carboxylic acid to allow for a wide variety of Ugi-type reactions to occur.β In addition to isocyanide, there are alternative reagents for the other three components: amine, isocyanide, and aldehyde or ketone. All these alternative components render the Ugi reaction an aptly diversity-oriented synthesis of a myriad of biologically active molecules and complex scaffolds. Consequently, this review will delve deeper into alternative components used in the Ugi MCRs, particularly over the past ten years.

Formamidation of a wide range of substituted and functionalized amines with CO and a base

We have developed a base mediated formamidation of amines with CO under mild conditions, which allows for the synthesis of a wide range of aromatic and aliphatic formamides in high yields and gram amounts in the absence of a transition metal.

Triazenyl Alkynes as Versatile Building Blocks in Multicomponent Reactions: Diastereoselective Synthesis of β-Amino Amides

Multicomponent reactions (MCRs) are powerful tool for the construction of polyfunctional molecules in an operationally simple and atom-economic manner, and the discovery of novel MCRs requests various building blocks. Herein, triazenyl alkynes were disclosed as versatile building blocks in a multicomponent reaction with carboxylic acids, aldehydes and anilines to furnish β-amino amides with the achievement of high diastereoselectivity and structural diversity. In this process, triazenyl alkynes were bifunctional so that the alkyne moiety acts as C2 fragment and triazene serves as directing group to modulate the transition state thus achieving high diastereoselectivity, in consistence with DFT calculations. Furthermore, the triazenyl group also enables diverse late-stage transformation. This protocol opens a new vision for the discovery of building block and rational design of MCRs.

Carbon dioxide based methodologies for the synthesis of fine chemicals

Rapid environmental changes triggered by the increase in the concentration of heat-absorbing gases such as CO2 in the atmosphere have become a major cause of concern. One of the ways to counter this growing threat will be to efficiently convert atmospheric CO2 into value-added products via the development of efficient transition-metal-catalyzed processes. Conversion of CO2 into bulk products such as CH3OH and methane as well as its incorporation into commercial polyurethane synthesis has been achieved and reviewed extensively. However, the efficient transformation of CO2 into fine chemicals and value-added chemicals has many fold advantages. Recent years have seen a rapid rise in the number of metal-mediated protocols to achieve this goal of converting CO2 into fine chemicals. These are essential developments given the requirement of several commodities and fine chemicals in various industrial processes and the utilization of atmospheric CO2 will help provide a sustainable solution to the current environmental problems. Accordingly, we present here a comprehensive compilation of catalytic processes, involving CO2 as the C1 source for reacting with substrates such as alkanes, alkenes, alkynes, amines, acid chlorides, alcohols, allyl boronates, alkenyl triflates, and many others to provide easy access to a wide variety of useful molecules. Such a technology would certainly prove to be beneficial in solving the problems associated with the environmental accumulation of CO2.

Facile Synthesis of Novel Hexahydroimidazo[1,2-a]pyridine Derivatives by One-Pot, Multicomponent Reaction under Ambient Conditions

An efficient one-pot multicomponent reaction for the synthesis of novel tetrasubstituted hexahydroimidazo[1,2-a]pyridines starting from readily available cinnamaldehydes, ethylenediamines, and 1,3-dicarbonyl compounds catalyzed by AcOH is described. Two new cycles and four new bonds are constructed with all reactants being efficiently utilized in this transformation. The products could be obtained in 1-3 h under ambient conditions exclusively as a single isomer (trans). Single-crystal X-ray analysis confirmed the trans derivative as the only isomer.

Ag2O-catalysed nucleophilic isocyanation: selective formation of less-stable benzylic isonitriles

Both primary and secondary benzylic isonitriles were exclusively produced by the Ag₂O-catalysed reaction of benzylic phosphates and trimethylsilyl cyanide without formation of the thermodynamically favoured regioisomers, benzylic nitriles.