Synthesis of functionalized quinolines via Ugi and Pd-catalyzed intramolecular arylation reactions

Access to Isoquinolin-2(1H)-yl-acetamides and Isoindolin-2-yl-acetamides from a Common MCR Precursor

We achieved a divergent synthesis of isoquinolin-2(1H)-yl-acetamides (16 examples, up to 90% yields) and regioselective isoindolin-2-yl-acetamides (14 examples, up to 93% yields) in moderate to good yields by reacting various substituted ethanones or terminal alkynes with Ugi-4CR intermediates via an ammonia-Ugi-4CR/Copper(I)-catalyzed annulation sequence reaction. The same intermediate thus gives 2D distant but 3D closely related scaffolds, which can be of high interest in exploiting chemistry space on a receptor. The scopes and limitations of these efficient sequence reactions are described, as well as gram-scale synthesis.

N-Oxide-Induced Ugi Reaction: A Rapid Access to Quinoline-C2-amino Amides via Deoxygenative C(sp2)-H Functionalization

A logic-based replacement of the carboxylic acid component of the Ugi reaction by quinoline N-oxides has been developed. In this approach, the carboxylic isostere, quinoline N-oxide, plays a vital role by shifting the equilibria toward the product side with irreversible addition onto the C2-position of the N-oxide. Thus, aldehydes react with amines, isocyanides, and quinoline N-oxides to furnish quinoline four-component Ugi adducts. The unique reactivity of N-oxides with Ugi components opens an efficient synthetic route for the preparation of biologically active compounds.

Isoquinolone-4-Carboxylic Acids by Ammonia-Ugi-4CR and Copper-Catalyzed Domino Reaction

Highly substituted isoquinolone-4-carboxylic acid is an important bioactive scaffold; however, it is challenging to access it in a general and short way. A Cu-catalyzed cascade reaction was successfully designed involving the Ugi postcyclization strategy by using ammonia and 2-halobenzoic acids as crucial building blocks. Privileged polysubstituted isoquinolin-1(2H)-ones were constructed in a combinatorial format with generally moderate to good yields. The protocol, with a ligand-free catalytic system, shows a broad substrate scope and good functional group tolerance toward excellent molecular diversity. Free 4-carboxy-isoquinolone is now for the first time generally accessible by a convergent multicomponent reaction protocol.

Efficient Synthesis of Fused Oxazepino-isoquinoline Scaffolds via an Ugi, Followed by an Intramolecular Cyclization

A mild and efficient protocol was developed for the synthesis of oxazepino-isoquinolines via a one-pot Ugi four-component reaction, followed by the intramolecular addition of the resulting alcohol to an alkyne moiety under microwave irradiation conditions. Notably, this process only required one purification step, providing facile access to two series of complex and potentially interesting biologically active scaffolds.

Synthesis of trifluoromethylated isoxazoles and their elaboration through inter- and intra-molecular C–H arylation

We report conditions for the preparation of a range of trifluoromethylated isoxazole building blocks through the cycloaddition reaction of trifluoromethyl nitrile oxide and their subsequent derivatisation.

Efficient Construction of Azaspiro[4.5]trienone Libraries via Tandem Ugi 4CC/Electrophilic ipso-Iodocyclization in One-Pot

A solution-phase parallel synthesis of pharmaceutically important azaspiro[4.5]trienones has been developed by performing tandem Ugi four-component condensation (U4CC), involving substituted p-anisidines, aldehydes, 3-alkyl/aryl-propiolic acids, and isocyanides, and iodine-mediated ipso-iodocyclization in one-pot. This highly atom economical process produced functionalized azaspiro[4.5]trienones in good to excellent overall yields and products were easily isolated by precipitation followed by crystallization. These vinyl-iodide bearing azaspiro[4.5]trienones were utilized for further modifications through Suzuki coupling and deiodination reaction to demonstrate the suitability of these products for various palladium catalyzed modifications. The present method provides an easy access to highly functionalized azaspiro[4.5]trienones that can be useful in drug discovery research.

Metal-mediated post-Ugi transformations for the construction of diverse heterocyclic scaffolds

The Ugi-4CR is by far one of the most successful multicomponent reactions leading to high structural diversity and molecular complexity. However, the reaction mostly affords a linear peptide backbone, enabling post-Ugi transformations as the only solution to rigidify the Ugi-adduct into more drug like species. Not surprisingly, the development of these transformations, leading to new structural frameworks, has expanded rapidly over the last few years. As expected, palladium-catalyzed reactions have received the foremost attention, yet other metals, particularly gold complexes, are fast catching up. This tutorial review outlines the developments achieved in the past decade, highlighting the modifications that are performed in a sequential or domino fashion with emphasis on major concepts, synthetic applications of the derived products as well as mechanistic aspects.

Diversity-oriented reconstruction of primitive diketopiperazine-fused tetrahydro-β-carboline ring systems via Pictet–Spengler/Ugi-4CR/deprotection-cyclization reactions

An expedient construction of tetrahydro-β-carbolinediketopiperazine ring systems, which are present in various indole alkaloids, is documented.

Rapid generation of privileged substructure-based compound libraries with structural diversity and drug-likeness

A library of privileged-substructure-based, heterocyclic compounds was constructed by a sequence of Ugi four-component reactions incorporating the indole motif and microwave-assisted cyclizations in branched pathways. Cheminformatic analysis confirmed that the library exhibited a high degree of structural diversity and good drug-likeness.

An efficient synthesis of (NH)-phenanthridinones via ligand-free copper-catalyzed annulation

An efficient and concise procedure for the ligand-free copper-catalyzed cascade reaction of C–O and C–N bond coupling was developed to afford various (NH)-phenanthridinones.

Construction of functionalized tricyclic dihydropyrazino-quinazolinedione chemotypes via an Ugi/N-acyliminium ion cyclization cascade

Dihydropyrazino-quinazolinedione chemotypes are complex and structurally challenging structures of biological interest, being found in the marine alkaloids such as brevianamide M-N and fumiquinazolines A-C. Herein we report the synthesis of this tricyclic system in three synthetic operations by means of an Ugi multi-component reaction (MCR) followed by a tandem N-acyliminium ion cyclization-intramolecular nucleophilic addition reaction sequence. Additional structural diversification for further library enrichment was also accomplished via sequential N-alkylation and N-acylation/sulfonation.

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.

Synthesis of dihydrophenanthridines by a sequence of Ugi-4CR and palladium-catalyzed intramolecular C-H functionalization

Background

Small polyfunctionalized heterocyclic compounds play important roles in the drug discovery process and in the isolation and structural identification of biological macromolecules. It is expected that ready access to diverse sets of heterocycles can not only help improving the known biological and pharmacokinetic properties of drugs, but also assist the discovery of molecules that exhibit biological effects beyond those associated with previously known macromolecules. By virtue of their inherent convergence, high productivity, their exploratory and complexity-generating power, multicomponent reactions (MCRs) are undoubtedly well suited for creating molecular diversity. The combination of MCRs with an efficient post-functionalization reaction has proven to be an efficient strategy to increase the skeleton diversity.

Results

The Ugi reaction of an o-iodobenzaldehyde (2), an aniline (3), an isocyanide (4), and a carboxylic acid (5) afforded α-acetamido-α-phenylacetamide (6) in good to excellent yields. The palladium-catalyzed intramolecular C-H functionalization of these adducts under ligandless conditions provided the functionalized dihydrophenanthridines (1).

Conclusion

Highly functionalized dihydrophenanthridines are synthesized in only two steps from readily accessible starting materials in good to excellent overall yields.