Two-Step Route to Diverse N-Functionalized Peptidomimetic-like Isatins through an Oxidation/Intramolecular Oxidative-Amidation Cascade of Ugi Azide and Ugi Three-Component Reaction Products

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.

Metal-free oxoammonium salt-mediated C(sp3)-H oxidative Ugi-azide multicomponent reaction

In this work, an efficient oxidative C(sp³)-H Ugi-azide multicomponent reaction of cyclic benzylic amines to the corresponding α-tetrazolo compounds using a TEMPO salt as mild hydride abstractor-type oxidant is reported. This simple one-pot approach allows the direct functionalization of N-heterocycles such as tetrahydroisoquinolines with a variety of isocyanides and NaN₃ as a practical azide source. The reaction proceeds at room temperature and without the need of acid additives, allowing for the use of sensitive substrates, while minimizing isocyanide polymerization to provide the desired heterocycle-tetrazole products in synthetically useful yields (up to 99%).

The Ugi three-component reaction and its variants

A broad variety of α-aminoamide-based compounds have been synthesized via the three-component version of the Ugi reaction (U-3CR) or by any of its variants (Ugi-Zhu-3CR, Orru-3CR, Ugi-4C-3CR, Ugi-Joullié-3CR, GBB-3CR, Ugi-Reissert-3CR, and so on).

5-Endo Trig Oxidative Radical Cyclizations of Ugi-3CR Products toward 1,4-Imidazolidinones

A 5-endo trig oxidative radical cyclization of benzylamine-derived Ugi three-component reaction products rapidly affords imidazolidinones with three diversity elements. This adaptation of our previously described multicomponent reaction-oxidation methodology further showcases manipulation of the diversity elements in multicomponent reaction products via oxidative radical cyclizations, which generates highly decorated privileged heterocycles.

Synthesis of N-heterocycles containing 1,5-disubstituted-1H-tetrazole via post-Ugi-azide reaction

Ugi-azide four-component reaction (UA-4CR) as development on Ugi four-component reaction (U-4CR) is the condensation reaction involving an aldehyde, an amine, an isocyanide, and an azide source. Nowadays, UA-4CR has been employed for the efficient and facile production of 1,5-disubstituted-1H-tetrazoles (1,5-DS-1H-Ts). Interestingly, the combination of 1,5-DS-1H-Ts with suitable post-transformations in a tandem manner results in the construction of various classes of heterocyclic compounds bearing 1,5-DS-1H-T moiety. This review aims to provide the application of diverse post-Ugi-azide reaction in the preparation of different N-heterocyclic compounds bearing 1,5-DS-1H-T such as substituted and fused 1,5-DS-1H-Ts.

Synthesis of a novel category of pseudo-peptides using an Ugi three-component reaction of levulinic acid as bifunctional substrate, amines, and amino acid-based isocyanides

The synthesis of a novel category of pseudo-peptides via intramolecular Ugi reaction of levulinic acid (4-oxopentanoic acid), aromatic and aliphatic amines, and amino acid-based isocyanides is reported. Levulinic acid was applied as a bifunctional substrate containing both carbonyl and acid moieties suitable for the Ugi reaction. This article provides a facile and convenient one-pot procedure for the synthesis of peptide-like heterocyclic molecules containing 2-pyrrolidone (γ-lactam), amide and ester functional groups with good to excellent yields.

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.

Efficient syntheses and anti-cancer activity of xenortides A-D including ent/epi-stereoisomers

A one-pot, two-step, total synthesis of naturally occurring xenortides A, B, C and D, (Xens A-D) isolated from the bacterium Xenorhabdus nematophila, and an entire complementary set of stereoisomers, has been achieved. Compounds were synthesized utilizing an isocyanide-based Ugi 4-CR followed by facile N-Boc deprotection. The reaction sequence took advantage of the chiral pool of N-Boc protected amino acids (l-Leu/Val and d-Leu/Val) with aryl isocyanides, phenyl acetaldehyde and methylamine giving the desired Xens A-D (A and B >98% ee) and all subsequent stereoisomers in reasonable yields upon deprotection followed by separation of diastereomers. Also, detailed mechanistic insights for diastereoselectivity of (-)-Xen A, as a model in the Ugi 4-CR, has been described. Moreover, for the first time, this focused library was screened for cytotoxicity against a panel of epithelial cancer cell lines as well as normal cell lines with an MTT proliferation assay. The structure-activity relationship (SAR) study demonstrated that tryptamides Xen B and D were more active than phenylethylamides Xen A and C. Furthermore, (-)-Xen B (IC50 = 19-25 μM) and ent-(+)-Xen D (IC50 = 21-26 μM) gave the highest cytotoxicity and they were also found to be non-toxic toward normal cells. Importantly, the SAR results indicate that the stereochemistry at C8 and C11 in (-)-Xen B and ent-(+)-Xen D play a critical role in cytotoxic activity.

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.

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.