Lewis acid-catalyzed formation of Ugi four-component reaction product from Passerini three-component reaction system without an added amine

First-row transition metal for isocyanide-involving multicomponent reactions (IMCR)

First-row transition metal catalyzed transformations that are able to construct complex molecules from simple, readily obtainable feedstocks have become a keystone of modern synthetic organic chemistry. Particularly, the multicomponent reaction (MCR) involving carbon-carbon (C-C) as well as carbon-heteroatom (C-X) bond formation plays an essential role in many chemical conversions, and insurgencies in these reactions powerfully improve the overall synthetic efficiency. Recently, MCRs emerges rapidly because of its greener sides like eco-friendly nature, swift and straightforward execution, high atom/step economy, and construction of aimed product with lowest or no by-product, usually in quantitative yield. Curiously, the exceptional divalent carbon atoms of isocyanides make them predominantly useful components in multicomponent reactions. As a result of widespread research over the past few decades, numerous well-designed and effective procedures for the first-row TM-catalyzed MCR to afford the various entities have been reported. These aspects are summarized in this review article. A particular focus on comparative discussion of various first-row transition-metal catalyzed isocyanide-based multicomponent reactions through mechanistic details included in the review article.

Selective Synthesis of 3-Substituted Pyrrolidinones by Enol-Passerini and Anomalous Enol-Passerini Condensations

Enols are used for the first time in a condensation with aldehydes and isocyanides to selectively give three- or pseudo-four-component adducts, depending on the reaction conditions. These new transformations have proven to be a convenient alternative for the synthesis of biologically relevant pyrrolidinones containing peptidic or pseudo-peptidic groups on carbon 3. More importantly, this work attests to the utility of heterocyclic enols containing conjugated electron-withdrawing groups as useful reagents in isocyanide-based multicomponent reactions.

Two Cycles with One Catch: Hydrazine in Ugi 4-CR and Its Postcyclizations

Isocyanide-based multicomponent reactions (IMCR) are by far the most versatile reactions that can construct relatively complex molecules by one-pot synthesis. More importantly, the development of post IMCR modifications significantly improves the scaffold’s diversity. Here, we describe the use of N-Boc protected hydrazine together with α-amino acid derived isocyanides in the Ugi tetrazole reaction and its post cyclization under both acidic and basic conditions. The cyclization in acidic conditions was conducted in a one pot fashion, which give 7-aminotetrazolopyrazinone (6) and tetrazolotriazepinone (7) cyclic products. The post cyclization under basic condition could selectively afford Boc-protected 7-aminotetrazolopyrazinone (8) products in yield of 38–87%.

Application of heterocyclic aldehydes as components in Ugi-Smiles couplings

Efficient one-pot Ugi–Smiles couplings are reported for the use of furyl-substituted aldehyde components. In the presence of these heterocyclic aldehydes, reactions tolerated variations in amine components and led to either isolated N-arylamide Ugi–Smiles adducts or N-arylepoxyisoindolines, products of tandem Ugi–Smiles Diels–Alder cyclizations, in moderate yields. A thienyl-substituted aldehyde was also a competent component for Ugi–Smiles adduct formation.

Radical Smiles Rearrangement: An Update

Over the decades the Smiles rearrangement and its variants have become essential synthetic tools in modern synthetic organic chemistry. In this mini-review we summarized some very recent results of the radical version of these rearrangements. The selected examples illustrate the synthetic power of this approach, especially if it is incorporated into a domino process, for the preparation of polyfunctionalized complex molecules.

OPHA (oxidation-Passerini-hydrolysis-alkylation) strategy: a four-step, one-pot improvement of the alkylative Passerini reaction

Multicomponent reactions are often recognized for their efficiency and convergency, if compared with multistep organic synthesis. Nevertheless, we here demonstrate that a four-step-one-pot approach (named OPHA strategy for the initials of the four steps involved) is not only able to afford compounds that could not be obtained by an alkylative Passerini reaction but also capable of generating them with minimal loss of atoms and high operational simplicity, as in a typical multicomponent approach.

Three-component coupling approach for the synthesis of diverse heterocycles utilizing reactive nitrilium trapping

The formation of an unexpected heterocyclic scaffold, a benzoxazole, in a three-component reaction between a ketone, isocyanide, and 2-aminophenol was encountered. This reaction involved a benzo[b][1,4]oxazine intermediate resulting from intramolecular attack of the aminophenol hydroxyl group on the nitrilium ion. Unlike previous literature examples, the trapped nitrilium benzo[b][1,4]oxazine could readily be subjected to ring opening with bis-nucleophiles. The reaction scope includes simple linear as well as complex cyclic ketones and substituted 2-aminophenols. A representative benzoxazole product could be further diversified to yield drug-like compounds.

N-Benzyl-2-(2-bromo-phen-yl)-2-(2-nitro-phen-oxy)acetamide

The title compound, C₂₁H₁₇BrN₂O₄, a 2-phen­oxy-2-phenyl­acetamide derivative, exhibits a stereogenic center but crystallizes as a racemate as indicated by the centrosymmetric space group. In the mol­ecular structure, the nitro-substituted benzene ring is coplanar [dihedral angle = 12.9 (1)°] with the plane formed by H—N—C(=O)—C=O due to intra­molecular N—H⋯O hydrogen-bond inter­actions.

Ugi-Smiles couplings: new entries to N-aryl carboxamide derivatives

The scope of the Ugi reaction has been extended by the use of phenols as carboxylic acid surrogates to afford N-aryl carboxamides. A Smiles rearrangement occurs as the last step of the mechanism instead of the classical final Mumm process. Various parameters concerning the nature of these inputs have been studied: the use of heteroaromatic derivatives, the substitution of the hydroxyl moiety by a thio entity (to afford functionalized thioamides), as well as the influence of the nature and position of substituents on the phenol. A three-component version (Passerini-Smiles) of this coupling has been developed as well. Following these couplings, various post-condensation transformations have been performed to reach more complex heteroaromatic fused systems. The easy functionalizations of phenols offer many opportunities for cyclization strategies: the reduction of the nitro group allows the formation of o-phenylenediamine derivatives, which, in turn, can be transformed into quinoxalines, benzotriazoles, and benzimidazoles. Various organometallic reactions of the Ugi-Smiles adducts have been successfully carried out, either from iodophenols (Heck couplings to give indoles, Ullmann reaction to form quinoxalines), or from allyl pyrimidines (azepine formation by RCM strategy) as starting phenol inputs. Finally, a new palladium-mediated oxidative cyclization led to the formation of tricyclic systems.