In situ generation of imines by the Staudinger/aza-Wittig tandem reaction combined with thermally induced Wolff rearrangement for one-pot three-component β-lactam synthesis

A new efficient protocol for diastereoselective three-component one-pot lactam synthesis involving the in situ generation of imines via the Staudinger/aza-Wittig tandem reaction combined with the Wolff-rearrangement and ketene-imine cycloaddition was developed to produce a series of 24 novel structurally diverse β-lactam- or 1,3-oxazine-products. It was shown that this synthesis can be performed both as a two step-procedure and true MCR with simultaneous loading of all reactants. The intramolecular version of the 1^st step provided facile access to seven-membered cyclic imines, which allowed further preparation of a series of rare tricyclic β-lactams. For the intermolecular version of the 1^st step (acyclic imine generation), it was shown that the outcome of the synthesis is different from that using pre-synthesized and isolated imines. Additionally, this is the first example of the implementation of the Staudinger/aza-Wittig tandem reaction for the preparation of four-membered heterocycles.

Structural diversity using amino acid "Customizable Units": conversion of hydroxyproline (Hyp) into nitrogen heterocycles

The ability of amino acid “customizable units” to generate structural diversity is illustrated by the conversion of 4-hydroxyproline (Hyp) units into a variety of nitrogen heterocycles. After a first common step, where the unit underwent a one-pot decarboxylation–alkylation reaction to afford 2-alkylpyrrolidines with high stereoselectivity, a divergent step was carried out. Thus, the deprotected 4-hydroxy group was used either to initiate a radical scission that afforded aliphatic β-amino aldehydes, or to carry out an elimination reaction, to give 2-alkyl-2,5-dihydro-1H-pyrroles. In the first case, the amines underwent a tandem reductive amination–cyclization to afford β-amino-δ-lactams, an efficient rigidifying unit in peptides. Different lactam N-substituents, such as alkylamines, peptides, and alkenyl chains suitable for olefin metathesis were introduced this way. In the second case, the pyrrole derivatives were efficiently converted into alkaloid and iminosugar derivatives in good global yields and with excellent stereoselectivity.

Catalytic Enantioselective Isocyanide-Based Reactions: Beyond Passerini and Ugi Multicomponent Reactions

The development of catalytic enantioselective isocyanide-based reactions is currently of great interest because the resulting products are valuable in organic synthesis, pharmacological chemistry, and materials science. This review assembles and comprehensively summarizes the recent achievements in this rapidly growing area according to the reaction types. Special attention is paid to the advantages, limitations, possible mechanisms, and synthetic applications of each reaction. In addition, a personal outlook on the opportunities for further exploration is given at the end.

"Doubly Customizable" Unit for the Generation of Structural Diversity: From Pure Enantiomeric Amines to Peptide Derivatives

Readily available, low-cost 4R-hydroxy-l-proline (Hyp) is introduced as a "doubly customizable" unit for the generation of libraries of structurally diverse compounds. Hyp can be cleaved at two points, followed by the introduction of new functionalities. In the first cycle, the removal and replacement of the carboxylic group are carried out, followed (second cycle) by the scission of the 4,5-position and manipulation of the resulting chains. In this way, three new chains are generated and can be transformed independently to afford a diversity of products with tailored substituents, such as β-amino aldehydes, diamines, β-amino acid derivatives, including N-alkylated ones, or modified peptides. Many of these products are high-profit compounds but, in spite of their commercial value, are still scarce. Moreover, the process takes place with stereochemical control, and either pure R or S isomers can be obtained with small variations of the synthetic route.

Cyclic Imines in Ugi and Ugi-Type Reactions

Ugi four-component reactions (U-4CRs) are widely recognized as being highly efficient for the synthesis of pseudopeptides. However, the products of these reactions are not so interesting as drug candidates because they are not conformationally restricted enough for a potent interaction with biological targets. One possible way to overcome this problem is to replace amine and oxo components in the U-4CRs with cyclic imines in so-called Joullié-Ugi three-component reactions (JU-3CRs). This approach provides a robust single-step route to peptide moieties connected to N-heterocyclic motifs that are found as core skeletons in many natural products and pharmaceutical compounds. JU-3CRs also provide much better diastereoselectivity than their four-component analogues. We survey here the redesign of many synthetic routes for the efficient preparation of a wide variety of three-, five-, six-, and seven-membered heterocyclic compounds connected to the peptide backbone. Additionally, in the Ugi reactions based on the cyclic imines, α-acidic isocyanides, or azides can be replaced with normal isocyanides or acids, respectively, leading to the synthesis of N-heterocycles attached to oxazoles or tetrazoles, which are of great pharmaceutical significance. This Review includes all research articles related to Ugi reactions based on the cyclic imines to the year 2020 and will be useful to chemists in designing novel synthetic routes for the synthesis of individual and combinatorial libraries of natural products and drug-like compounds.

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%.

Metal-free, direct conversion of α-amino acids into α-keto γ-amino esters for the synthesis of α,γ-peptides

An efficient, metal-free synthesis of unusual α-keto γ-amino esters from α-amino acids is achieved by a radical scission–oxidation–addition of silyloxy acrylates procedure, where no purification of the reaction intermediates is needed. The process allows the synthesis of α,γ-peptides.

A N,N'-dioxide/Mg(OTf)2 complex catalyzed enantioselective α-addition of isocyanides to alkylidene malonates

A simple and efficient catalytic asymmetric α-addition of isocyanides to alkylidene malonates was realized using a chiral N,N′-dioxide/Mg^II catalyst. A variety of 2-alkyl-5-aminoxazoles were obtained in up to 99% yield and 96% ee, and could be converted to imides and dipeptides.

A highly efficient catalytic asymmetric α-addition of isocyanides to alkylidene malonates was accomplished. The process was based on the utilization of a chiral N,N′-dioxide/Mg^II catalyst, delivering a variety of 2-alkyl-5-aminooxazoles in up to 99% yield and 96% ee under mild reaction conditions. Besides, a chiral imide and dipeptide could be easily obtained by ring-opening of the oxazole product, both of which are important structural motifs for many biologically active compounds. Based on the experimental investigations and previous work, a possible transition state model was proposed.

Diastereoselective Passerini Reaction of Biobased Chiral Aldehydes: Divergent Synthesis of Various Polyfunctionalized Heterocycles

Lewis acid catalyzed Passerini reactions on chiral aldehydes derived from desymmetrized erythritol take place with unprecedented diastereoselectivity. The resulting adducts have been selectively and efficiently converted into a variety of densely functionalized, polyoxygenated heterocycles.

Amino-functionalized iminocyclitols: synthetic glycomimetics of medicinal interest

A review on the syntheses and biological activities of unnatural glycomimetics highlighting the effect of replacement of hydroxyl groups of natural iminosugars by amino functionalities is presented.

Stereoselective synthesis of functionalized 1,2,3,4-tetrahydroisoquinolines (THIQs) via highly diastereoselective Ugi three-component reactions (U3CRs) with chiral 3,4-dihydroisoquinolines (DHIQs)

A highly diastereoselective Ugi three-component reaction (U3CR) of chiral 3,4-dihydroisoquinolines has been developed to synthesize enantiopure 1,2,3,4-tetrahydroisoquinolines (THIQs).

Total synthesis of sandramycin and its analogues via a multicomponent assemblage

The total synthesis of sandramycin has been accomplished by using a Staudinger/aza-Wittig/diastereoselective Ugi three-component reaction sequence as a key step to obtain a linear pentadepsipeptide. Subsequent [5 + 5] coupling of the penptapeptide, macrolactamization, and introduction of the quinaldin chromophores afforded sandramycin. Dihydroxy and diacetoxy analogues were also prepared, and the cytotoxic activity of these analogues against a range of human cancer cell lines was evaluated.

Recent developments in asymmetric multicomponent reactions

Multicomponent reactions (MCRs) receive increasing attention because they address both diversity and complexity in organic synthesis. Thus, in principle diverse sets of relatively complex structures can be generated from simple starting materials in a single reaction step. The ever increasing need for optically pure compounds for pharmaceutical and agricultural applications as well as for catalysis promotes the development of asymmetric multicomponent reactions. In recent years, asymmetric multicomponent reactions have been applied to the total synthesis of various enantiopure natural products and commercial drugs, reducing the number of required reaction steps significantly. Although many developments in diastereoselective MCRs have been reported, the field of catalytic enantioselective MCRs has just started to blossom. This critical review describes developments in both diastereoselective and catalytic enantioselective multicomponent reactions since 2004. Significantly broadened scopes, new techniques, more environmentally benign methods and entirely novel MCRs reflect the increasingly inventive paths that synthetic chemist follow in this field. Until recently, enantioselective transition metal-catalyzed MCRs represented the majority of catalytic enantioselective MCRs. However, metal contamination is highly undesirable for drug synthesis. The emergence of organocatalysis greatly influences the quest for new asymmetric MCRs.

Long-range diastereoselectivity in Ugi reactions of 2-substituted dihydrobenzoxazepines

The Ugi reaction of 2-substituted dihydrobenzoxazepines was found to proceed with unexpectedly good diastereoselectivitiy (diastereoisomeric ratios up to 9:1), despite the large distance between the pre-existing stereogenic centre and the newly generated one. This result represents the first good 1,4 asymmetric induction in an Ugi reaction as well as the first example of diastereoselective Ugi reaction of seven membered cyclic imines. It allows the diversity-oriented synthesis of various tetrahydro[f][1,4]benzoxazepines.