5-Aminooxazole as an Internal Traceless Activator of C-Terminal Carboxylic Acid: Rapid Access to Diversely Functionalized Cyclodepsipeptides

A Comparative Synthetic Strategy Perspective on α-Amino Acid- and Non-Amino Acid-Derived Synthons towards Total Syntheses of Selected Natural Macrolides

Macrocyclic alkaloids (macrolides) and cyclopeptides have an immense range of applications in drug discovery research because of their natural abundance and potential biological and physicochemical properties. Presently, more than 100 approved drugs or clinical drug candidates contain macrocyclic scaffolds as the biologically active component. This review provides an interesting perspective about the use of amino acid-derived chiral pools versus other methods derived from miscellaneous synthons towards the total synthesis of non-peptidic macrolides. The synthetic routes and the key strategies involved in the total syntheses of ten natural macrolides have been discussed. Both the amino acid-derived and non-amino acid-derived synthetic routes have been illustrated to present a comparative study between the two approaches.

Multicomponent Reaction Toolbox for Peptide Macrocyclization and Stapling

In the past decade, multicomponent reactions have experienced a renaissance as powerful peptide macrocyclization tools enabling the rapid creation of skeletal complexity and diversity with low synthetic cost. This review provides both a historical and modern overview of the development of the peptide multicomponent macrocyclization as a strategy capable to compete with the classic peptide cyclization methods in terms of chemical efficiency and synthetic scope. We prove that the utilization of multicomponent reactions for cyclizing peptides by either their termini or side chains provides a key advantage over those more established methods; that is, the possibility to explore the cyclic peptide chemotype space not only at the amino acid sequence but also at the ring-forming moiety. Owing to its multicomponent nature, this type of peptide cyclization process is well-suited to generate diversity at both the endo- and exo-cyclic fragments formed during the ring-closing step, which stands as a distinctive and useful characteristic for the creation and screening of cyclic peptide libraries. Examples of the novel multicomponent peptide stapling approach and heterocycle ring-forming macrocyclizations are included, along with multicomponent methods incorporating macrocyclization handles and the one-pot syntheses of macromulticyclic peptide cages. Interesting applications of this strategy in the field of drug discovery and chemical biology are provided.

Recent advances in the synthesis of cyclic pseudopeptides

Constrained peptides pose tremendous value in drug discovery. For example, owing to their large surface areas, they offer novel ways at inhibiting protein-protein interactions. As this field has grown, it has become desirable to introduce non-peptidic functionality into these rings to enable differentiated structure activity relationships and improved pharmacokinetic properties. Recent advances in the synthesis of cyclic pseudopeptides include macrocyclization through cysteine alkylation, multicomponent reactions, decarboxylative couplings, and novel stapling chemistry.

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.

Mechanistic investigation of aziridine aldehyde-driven peptide macrocyclization: the imidoanhydride pathway

Aziridine aldehyde dimers, peptides, and isocyanides participate in a multicomponent reaction to yield peptide macrocycles. We have investigated the selectivity and kinetics of this process and performed a detailed analysis of its chemoselectivity. While the reactants encompass all of the elements of the traditional Ugi four-component condensation, there is a significant deviation from the previously proposed mechanism. Our results provide evidence for an imidoanhydride pathway in peptide macrocyclization and lend justification for the diastereoselectivity and high effective molarity observed in the reaction.

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

In the recent past, the design and synthesis of peptide mimics (peptidomimetics) has received much attention. This because they have shown in many cases enhanced pharmacological properties over their natural peptide analogues. In particular, the incorporation of cyclic constructs into peptides is of high interest as they reduce the flexibility of the peptide enhancing often affinity for a certain receptor. Moreover, these cyclic mimics force the molecule into a well-defined secondary structure. Constraint structural and conformational features are often found in biological active peptides. For the synthesis of cyclic constrained peptidomimetics usually a sequence of multiple reactions has been applied, which makes it difficult to easily introduce structural diversity necessary for fine tuning the biological activity. A promising approach to tackle this problem is the use of multicomponent reactions (MCRs), because they can introduce both structural diversity and molecular complexity in only one step. Among the MCRs, the isocyanide-based multicomponent reactions (IMCRs) are most relevant for the synthesis of peptidomimetics because they provide peptide-like products. However, these IMCRs usually give linear products and in order to obtain cyclic constrained peptidomimetics, the acyclic products have to be cyclized via additional cyclization strategies. This is possible via incorporation of bifunctional substrates into the initial IMCR. Examples of such bifunctional groups are N-protected amino acids, convertible isocyanides or MCR-components that bear an additional alkene, alkyne or azide moiety and can be cyclized via either a deprotection-cyclization strategy, a ring-closing metathesis, a 1,3-dipolar cycloaddition or even via a sequence of multiple multicomponent reactions. The sequential IMCR-cyclization reactions can afford small cyclic peptide mimics (ranging from four- to seven-membered rings), medium-sized cyclic constructs or peptidic macrocycles (>12 membered rings). This review describes the developments since 2002 of IMCRs-cyclization strategies towards a wide variety of small cyclic mimics, medium sized cyclic constructs and macrocyclic peptidomimetics.

Exploiting the divergent reactivity of α-isocyanoacetate: multicomponent synthesis of 5-alkoxyoxazoles and related heterocycles

A novel multicomponent synthesis of 5-alkoxyoxazoles, based on a new reactivity profile of α-isocyanoacetates, is described. Thus, simply heating a solution of amine, aldehyde, and α-(EWG-phenyl)-α-isocyanoacetate or α-(4-pyridyl)-α-isocyanoacetate (EWG=electron-withdrawing group) in toluene provided 5-alkoxyoxazoles in good to excellent yields. Reaction of the 5-alkoxyoxazoles with various α,β-unsaturated acyl chlorides led to the formation of epoxytetrahydropyrrolo[3,4-b]pyridin-5-ones by a domino N-acylation/Diels-Alder cycloaddition sequence. Subsequent fragmentation under basic conditions provided 6,7-dihydro-5H-pyrrolo[3,4-b]pyridin-5-ones. A four-component synthesis of the pyridin-5-one compounds, without isolation of the 5-alkoxyoxazole, was subsequently developed.

Ammonium chloride promoted three-component synthesis of 5-iminooxazoline and its subsequent transformation to macrocyclodepsipeptide

A three-component reaction of an alpha,alpha-disubstituted alpha-isocyanoacetamide, an aldehyde and an amino alcohol afforded the 5-iminooxazoline, which, upon saponification, cyclized under acidic conditions to provide the macrocyclodepsipeptide in good overall yield.

Rapid access to oxindoles by the combined use of an Ugi four-component reaction and a microwave-assisted intramolecular Buchwald-Hartwig amidation reaction

A two-step sequence involving an Ugi four-component reaction (Ugi-4CR) and a palladium-catalyzed intramolecular amidation of aryl iodide has been developed for rapid access to functionalized oxindole (1). Microwave heating was used to accelerate and to improve the efficiency of the intramolecular Buchwald-Hartwig reaction.

One-pot synthesis of macrocycles by a tandem three-component reaction and intramolecular [3+2] cycloaddition

By combining three appropriately designed simple substrates, a programmed sequence involving an alpha-isocyano acetamide-based three-component reaction followed by a copper-catalyzed intramolecular [3+2] cycloaddition of alkyne and azide took place to afford complex macrocycles in moderate to good yields. One macrocycle and two heterocycles were produced with concurrent formation of five chemical bonds in this operationally simple process.

Rapid Synthesis of Cyclodepsipeptides Containing a Sugar Amino Acid or a Sugar Amino Alcohol by a Sequence of a Multicomponent Reaction and Acid-Mediated Macrocyclization

Cyclodepsipeptides incorporating a sugar amino acid (alcohol) have been synthesized. A three-component reaction of a sugar amino acid (SAA) derivative, an aldehyde, and a dipeptide isonitrile in refluxing methanol afforded the corresponding 5-aminooxazole which, after saponification, underwent a trifluoroacetic acid promoted macrocyclization to furnish the cyclic sugar amino acids.