Daniel G. Rivera

Multiple Multicomponent Macrocyclizations (MiBs): A Strategic Development Toward Macrocycle Diversity

Macrocycles are of high significance in areas as diverse as drug development and supramolecular chemistry. They can be considered as privileged molecules because they can combine flexibility and conformational bias. They allow a certain conformational adaptation for binding and at the same time can have an improved overall energy term while binding, compared to linear molecules. Recently, a diversityoriented macrocyclization strategy termed multiple multicomponent macrocyclization including bifunctional building blocks (MiB) was developed which allows producing constitutionally diverse and complex macrocycles from simple building blocks in one pot. The efficient search for novel molecular ligands of biological targets remains a continuing goal in drug discovery and chemical biology.1-3 In the past, the predominant interest of medicinal chemists in synthetic ligands has been devoted to small rings (especially heterocycles) because of their known capability to interact with defined protein motifs and their ease of preparation. Huge libraries, including combinatorial ones,4,5 have been synthesized by means of wellestablished processes and screened for biological activity. Lately, macrocycles have attracted increasing attention also by virtue of both their high success rate in medicinal and recognition chemistry and their widespread occurrence in nature.6-10 The demand for bioactive compounds with new application profiles has triggered the search for molecules with biological features that simple 5/6/7-ring (hetero)cycles do not bear.8-12 Macrocycles are usually endowed with a proper combination of more than one binding domain, conformational preorganization, and flexibility.8,13,14 Their structural, physicochemical, and biological features provide recognition and binding properties not found in linear or small ring counterparts.14 For example, their often increased biological stability compared to acyclic analogues (e.g., cyclopeptides compared to peptides) makes them a fascinating paradigm to design biologically active molecules.8-14 Combinatorial synthetic chemistry in the macrocycle field does not yet reflect the tremendous impact of naturally occurring macrocycles in areas such as antibiotics, immunosuppressants, ion chelators, or membrane active compounds, where their success rate appears to be overproportional (in relative terms) compared to other drug types.6-9,15

Multiple multicomponent macrocyclizations including bifunctional building blocks (MiBs) based on Staudinger and Passerini three-component reactions.

Multiple multicomponent macrocyclizations including bifunctional buildings blocks (MiBs) so far have relied almost exclusively on Ugi reactions. The efficient expansion to non-Ugi-MiBs is exemplified by the synthesis of tetra-beta-lactam and bis-alpha-acyloxy carboxamide macrocycles based on multiple Staudinger and Passerini three-component reactions (3CR), respectively. A recent variation of the Passerini-3CR that involves primary alcohols, isocyanides, and carboxylic acids under oxidative conditions is successfully adapted to this procedure.

Terpene‐Derived Bifunctional Thioureas in Asymmetric Organocatalysis

Chiral hydrogen‐bond donors have played a crucial role in the progress of asymmetric catalysis as an essential tool in modern organic synthesis. Among these donors, thioureas have been found to be one of the most powerful systems in organocatalytic approaches. Within the last three years, a new generation of bifunctional thiourea catalysts has emerged as an effective way to promote highly demanding chemical transformations, frequently with success rates that are intriguingly higher than with other traditional thioureas. This new class of organocatalysts incorporates a bulky, rigid, and chiral terpene skeleton as one of the thiourea substituents, thus providing an additional and effective rigidifying (entropic) effect on the electrophile activation profile of thioureas. Structural integrations, either with additional Lewis basic nucleophile‐activating motifs or with amino groups for enamine catalysis, have enabled their implementation in a wide repertoire of stereocontrolled catalytic processes. This Review charts the development of these new catalysts, as well as their applications in a wide variety of reactions and reaction sequences, with the hope of encouraging further progress in the field of thiourea‐catalyst discovery and of bringing attention to the countless possibilities that are opened up by the tremendous success of terpene‐derived thiourea catalysts.

Rapid Access to N-Substituted Diketopiperazines by One-Pot Ugi-4CR/Deprotection+Activation/Cyclization (UDAC)

The most efficient diversity generating approaches to heterocycles are combinations of a multicomponent (MCR) with a cyclization reaction, for example, by Ugi-deprotection-cylization (UDC) protocols. If the desired post-Ugi reaction requires more than one deprotection, for example of two initially protected Ugi-reactive groups, or if it requires additional activation, for example, by an Ugi-activation-cyclization (UAC), either the isolation of intermediates or a sequential process or both become necessary. A recently introduced convertible isonitrile reagent allows a mild and chemoselective in situ post-Ugi activation of the isonitrile-born carboxylate with simultaneous deprotection of the nucleophilic amine, that is, liberation and activation of two Ugi-reactive groups, if desired also under subsequent lactam formation. This is exemplified by the synthesis of peptide-peptoid diketopiperazines.

A Multiple Multicomponent Approach to Chimeric Peptide–Peptoid Podands

The success of multi-armed, peptide-based receptors in supramolecular chemistry traditionally is not only based on the sequence but equally on an appropriate positioning of various peptidic chains to create a multivalent array of binding elements. As a faster, more versatile and alternative access toward (pseudo)peptidic receptors, a new approach based on multiple Ugi four-component reactions (Ugi-4CR) is proposed as a means of simultaneously incorporating several binding and catalytic elements into organizing scaffolds. By employing α-amino acids either as the amino or acid components of the Ugi-4CRs, this multiple multicomponent process allows for the one-pot assembly of podands bearing chimeric peptide-peptoid chains as appended arms. Tripodal, bowl-shaped, and concave polyfunctional skeletons are employed as topologically varied platforms for positioning the multiple peptidic chains formed by Ugi-4CRs. In a similar approach, steroidal building blocks with several axially-oriented isocyano groups are synthesized and utilized to align the chimeric chains with conformational constrains, thus providing an alternative to the classical peptido-steroidal receptors. The branched and hybrid peptide-peptoid appendages allow new possibilities for both rational design and combinatorial production of synthetic receptors. The concept is also expandable to other multicomponent reactions.

Multicomponent Combinatorial Development and Conformational Analysis of Prolyl Peptide-Peptoid Hybrid Catalysts: Application in the Direct Asymmetric Michael Addition

A solution-phase combinatorial approach based on the Ugi four-component reaction was implemented for the development of new prolyl peptide-peptoid hybrid catalysts. Three different elements of diversity were varied during the creation of the set of catalysts: the amine, oxo, and isocyano components. The multicomponent nature of this process enabled the straightforward generation of a series of peptide-peptoid hybrids having the generic sequence Pro-N-R(1)-Xaa-NHR(3), with Xaa being either Gly (R(2) = H) or Aib (R(2) = gem-Me) and R(1) and R(3) either alkyl or amino acid substituents. The catalytic behavior of the peptide-peptoid hybrids was assessed in the asymmetric conjugate addition of aldehydes to nitroolefins, where most of the catalysts showed great efficacy and rendered the Michael adducts with good to excellent enantio- and diastereoselectivity. A molecular modeling study was performed for two distinct catalysts aiming to understand their conformational features. The conformational analysis provided important information for understanding the remarkable stereocontrol achieved during the organocatalytic transformation.

Multicomponent synthesis of Ugi-type ceramide analogues and neoglycolipids from lipidic isocyanides.

Unique types of ceramide and glycolipid architectures were obtained by means of Ugi reactions incorporating lipidic isocyanides as surrogates of sphingolipids. The multicomponent nature of this approach allowed for a highly efficient assembly process, wherein two of the components provided the lipidic tails while a third one incorporated either the functionality suitable for the conjugation to sugar or the sugar moiety itself. Two dissimilar strategies were implemented: (i) the initial assembly of ceramide analogues followed by glycosylation to produce a glycolipid skeleton and (ii) the one-pot construction of glycolipid frameworks by condensation of lipidic isocyanides either with lipidic amines and oligosaccharidic acids or with fatty acids and oligosaccharidic amines. Whereas both approaches are amenable for accessing analogues of anticancer glycolipids, the latter one proved to have greater potential owing to its more straightforward and efficient character. Overall, the methodology developed shows great promise toward the massive (eventually combinatorial) production of neoglycolipids suitable for biological screening.

Macrocyclization of Peptide Side Chains by the Ugi Reaction: Achieving Peptide Folding and Exocyclic N-Functionalization in One Shot.

The cyclization of peptide side chains has been traditionally used to either induce or stabilize secondary structures (β-strands, helices, reverse turns) in short peptide sequences. So far, classic peptide coupling, nucleophilic substitution, olefin metathesis, and click reactions have been the methods of choice to fold synthetic peptides by means of macrocyclization. This article describes the utilization of the Ugi reaction for the side chain-to-side chain and side chain-to-termini macrocyclization of peptides, thus enabling not only access to stable folded structures but also the incorporation of exocyclic functionalities as N-substituents. Analysis of the NMR-derived structures revealed the formation of helical turns, β-bulges, and α-turns in cyclic peptides cross-linked at i, i + 3 and i, i + 4 positions, proving the folding effect of the multicomponent Ugi macrocyclization. Molecular dynamics simulation provided further insights on the stability and molecular motion of the side chain cross-linked peptides.

Highly Stereoselective Synthesis of Natural‐Product‐Like Hybrids by an Organocatalytic/Multicomponent Reaction Sequence

In an endeavor to provide an efficient route to natural product hybrids, described herein is an efficient, highly stereoselective, one-pot process comprising an organocatalytic conjugate addition of 1,3-dicarbonyls to α,β-unsaturated aldehydes followed by an intramolecular isocyanide-based multicomponent reaction. This approach enables the rapid assembly of complex natural product hybrids including up to four different molecular fragments, such as hydroquinolinone, chromene, piperidine, peptide, lipid, and glycoside moieties. The strategy combines the stereocontrol of organocatalysis with the diversity-generating character of multicomponent reactions, thus leading to structurally unique peptidomimetics integrating heterocyclic, lipidic, and sugar moieties.

Cyclic Peptidomimetics and Pseudopeptides from Multicomponent Reactions

Multicomponent reactions (MCRs) that provide in the final product amides are suitable to produce peptides and peptide-like moieties. The Passerini and Staudinger reactions provide one amide bond, and the Ugi-four-component reaction generates two amides from three or even four (or more) components, respectively. The Ugi-reaction thus is most important to produce peptides and peptoids while the Passerini reaction is useful to generate depsipeptoid moieties. In order to produce cyclic peptides and pseudopeptides, the linear peptidic MCR products have to be cyclized, usually with the help of bifunctional or activatable building blocks. Orthogonal but cyclizable secondary functionalities that need no protection in isonitrile MCRs commonly include alkenes (for ring closing metathesis), azide/alkyne (for Huisgen click reactions) or dienes and enoates (Diels-Alder) etc. If MCR-reactive groups are to be used also for the cyclisation, monoprotected bifunctional building blocks are used and deprotected after the MCR, e.g. for Ugi reactions as Ugi-Deprotection-Cyclisation (UDC). Alternatively one of the former building blocks or functional groups generated by the MCR can be activated. Most commonly these are activated amides (from so-called convertible isonitriles) which can be used e.g. for Ugi-Activation-Cyclisation (UAC) protocols, or most recently for a simultaneous use of both strategies Ugi-Deprotection/Activation-Cyclisation (UDAC). These methods mostly lead to small, medicinally relevant peptide turn mimics. In an opposing strategy, the MCR is rather used as ring-closing reaction, thereby introducing a (di-)peptide moiety. Most recently these processes have been combined to use MCRs for both, linear precursor synthesis and cyclisation. These multiple MCR approaches allow the most efficient and versatile one pot synthesis of macrocyclic pseudopeptides known to date.