Ring-Closing Alkyne Metathesis Approach toward the Synthesis of Alkyne Mimics of Thioether A-, B-, C-, and DE-Ring Systems of the Lantibiotic Nisin Z

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple-bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, 95 Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.

The Ascent of Alkyne Metathesis to Strategy-Level Status

For numerous enabling features and strategic virtues, contemporary alkyne metathesis is increasingly recognized as a formidable synthetic tool. Central to this development was the remarkable evolution of the catalysts during the past decades. Molybdenum alkylidynes carrying (tripodal) silanolate ligands currently set the standards; their functional group compatibility is exceptional, even though they comprise an early transition metal in its highest oxidation state. Their performance is manifested in case studies in the realm of dynamic covalent chemistry, advanced applications to solid-phase synthesis, a revival of transannular reactions, and the assembly of complex target molecules at sites, which one may not intuitively trace back to an acetylenic ancestor. In parallel with these innovations in material science and organic synthesis, new insights into the mode of action of the most advanced catalysts were gained by computational means and the use of unconventional analytical tools such as ⁹⁵Mo and ¹⁸³W NMR spectroscopy. The remaining shortcomings, gaps, and desiderata in the field are also critically assessed.

Macrocyclization strategies for cyclic peptides and peptidomimetics

Peptides are a growing therapeutic class due to their unique spatial characteristics that can target traditionally "undruggable" protein-protein interactions and surfaces. Despite their advantages, peptides must overcome several key shortcomings to be considered as drug leads, including their high conformational flexibility and susceptibility to proteolytic cleavage. As a general approach for overcoming these challenges, macrocyclization of a linear peptide can usually improve these characteristics. Their synthetic accessibility makes peptide macrocycles very attractive, though traditional synthetic methods for macrocyclization can be challenging for peptides, especially for head-to-tail cyclization. This review provides an updated summary of the available macrocyclization chemistries, such as traditional lactam formation, azide-alkyne cycloadditions, ring-closing metathesis as well as unconventional cyclization reactions, and it is structured according to the obtained functional groups. Keeping peptide chemistry and screening in mind, the focus is given to reactions applicable in solution, on solid supports, and compatible with contemporary screening methods.

Alkyne-Bridged α-Conotoxin Vc1.1 Potently Reverses Mechanical Allodynia in Neuropathic Pain Models

Several Conus-derived venom peptides are promising lead compounds for the management of neuropathic pain, with α-conotoxins being of particular interest. Modification of the interlocked disulfide framework of α-conotoxin Vc1.1 has been achieved using on-resin alkyne metathesis. Although introduction of a metabolically stable alkyne motif significantly disrupts backbone topography, the structural modification generates a potent and selective GABAB receptor agonist that inhibits Cav2.2 channels and exhibits dose-dependent reversal of mechanical allodynia in a behavioral rat model of neuropathic pain. The findings herein support the hypothesis that analgesia can be achieved via activation of GABABRs expressed in dorsal root ganglion (DRG) sensory neurons.

Peptide macrocyclization by transition metal catalysis

Peptide macrocyclization has traditionally relied on lactam, lactone and disulfide bond-forming reactions that aim at introducing conformational constraints into small peptide sequences. With the advent of ruthenium-catalyzed ring-closing metathesis and copper-catalyzed alkyne-azide cycloaddition, peptide chemists embraced transition metal catalysis as a powerful macrocyclization tool with relevant applications in chemical biological and peptide drug discovery. This article provides a comprehensive overview of the reactivity and methodological diversification of metal-catalyzed peptide macrocyclization as a special class of late-stage peptide derivatization method. We report the evolution from classic palladium-catalyzed cross-coupling approaches to more modern oxidative versions based on C-H activation, heteroatom alkylation/arylation and annulation processes, in which aspects such as chemoselectivity and diversity generation at the ring-closing moiety became dominant over the last years. The transit from early cycloadditions and alkyne couplings as ring-closing steps to very recent 3d metal-catalyzed macrocyclization methods is highlighted. Similarly, the new trends in decarboxylative radical macrocyclizations and the interplay between photoredox and transition metal catalysis are included. This review charts future perspectives in the field hoping to encourage further progress and applications, while bringing attention to the countless possibilities available by diversifying not only the metal, but also the reactivity modes and tactics to bring peptide functional groups together and produce structurally diverse macrocycles.

Olefin Cross Metathesis as a Versatile Method for the Facile Functionalization of Porous Polymers

Olefin cross metathesis has been employed for the first time for the postpolymerization chemical modification of porous polymers. High quality microspheres of poly(divinylbenzene) were synthesized by the precipitation polymerization of divinylbenzene-55 in porogenic solvents, and the olefin cross metathesis reactions of the pendent (polymer-bound) vinyl groups not consumed by polymerization were performed with diverse coupling partners in dichloromethane using the Grubbs 2 catalyst, leading to microspheres decorated with a wide range of functional groups.

Clickable coupling of carboxylic acids and amines at room temperature mediated by SO2F2: a significant breakthrough for the construction of amides and peptide linkages

The construction of amide bonds and peptide linkages is one of the most fundamental transformations in all life processes and organic synthesis. The synthesis of structurally ubiquitous amide motifs is essential in the assembly of numerous important molecules such as peptides, proteins, alkaloids, pharmaceutical agents, polymers, ligands and agrochemicals. A method of SO2F2-mediated direct clickable coupling of carboxylic acids with amines was developed for the synthesis of a broad scope of amides in a simple, mild, highly efficient, robust and practical manner (>110 examples, >90% yields in most cases). The direct click reactions of acids and amines on a gram scale are also demonstrated using an extremely easy work-up and purification process of washing with 1 M aqueous HCl to provide the desired amides in greater than 99% purity and excellent yields.

Well-Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

Although alkyne metathesis has been known for 50 years, rapid progress in this field has mostly occurred during the last two decades. In this article, the development of several highly efficient and thoroughly studied alkyne metathesis catalysts is reviewed, which includes novel well-defined, in situ formed and heterogeneous systems. Various alkyne metathesis methodologies, including alkyne cross-metathesis (ACM), ring-closing alkyne metathesis (RCAM), cyclooligomerization, acyclic diyne metathesis polymerization (ADIMET), and ring-opening alkyne metathesis polymerization (ROAMP), are presented, and their application in natural product synthesis, materials science as well as supramolecular and polymer chemistry is discussed. Recent progress in the metathesis of diynes is also summarized, which gave rise to new methods such as ring-closing diyne metathesis (RCDM) and diyne cross-metathesis (DYCM).

Diversity-Oriented Synthesis of Cyclic Azapeptides by A3 -Macrocyclization Provides High-Affinity CD36-Modulating Peptidomimetics

Macrocyclization has enabled the use of peptides in drug discovery creating a need for methods to synthesize diverse peptide macrocycles. Azapeptides have advanced to clinically used drugs, however, few cyclic azapeptides have been studied. A multiple component "A³ -macrocyclization" strategy is described for the preparation of diverse cyclic azapeptides and is demonstrated by the synthesis of 15 growth hormone releasing hormone-6 (GHRP-6) analogs. Certain cyclic aza-GHRP-6 analogs exhibited unprecedented affinity for the CD36 receptor, and capacity to modulate Toll-like receptor agonist-induced overproduction of nitric oxide, and reduce pro-inflammatory cytokine and chemokine production in macrophages.

Orthogonal ring-closing alkyne and olefin metathesis for the synthesis of small GTPase-targeting bicyclic peptides

Bicyclic peptides are promising scaffolds for the development of inhibitors of biological targets that proved intractable by typical small molecules. So far, access to bioactive bicyclic peptide architectures is limited due to a lack of appropriate orthogonal ring-closing reactions. Here, we report chemically orthogonal ring-closing olefin (RCM) and alkyne metathesis (RCAM), which enable an efficient chemo- and regioselective synthesis of complex bicyclic peptide scaffolds with variable macrocycle geometries. We also demonstrate that the formed alkyne macrocycle can be functionalized subsequently. The orthogonal RCM/RCAM system was successfully used to evolve a monocyclic peptide inhibitor of the small GTPase Rab8 into a bicyclic ligand. This modified peptide shows the highest affinity for an activated Rab GTPase that has been reported so far. The RCM/RCAM-based formation of bicyclic peptides provides novel opportunities for the design of bioactive scaffolds suitable for the modulation of challenging protein targets.

Bicyclic peptides can inhibit biological targets hard to address with small molecules. Here, the authors combine two orthogonal ring-closing reactions to produce bicyclic peptides with improved bioactivity thereby providing a strategy that can greatly improve the structural diversity of such peptides.

Synthesis of nisin AB dicarba analogs using ring-closing metathesis: influence of sp(3) versus sp(2) hybridization of the α-carbon atom of residues dehydrobutyrine-2 and dehydroalanine-5 on the lipid II binding affinity

Herein the synthesis of two nisin AB dicarba analogs is described, focusing on amino acid modifications at positions 2 and 5. The nisin mimics were synthesized by a combination of solid phase synthesis of the linear peptides, followed by macrocyclization via ring-closing metathesis and fragment assembly by means of solution phase chemistry. The two N-terminal nisin AB-fragment mimics contain either the native dehydrobutyrine (Dhb)/dehydroalanine (Dha) amino acid residues or alanine at position 2 and 5, respectively. The native dehydrobutyrine at position 2 and dehydroalanine at position 5 were introduced as their precursors, namely threonine and serine, respectively, and subsequent dehydration was carried out by EDCI/CuCl as the condensing agent. Both AB-fragment mimics were analyzed in a lipid II binding assay and it was found that the Ala2/Ala5 AB-mimic (2) showed a reduced activity, while the Dhb2/Dha5 AB-mimic (3) was as active as the native AB-fragment (1).

Hit 'em where it hurts: The growing and structurally diverse family of peptides that target lipid-II

Understanding the mode of action of antibiotics is becoming more and more important in the time that microorganisms start to develop resistance. One very well validated target of several classes of antibiotics is the peptidoglycan precursor lipid II. In this review different classes of lipid II targeting antibiotics will be discussed in detail, including the lantibiotics, human invertebrate defensins and the recently discovered teixobactin. By hitting bacteria where it hurts, at the level of lipid II, we expect to be able to develop efficient antibacterial agents in the future. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Multipronged approach for engineering novel peptide analogues of existing lantibiotics

INTRODUCTION

Lantibiotics are a class of ribosomally and post-translationally modified peptide antibiotics that are active against a broad spectrum of Gram-positive bacteria. Great efforts have been made to promote the production of these antibiotics, so that they can one day be used in our antimicrobial arsenal to combat multidrug-resistant bacterial infections.

AREAS COVERED

This review provides a synopsis of lantibiotic research aimed at furthering our understanding of the structural limitation of lantibiotics as well as identifying structural regions that can be modified to improve the bioactivity. In vivo, in vitro and chemical synthesis of lantibiotics has been useful for engineering novel variants with enhanced activities. These approaches have provided novel ways to further our understanding of lantibiotic function and have advanced the objective to develop lantibiotics for the treatment of infectious diseases.

EXPERT OPINION

Synthesis of lantibiotics with enhanced activities will lead to the discovery of new promising drug candidates that will have a long lasting impact on the treatment of Gram-positive infections. The current body of literature for producing structural variants of lantibiotics has been more of a 'proof-of-principle' approach and the application of these methods has not yet been fully utilized.

Synthesis of bisarylethyne–peptide conjugates

Convenient preparation of bisarylethyne–peptide conjugates, and a mild procedure for the H- or D-reduction of the bisarylethyne triple bond.

Recent advances in synthetic analogues of lantibiotics: What can we learn from these?

The lantibiotics are a family of antibacterial cyclic peptides distinguished by one or more thioether linkages between amino acid side chains, and by unique modes of action. Recent developments in the chemical synthesis, mutagenesis and mutasynthesis of these peptides are providing insights into the structural requirements for antibacterial activity and into the mode of action, as well as having the potential to produce analogues with greater stability, potency and bioavailability. This Review provides a survey of these recent advances.

The challenge of the lantibiotics: synthetic approaches to thioether-bridged peptides

The unique antibacterial properties and structural complexity of the lantibiotics has stimulated considerable interest in the development of methodology to synthesise these peptides. One of the most challenging issues has been the synthesis of polycyclic peptides with multiple thioether bridges between side-chains, which are a characteristic feature of the lantibiotics. In this perspective, the different approaches to this problem, including solution-phase synthesis, solid-phase synthesis, biomimetic approaches and biotransformation strategies, are reviewed, highlighting the advances resulting from each of these approaches.

Peptides and proteins as a continuing exciting source of inspiration for peptidomimetics

Despite their enormous diversity in biological function and structure, peptides and proteins are endowed with properties that have induced and stimulated the development of peptidomimetics. Clearly, peptides can be considered as the "stem" of a phylogenetic molecular development tree from which branches of oligomeric peptidomimetics such as peptoids, peptidosulfonamides, urea peptidomimetics, as well as β-peptides have sprouted. It is still a challenge to efficiently synthesize these oligomeric species, and study their structural and biological properties. Combining peptides and peptidomimetics led to the emergence of peptide-peptidomimetic hybrids in which one or more (proteinogenic) amino acid residues have been replaced with these mimetic residues. In scan-like approaches, the influence of these replacements on biological activity can then be studied, to evaluate to what extent a peptide can be transformed into a peptidomimetic structure while maintaining, or even improving, its biological properties. A central issue, especially with the smaller peptides, is the lack of secondary structure. Important approaches to control secondary structure include the introduction of α,α-disubstituted amino acids, or (di)peptidomimetic structures such as the Freidinger lactam. Apart from intra-amino acid constraints, inter-amino acid constraints for formation of a diversity of cyclic peptides have shaped a thick branch. Apart from the classical disulfide bridges, the repertoire has been extended to include sulfide and triazole bridges as well as the single-, double- and even triple-bond replacements, accessible by the extremely versatile ring-closing alkene/alkyne metathesis approaches. The latter approach is now the method of choice for the secondary structure that presents the greatest challenge for structural stabilization: the α-helix.

Stereoselective disulfide formation stabilizes the local peptide conformation in nisin mimics

Nisin is a polymacrocyclic peptide antimicrobial with high activity against Gram-positive bacteria. Lanthionine and methyllanthionine bridges, closing the macrocycles, are stabilized by thioether bonds, formed between cysteines and dehydrated serine or threonine. The role of polypeptide backbone conformation in the formation of macrocycles A and B within cysteine mutants of nisin residues 1−12 is investigated here by molecular dynamics simulations. Enantiomeric combinational space of Cys3 and Cys7 and of Cys8 and Cys11 is examined for the preference of disulfide bond formation over helical turn formation within this region. A clear preference for spontaneous disulfide formation and closure of rings 3,7 and 8,11 is demonstrated for the D-Cys3, D-Cys7, L-Cys8, L-Cys11 nisin homologue, while interlinked rings A and B are obtained through disulfide bridges between L-Cys3 and D-Cys8 and between D-Cys7 and D-Cys11. This study offers a simple designer approach to solid phase synthesis of macrocyclic peptides and lantibiotic analogues.

All-atom model for stabilization of alpha-helical structure in peptides by hydrocarbon staples

Recent work has shown that the incorporation of an all-hydrocarbon "staple" into peptides can greatly increase their alpha-helix propensity, leading to an improvement in pharmaceutical properties such as proteolytic stability, receptor affinity, and cell permeability. Stapled peptides thus show promise as a new class of drugs capable of accessing intractable targets such as those that engage in intracellular protein-protein interactions. The extent of alpha-helix stabilization provided by stapling has proven to be substantially context dependent, requiring cumbersome screening to identify the optimal site for staple incorporation. In certain cases, a staple encompassing one turn of the helix (attached at residues i and i+4) furnishes greater helix stabilization than one encompassing two turns (i,i+7 staple), which runs counter to expectation based on polymer theory. These findings highlight the need for a more thorough understanding of the forces that underlie helix stabilization by hydrocarbon staples. Here we report all-atom Monte Carlo folding simulations comparing unmodified peptides derived from RNase A and BID BH3 with various i,i+4 and i,i+7 stapled versions thereof. The results of these simulations were found to be in quantitative agreement with experimentally determined helix propensities. We also discovered that staples can stabilize quasi-stable decoy conformations, and that the removal of these states plays a major role in determining the helix stability of stapled peptides. Finally, we critically investigate why our method works, exposing the underlying physical forces that stabilize stapled peptides.

Total synthesis and biological evaluation of amphidinolide V and analogues

The awesome power of metathesis is illustrated by a concise synthesis of the extremely scarce marine natural product amphidinolide V, which hinges on a sequence of ring-closing alkyne metathesis followed by intermolecular enyne metathesis with ethylene (see scheme). As a complete set of conceivable stereoisomers was prepared, the constitution and absolute configuration of this macrolide could be established and first insights into structure-activity relationships governing its cytotoxicity were obtained.A sequence of ring-closing alkyne metathesis followed by an intermolecular enyne metathesis of the resulting cycloalkyne with ethene was used to forge the macrocyclic skeleton and to set the vicinal exo-methylene branches characteristic for the cytotoxic marine natural product amphidinolide V (1). Comparison of the synthetic material with an authentic sample of this extremely scarce metabolite isolated from a dinoflagellate of the Amphidinium sp. eliminated any doubts about its structure and allowed the absolute configuration of amphidinolide V to be determined as 8R,9S,10S,13R. Moreover, the flexibility inherent to the underlying synthesis blueprint also opened access to a comprehensive set of diastereomers of 1 as well as to synthetic analogues differing from the natural lead in the lipophilic chains appended to the macrocyclic core. This set of designed analogues gave first insights into structure-activity relationships, which revealed that the stereostructure of the macrolactone is a highly critical parameter, whereas the examined alterations of the side chain did not diminish the cytotoxicity of the compounds to any notable extent.

Synthesis of bicyclic alkene-/alkane-bridged nisin mimics by ring-closing metathesis and their biochemical evaluation as lipid II binders: toward the design of potential novel antibiotics

This report describes the design, synthesis, and biochemical evaluation of alkene- and alkane-bridged AB(C)-ring mimics of the lantibiotic nisin. Nisin belongs to a class of natural antimicrobial peptides, and has a unique mode of action: its AB(C)-ring system binds to the pyrophosphate moiety of lipid II. This mode of action was the rationale for the design of smaller nisin-derived peptides to obtain novel potential antibiotics. As a conformational constraint the thioether bridge was mimicked by an alkene- or alkane isostere. The peptides of the linear individual ring precursors were synthesized on solid support or in solution, and cyclized by ring-closing metathesis in solution with overall yields of between 36 and 89 %. The individual alkene-bridged macrocycles were assembled in solution by using carbodiimide-based synthesis protocols for the corresponding AB(C)-ring mimics. These compounds were tested for their binding affinity toward lipid II by evaluation of their potency to inhibit nisin-induced carboxyfluorescein release from large unilamellar vesicles. It was found that these AB(C)-ring mimics were not able to induce membrane leakage; however, they acted by inhibiting nisin-induced carboxyfluorescein release; this indicates their affinity toward lipid II. These results imply that an alkene or alkane moiety is a suitable thioether bridge mimic.

Total Synthesis of Myxovirescin A1

A convergent total synthesis of the antibiotic macrolide myxovirescin A1 (1) is described that is largely based on reagent- and catalyst-controlled transformations. This includes a highly regioselective Negishi reaction of dibromo-alkene 48 with an alkynylzinc reagent, and a palladium catalyzed alkyl-Suzuki coupling of the resulting enyne derivative 12 with the 9-BBN-adduct derived from alkene 61. The latter was obtained via an asymmetric hydrogenation of the chlorinated beta-ketoester 49 and an anti-selective oxyallylation of the functionalized aldehyde 53 as the key steps. The preparation of the bis-borylated allyl-donor 57 used in the oxyallylation step, however, required careful optimization and led to important insights into the nature of the classical hydroborating agent "di(isopinocampheyl)borane (Ipc2BH)". It was unambiguously shown by X-ray crystallography that in the solid state this compound is dimeric, but it is prone to undergo an essentially quantitative mono-deborylation when dissolved in CH2Cl2 or benzene; its composition in ethereal solvents is even more complex as evident from 11B NMR data. Product 71 derived from 12 and 61 was elaborated into the enyne-yne derivative 75, which served as the substrate for an exquisitely selective ring closing alkyne metathesis reaction (RCAM) catalyzed by the molybdenum tris-amido complex 20 activated in situ with CH2Cl2. The resulting cyclic enyne 76 was subjected to a ruthenium catalyzed trans-hydrosilylation/proto-desilylation tandem. Although [Cp*Ru(MeCN)3]PF6 had previously been recommended as catalyst of choice for trans-hydrosilylation reactions of internal alkynes, this complex failed to afford the desired product, whereas its sterically less hindered congener [CpRu(MeCN)3]PF6 permitted the reaction to be performed in appreciable yield, but at the expense of a lower stereoselectivity. AgF-mediated proto-desilylation of the isomeric silanes 79 and 80 followed by cleavage of the remaining acetal protecting groups afforded myxovirescin A1 and its hitherto unknown 14Z-isomer 81, respectively.

A microwave enhanced cross-metathesis approach to peptidomimetics

Functionalization of amino acid C- and N-termini with appropriate olefinic moieties allows for the generation of a peptidomimetic via a stereoselective cross-metathesis.

Step-wise and pre-organization induced synthesis of a crossed alkene-bridged nisin Z DE-ring mimic by ring-closing metathesis

This paper describes two approaches for the synthesis of a crossed alkene-bridged mimic of the thioether ring system of the nisin Z DE-fragment. The first approach comprised the stepwise total synthesis featuring a cross metathesis and a macrolactamization on a solid support followed by a ring-closing metathesis in solution. Via this route the title compound was obtained in an overall yield of 7% (85% on average for 16 reaction steps). In the second approach, the linear precursor peptide was subjected to ring-closing metathesis and the bicyclic peptide with the correct side chain connectivity pattern was obtained in yields up to 95%. The preferred formation of the bicyclic crossed alkene-bridged mimic of the DE-ring suggests a favorable pre-organization of the linear precursor peptide.

Total Syntheses of the Actin-Binding Macrolides Latrunculin A, B, C, M, S and 16-epi-Latrunculin B

The latrunculins are highly selective actin-binding marine natural products and as such play an important role as probe molecules for chemical biology. A short, concise and largely catalysis-based approach to this family of bioactive macrolides is presented. Specifically, the macrocyclic skeletons of the targets were forged by ring-closing alkyne metathesis (RCAM) or enyne-yne metathesis of suitable diyne or enyne-yne precursors, respectively. This transformation was best achieved with the aid of [(tBu)(Me(2)C(6)H(3))N](3)Mo (37) as precatalyst activated in situ with CH(2)Cl(2), as previously described. This catalyst system is strictly chemoselective for the triple bond and does not affect the olefinic sites of the substrates. Moreover, the molybdenum-based catalyst turned out to be broader in scope than the Schrock alkylidyne complex [(tBuO)(3)W[triple chemical bond]CCMe(3)] (38), which afforded cycloalkyne 35 in good yield but failed in closely related cases. The required metathesis precursors were assembled in a highly convergent fashion from three building blocks derived from acetoacetate, cysteine, and (+)-citronellene. The key fragment coupling can either be performed via a titanium aldol reaction or, preferentially, by a sequence involving a Horner-Wadsworth-Emmons olefination followed by a protonation/cyclization/diastereoselective hydration cascade. Iron-catalyzed C--C-bond formations were used to prepare the basic building blocks in an efficient manner. This synthesis blueprint gave access to latrunculin B (2), its naturally occurring 16-epimer 3, as well as the even more potent actin binder latrunculin A (1) in excellent overall yields. Because of the sensitivity of the 1,3-diene motif of the latter, however, the judicious choice of protecting groups and the proper phasing of their cleavage was decisive for the success of the total synthesis. Since latrunculin A and B had previously been converted into latrunculin S, C and M, respectively, formal total syntheses of these congeners have also been achieved. Finally, a previously unknown acid-catalyzed degradation pathway of these bioactive natural products is described. The cysteine-derived ketone 18, the tetrahydropyranyl segment 31 serving as the common synthesis platform for the preparation of all naturally occurring latrunculins, as well as the somewhat strained cycloalkyne 35 formed by the RCAM reaction en route to 2 were characterized by X-ray crystallography.

Latrunculin Analogues with Improved Biological Profiles by “Diverted Total Synthesis”: Preparation, Evaluation, and Computational Analysis

Deliberate digression from the blueprint of the total syntheses of latrunculin A (1) and latrunculin B (2) reported in the accompanying paper allowed for the preparation of a focused library of "latrunculin-like" compounds, in which all characteristic structural elements of these macrolides were subject to pertinent molecular editing. Although all previously reported derivatives of 1 and 2 were essentially devoid of any actin-binding capacity, the synthetic compounds presented herein remain fully functional. One of the designer molecules with a relaxed macrocyclic backbone, that is compound 44, even surpasses latrunculin B in its effect on actin while being much easier to prepare. This favorable result highlights the power of "diverted total synthesis" as compared to the much more widely practiced chemical modification of a given lead compound by conventional functional group interconversion. A computational study was carried out to rationalize the observed effects. The analysis of the structure of the binding site occupied by the individual ligands on the G-actin host shows that latrunculin A and 44 both have similar hydrogen-bond network strengths and present similar ligand distortion. In contrast, the H-bond network is weaker for latrunculin B and the distortion of the ligand from its optimum geometry is larger. From this, one may expect that the binding ability follows the order 1 >/= 44 > 2, which is in accord with the experimental data. Furthermore, the biological results provide detailed insights into structure/activity relationships characteristic for the latrunculin family. Thus, it is demonstrated that the highly conserved thiazolidinone ring of the natural products can be replaced by an oxazolidinone moiety, and that inversion of the configuration at C16 (latrunculin B numbering) is also well accommodated. From a purely chemical perspective, this study attests to the maturity of ring-closing alkyne metathesis (RCAM) catalyzed by a molybdenum alkylidyne complex generated in situ, which constitutes a valuable tool for advanced organic synthesis and natural product chemistry.