Macrocyclization by Ring-Closing Metathesis in the Total Synthesis of Natural Products: Reaction Conditions and Limitations

A remote C-C bond cleavage-enabled skeletal reorganization: Access to medium-/large-sized cyclic alkenes

Although great success has been achieved in selective C-C bond cleavage via the intramolecular radical remote migration process of several carbon-based groups, the development of the radical-based remote vinyl migration process remains a formidable challenge because of the energetically unfavorable process. To address this problem, we report here, for the first time, a novel C-C bond reorganization strategy via an unprecedented radical 1,3-, 1,4-, or 1,5-vinyl migration triggered by various types of fluoroalkylation of alkenes for the efficient realization of 1,2-fluoroalkylalkenylation reaction. This strategy provides an expedient and broadly applicable platform to access skeletally and functionally diverse fluoroalkyl-containing medium- and large-sized cyclic alkenes with excellent chemo-, regio-, and stereoselectivity. The broad substrate scope, which covers distinctly electron-neutral or electron-deficient alkenyl migrating groups and various fluoroalkyl radical precursors, the excellent functional group tolerance, the remarkable selectivity, and the operational simplicity, as well as versatile transformations of the products, make this approach practical and attractive.

Total Synthesis of Kanamienamide

Kanamienamide is a novel enol ether containing enamide with a single digit micromolar inhibitory activity against cancer cell lines. An efficient and convergent total synthesis of kanamienamide has been developed for the first time, which features a Cu-mediated amide coupling with vinyl iodide at the late stage. Other key transformations include Evans asymmetric alkylation, CBS asymmetric reduction, ring-closing metathesis reaction, and Stork-Zhao-Wittig olefination. This strategy is amenable for facile analogue preparation and SAR studies.

A Direct Arylation-Cyclisation Reaction for the Construction of Medium-Sized Rings

A strategy for assembling biaryls linked through a medium-sized ring is herein presented. π-Complexation of fluoroarenes to chromium tricarbonyl activates the molecule towards both C-H activation and nucleophilic aromatic substitution without covalently altering the molecular connectivity of the arene. The construction of bridged biaryl molecules with 6-10-membered core rings is achieved through a one-pot C-H arylation/nucleophilic aromatic substitution sequence. The methodology is applicable to the synthesis of heterocyclic as well as fully carbocyclic rings.

A Highly Efficient Synthesis of Z-Macrocycles Using Stereoretentive, Ruthenium-Based Metathesis Catalysts

A highly efficient, Z-selective ring-closing metathesis system for the formation of macrocycles using a stereoretentive, ruthenium-based catalyst supported by a dithiolate ligand is reported. The catalyst is remarkably active as observed in initiation experiments showing complete catalyst initiation at -20 °C within 10 minutes. Macrocyclization reactions generated Z-products from easily accessible diene starting materials bearing a Z-olefin moiety. This approach provides a more efficient and selective route to Z-macrocycles relative to previously reported systems. Reactions were completed within shorter reaction times, and turnover numbers of up to 100 could be achieved. Macrocyclic lactones ranging in size from twelve- to seventeen-membered rings were synthesized in moderate to high yields (67-79 %) with excellent Z-selectivity (95-99 %).

A Short Covalent Synthesis of an All-Carbon-Ring [2]Rotaxane

While the current supramolecular syntheses of [2]rotaxanes are generally efficient, the final product always retains the functional groups required for non-covalent preorganization. A short and high-yielding covalent-template-assisted approach is reported for the synthesis of a [2]rotaxane. A terephthalic acid template core preorganizes the covalently connected ring precursor fragments to induce a clipping-type cyclization over the thread moiety. Cleavage of the temporary ester bonds that connect the ring and thread fragments liberates the [2]rotaxane.

Synthesis and biological analysis of truncated calyculone H

Herein, we report for the first time the design and linear synthesis of a truncated calyculone H (7) that lacks the telltale isopropyl/isopropylene groups, whereas the 12-membered macrocycle remains intact. Key steps for the framework of target molecule include allylic oxidation using SeO2, Sharpless asymmetric epoxidation, Barbier zinc allylation, and ring-closing metathesis (RCM) reactions. A second truncated "calyculone-like" analogue, 27, with a different oxidation pattern around the ring was also synthesized following a similar strategy. Screening for in vitro cytotoxicity against a panel of 60 human cancer cell lines revealed that 7 was as potent if not more so (for a few cell lines) than the natural product calyculone A (2).

Ring-Expansion Approach to Medium-Sized Lactams and Analysis of Their Medicinal Lead-Like Properties

Medium-sized rings are widely considered to be under-represented in biological screening libraries for lead identification in medicinal chemistry. To help address this, a library of medium-sized lactams has been generated by using a simple, scalable and versatile ring-expansion protocol. Analysis of the library by using open-access computational tool LLAMA suggested that these lactams and their derivatives have highly promising physicochemical and 3D spatial properties and thus have much potential in drug discovery.

Macrocyclic Peptidomimetics Prepared by Ring-Closing Metathesis and Azide–Alkyne Cycloaddition

Macrocycles are finding increasing use as a means to define the backbone geometries of peptides and peptidomimetics. Ring-closing metathesis and CuI-catalyzed azide–alkyne cycloaddition are particularly useful for introducing such rings and they do so in high yield and with a good functional group tolerance and compatibility. Here, we present an overview of the use of these two methods, with reference to selected examples and particular reference to β-strand peptidomimetics for use as protease inhibitors.

A redox-switchable ring-closing metathesis catalyst

A ring-closing metathesis catalyst was arrested upon reduction of a redox-active ligand; subsequent oxidation restored catalytic activity.

Catalytic macrolactonizations for natural product synthesis

Non-seco-acid-based catalytic macrolactonization strategies and methods and their applications in total syntheses of natural products are highlighted.

Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C-C Bond Formation

Diverse hydrogen-mediated C-C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C-C couplings are shown to evoke a step-change in efficiency.

Ruthenium-Catalyzed Metathesis Cascade Reactions in Natural Products Synthesis

In this account, we provide a brief summary of recent developments in ruthenium-catalyzed metathesis cascade reactions towards the total synthesis of natural products. We also highlight recent progress from our own laboratory regarding the synthesis of securinega alkaloids and humulanolides, which has resulted in the development of novel ruthenium-catalyzed metathesis cascade reactions. Inspired and guided by the pioneering and elegant research conducted in this area, we developed a regio-controlled relay dienyne metathesis cascade reaction and a cyclobutene-promoted RCM/ROM/RCM cascade reaction for the synthesis of securinega alkaloids and humulanolides, respectively.

Total Syntheses of Nannocystins A and A0, Two Elongation Factor 1 Inhibitors

Asymmetric total syntheses of nannocystins A and A0 were achieved in a convergent route starting from simple materials. Nannocystin family natural products bear potent anticancer activity as elongation factor 1 inhibitors. In this synthesis, the challenging tertiary amide bond was constructed by peptide coupling between an acyl chloride and a secondary amine. A late-stage ring-closing metathesis reaction successfully rendered the macrocycle. This efficient synthetic strategy should be applicable to other nannocystins and analogues and therefore should benefit future structure-activity relationship studies.

Total Synthesis of (+)-SCH 351448: Efficiency via Chemoselectivity and Redox-Economy Powered by Metal Catalysis

The polyketide natural product (+)-SCH 351448, a macrodiolide ionophore bearing 14 stereogenic centers, is prepared in 14 steps (LLS). In eight prior syntheses, 22-32 steps were required. Multiple chemoselective and redox-economic functional group interconversions collectively contribute to a step-change in efficiency.

Total Synthesis and Complete Stereostructure of a Marine Macrolide Glycoside, (-)-Lyngbyaloside B

We have described in detail the total synthesis of both the proposed and correct structures of (-)-lyngbyaloside B, which facilitated the elucidation of the complete stereostructure of this natural product. Our study began with the total synthesis of 13-demethyllyngbyaloside B, in which an esterification/ring-closing metathesis (RCM) strategy was successfully used for the efficient construction of the macrocycle. We also established reliable methods for the introduction of the conjugated diene side chain and the l-rhamnose residue onto the macrocyclic framework. However, the esterification/RCM strategy proved ineffective for the parent natural product because of the difficulties in acylating the sterically encumbered C-13 tertiary alcohol; macrolactionization of a seco-acid was also extensively investigated under various conditions without success. We finally completed the total synthesis of the proposed structure of (-)-lyngbyaloside B by means of a macrolactonization that involves an acyl ketene as the reactive species. However, the NMR spectroscopic data of our synthetic material did not match those of the authentic material, which indicated that the proposed structure must be re-examined. Inspection of the NMR spectroscopic data of the natural product and molecular mechanics calculations led us to postulate that the configuration of the C-10, C-11, and C-13 stereogenic centers had been incorrectly assigned in the proposed structure. Finally, our revised structure of (-)-lyngbyaloside B was unambiguously verified through total synthesis.

Contemporary Strategies for the Synthesis of Tetrahydropyran Derivatives: Application to Total Synthesis of Neopeltolide, a Marine Macrolide Natural Product

Tetrahydropyrans are structural motifs that are abundantly present in a range of biologically important marine natural products. As such, significant efforts have been paid to the development of efficient and versatile methods for the synthesis of tetrahydropyran derivatives. Neopeltolide, a potent antiproliferative marine natural product, has been an attractive target compound for synthetic chemists because of its complex structure comprised of a 14-membered macrolactone embedded with a tetrahydropyran ring, and twenty total and formal syntheses of this natural product have been reported so far. This review summarizes the total and formal syntheses of neopeltolide and its analogues, highlighting the synthetic strategies exploited for constructing the tetrahydropyran ring.

Synthetic Strategy and Anti-Tumor Activities of Macrocyclic Scaffolds Based on 4-Hydroxyproline

A series of novel 13- to 15-member hydroxyproline-based macrocycles, which contain alkyl-alkyl ether and alkyl-aryl ether moieties, have been synthesized by the strategy of macrocyclization utilising azide-alkyne cycloaddition, Mitsunobu protocol and amide formation. Their anti-tumor activities towards A549, MDA-MB-231 and Hep G2 cells were screened in vitro by an MTT assay. The results indicated that 13-member macrocycle 33 containing alkene chain showed the best results, exhibiting the highest inhibitory effects towards lung cancer cell line A549, which was higher than that of the reference cisplatin (IC50 value = 2.55 µmol/L).

An entropy-driven ring-opening metathesis polymerization approach towards main-chain liquid crystalline polymers

In this manuscript, an entropy-driven ring-opening metathesis polymerization (ED-ROMP) approach is applied for the first time in producing main-chain liquid crystalline polymers by polymerizing a macrocyclic olefin monomer.

Metal-templated synthesis of intertwined, functionalized strands as precursors to molecularly woven materials

A novel approach toward molecularly woven materials is presented, based on metal-templated precursors containing intertwined strands with functional endings.

Profluorescent substrates for the screening of olefin metathesis catalysts

Herein we report on a 96-well plate assay based on the fluorescence resulting from the ring-closing metathesis of two profluorophoric substrates. To demonstrate the validity of the approach, four commercially available ruthenium-metathesis catalysts were evaluated in six different solvents. The results from the fluorescent assay agree well with HPLC conversions, validating the usefulness of the approach.

Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (-)-Nitidasin

Astellatol and nitidasin belong to a subset of sesterterpenoids that share a sterically encumbered trans-hydrindane motif with an isopropyl substituent. In addition, these natural products feature intriguing polycyclic ring systems, posing significant challenges for chemical synthesis. Herein, the evolution of our stereoselective strategy for isopropyl trans-hydrindane sesterterpenoids is detailed. These endeavors included the synthesis of several building blocks, enabling studies toward all molecules of this terpenoid subclass, and of advanced intermediates of our initial route toward a biomimetic synthesis of astellatol. These findings provided the basis for a second-generation and a third-generation approach toward astellatol that eventually culminated in the enantioselective total synthesis of (-)-nitidasin. In particular, a series of substrate-controlled transformations to install the ten stereogenic centers of the target molecule was orchestrated and the carbocyclic backbone was forged in a convergent fashion. Furthermore, the progress toward the synthesis of astellatol is disclosed and insights into some observed yet unexpected diastereoselectivities by detailed quantum-mechanical calculations are provided.