Ring-closing Metathesis Strategy Research Articles

Molecular Acrobatics in Polycyclic Frames: Synthesis of "Kurmanediol" via Post-synthetic Modification of Cage Molecules by Olefinic Metathesis.

We report late-stage ring-opening metathesis (ROM), ring-rearrangement metathesis (RRM), and ring-closing metathesis (RCM) approaches to generate expanded pentacycloundecane (PCUD) cage derivatives. These higher-order intricate polycyclic cage systems are aesthetically pleasing and structurally intriguing. Their assembly maintains molecular symmetry during the entire synthetic sequence. To this end, metathesis-based catalysts are used to execute the ROM, RRM, and RCM strategies. The synthetic approach to these cage polycycles involves the Diels-Alder reaction, [2 + 2] photocycloaddition, RRM, ROM, and RCM as key steps.

Synthesis of Bioactive Macrocycles Involving Ring-Closing Metathesis Strategy

AbstractThis review reports the synthesis of various bioactive macrocycles, involving ring-closing metathesis as a key step, developed since ca. 2000. These macrocycles exhibited biological activities such as antiviral, antifungal, antibacterial, and anticancer activities, and more. Thus, their syntheses and utilization are essential for both synthetic organic and medicinal chemists.

Synthesis of the Macrolactone Cores of Maltepolides via a Diene-Ene Ring-Closing Metathesis Strategy.

Synthesis of the C19-truncated maltepolide E has been accomplished via a diene-ene ring-closing metathesis (RCM) strategy without damage to the C11-C14 alkenyl epoxy unit. Upon release of the C17-OH group, it attacked at the C14 position with double bond migration and epoxide ring opening to furnish the C19-truncated maltepolides A and B as proposed for the biosynthesis of maltepolides.

E- and Z-trisubstituted macrocyclic alkenes for natural product synthesis and skeletal editing.

Many therapeutic agents are macrocyclic trisubstituted alkenes but preparation of these structures is typically inefficient and non-selective. A possible solution would entail catalytic macrocyclic ring-closing metathesis, but these transformations require high catalyst loading, conformationally rigid precursors and are often low yielding and/or non-stereoselective. Here we introduce a ring-closing metathesis strategy for synthesis of trisubstituted macrocyclic olefins in either stereoisomeric form, regardless of the level of entropic assistance. The goal was achieved by addressing several unexpected difficulties, including complications arising from pre-ring-closing metathesis alkene isomerization. The power of the method is highlighted by two examples. The first is the near-complete reversal of substrate-controlled selectivity in the formation of a macrolactam related to an antifungal natural product. The other is a late-stage stereoselective generation of an E-trisubstituted alkene in a 24-membered ring, en route to the cytotoxic natural product dolabelide C.

Relay ring-closing metathesis strategies towards the synthesis of the ABC tricycle of Taxol

Two synthetic routes towards the ABC tricycle of Taxol are presented. They both involve a relay ring-closing metathesis reaction to make the central B ring in a convergent fashion. In the first approach, the extender arm is positioned on the A ring, and the C ring bears the relay tether in the second route. Metathesis precursors with diverse extender arms were efficiently synthesised; unfortunately, the crucial metathesis reactions failed to deliver the target compounds in all cases.

Diastereoselective synthesis of trisubstituted olefins using a silicon-tether ring-closing metathesis strategy.

The diastereoselective synthesis of trisubstituted olefins with concomitant C-C bond formation is still a difficult challenge, and olefin metathesis reactions for the formation of such alkenes are usually not high yielding or/and diastereoselective. Herein we report an efficient and diastereoselective synthesis of trisubstituted olefins flanked by an allylic alcohol, by a silicon-tether ring-closing metathesis strategy. Both E- and Z-trisubstituted alkenes were synthesised, depending on the method employed to cleave the silicon tether. Furthermore, this methodology features a novel Peterson olefination for the synthesis of allyldimethylsilanes. These versatile intermediates were also converted into the corresponding allylchlorodimethylsilanes, which are not easily accessible in high yields by other methods.

Divergent synthesis of oxacyclophenylpropanoids from biomass-derived eugenol

A divergent synthesis of new oxacyclophenylpropanoids from the readily available and biomass-derived starting material eugenol is reported. The synthetic routes feature two important steps, including (i) DDQ-mediated selective oxidation of an allyl group and subsequent Claisen-rearrangement for the formation of key intermediates 1 and 11, and (ii) five-membered or six-membered ring formation via either palladium-catalyzed cyclization or ring-closing metathesis (RCM) strategies.

Advanced Approaches to Post-Assembly Modification of Peptides by Transition-Metal-Catalyzed Reactions

We have summarized diverse synthetic approaches for the modification of peptides by employing transition-metal-catalyzed reactions. These methods can deliver unusual peptides suitable for peptidomimetics. To this end, several popular reactions such as Diels–Alder, 1,3-dipolar cycloaddition, [2+2+2] cyclotrimerization, metathesis, Suzuki­–Miyaura cross-coupling, and Negishi coupling have been used to assemble modified peptides by post-assembly chemical modification strategies. 1 Introduction 2 Synthesis of a Cyclic α-Amino Acid Derivative via a Ring-Closing Metathesis Protocol 3 Peptide Modification Using a Ring-Closing Metathesis Strategy 4 Peptide Modification via a [2+2+2] Cyclotrimerization Reaction 5 Peptide Modification by Using [2+2+2] Cyclotrimerization and Suzuki Coupling 6 Peptide Modification via a Suzuki–Miyaura Cross-Coupling 7 Peptide Modification via Cross-Enyne Metathesis and a Diels–Alder­ Reaction as Key Steps 8 Peptide Modification via 1,3-Dipolar Cycloaddition Reactions 9 Modified Peptides via Negishi Coupling 10 A Modified Dipeptide via Ethyl Isocyanoacetate 11 Conclusions

Access to Multisubstituted 2(5 H)-Furanones Using Hydrogen Bonding-Promoted Ring-Closing Metathesis and Polyamine Workup.

Structurally complex 2(5 H)-furanones are potentially challenging targets for ring-closing metathesis (RCM). A hydrogen bonding-guided RCM strategy was developed in this study to provide 3-substituted and 3,4-disubstituted 2(5 H)-furanones in moderate to high yields with broad functional group tolerance. A workup procedure using ethylenediamine-derived polyamines such as tetraethylenepentylamine was also established to effectively remove Ru residues in products.

Synthetic Studies on Pyrroloindolizidine Skeleton Based on Gold–Catalyzed Hydroamination–Enamine Cyclization–Ring–Closing Metathesis Strategy

Synthetic Studies on Pyrroloindolizidine Skeleton Based on Gold–Catalyzed Hydroamination–Enamine Cyclization–Ring–Closing Metathesis Strategy

Novel Synthesis of Trisubstituted Olefins for the Preparation of the C16-C30 Fragment of Dolabelide C.

A silicon-tether ring-closing metathesis strategy is reported for the synthesis of trisubstituted olefins flanked by allylic or homoallylic alcohols, which are difficult to obtain by classical ring-closing or cross-metathesis reactions. In addition, a novel Peterson olefination reaction has been developed for the preparation of the allyldimethylsilane precursors, which are versatile synthetic intermediates. This method was then applied to the synthesis of the C16-C30 fragment of dolabelide C.

Total Synthesis of the Bis-silyl Ether of (+)-15-epi-Aetheramide A.

Synthesis of the macrolactone depsipeptide aetheramide A was attempted by three different approaches. The first approach to form the macrolactone involving macrolactonization to form the C1-C21 bond and the second approach using a ring-closing metathesis (RCM) strategy to form the C10-C11 olefinic bond failed. The third approach starting from R-mandelic acid, involving the RCM reaction to install the C18-C19 ring junction, was successful in assembling the macrolactone.

Development of an O-Vinylation–Ring-Closing Metathesis Strategy to Access 3,3′-3,4-Dihydropyrans

Dihydropyrans are common structural motifs that appear in both natural products and pharmaceuticals and are intermediates for the synthesis of tetrahydropyrans. Currently, no reports exist in the literature for the synthesis of 3,3′-differentially disubstituted-3,4-dihydro-2-pyrans. We describe an approach employing abundant esters as starting materials that allows access to these heterocyclic scaffolds through a unique O-vinylation–RCM sequence.

A-Ring Synthons of 19-Nor Type Vitamin D Derivatives

Various 19-nor vitamin D analogs, which lack the methylene group at C19, exhibit significant vitamin D (VD)-related biological activities, but generally show a reduced calcemic side-effect compared with VD itself. Among them, paricalcitol is already used clinically for treatment and prevention of secondary hyperparathyroidism associated with chronic renal failure. Therefore, considerable synthetic efforts have been directed towards 19-nor VD analogs, focusing especially on A-ring synthons suitable for use in convergent synthetic strategies based on coupling of CD-ring synthons and A-ring synthons. For example, we have recently developed a new synthetic route to A-ring synthons from linear dienes based upon a ring-closing olefin metathesis strategy. Here, we review recent synthetic approaches to A-ring synthons for 19-nor VD derivatives.

A ring closing metathesis strategy for carbapyranosides of xylose and arabinose

The synthesis of β-carba-xylo and arabino pyranosides of cholestanol is described. The synthetic strategy, which is analogous to the Postema approach to C-glycosides, centers on the ring closing metathesis of an enol ether–alkene precursor to give a cyclic enol ether that is elaborated to a carba-pyranoside via hydroboration–oxidation on the olefin. The method, which is attractive for its modularity and stereoselectivity, may find wider applications to carba-hexopyranosides and other complex cycloalkyl ether frameworks.

A ring closing metathesis strategy for carbapyranosides of xylose and arabinose

The synthesis of β-carba-xylo and arabino pyranosides of cholestanol is described. The synthetic strategy, which is analogous to the Postema approach to C-glycosides, centers on the ring closing metathesis of an enol ether–alkene precursor to give a cyclic enol ether that is elaborated to a carba-pyranoside via hydroboration–oxidation on the olefin. The method, which is attractive for its modularity and stereoselectivity, may find wider applications to carba-hexopyranosides and other complex cycloalkyl ether frameworks.

A relay ring-opening/double ring-closing metathesis strategy for the bicyclic macrolide-butenolide core structures

A concise strategy has been developed for the synthesis of the bicyclic macrolide-butenolide core structures of various natural products with the macrolide ring size ranging from 12- to 16-membered. The bicyclic structure was easily assembled using the relay ring-opening/double ring-closing metathesis strategy. An efficient synthesis of (±)-desmethyl manshurolide has been achieved as an application of this strategy.

ChemInform Abstract: Model Studies for a Ring‐Closing Metathesis Approach to the Bafilomycin Macrolactone Core from a 2,2‐Dimethoxy Tetraenic Ester Precursor.

A ring-closing metathesis strategy is reported for the construction of the 16-membered macrolactone core of the bafilomycins. One decisive key feature is the presence of a 2,2-dimethoxyketal functionality at C-2 that provides the required flexibility to the tetraenic ester precursor, allowing the ring-closing metathesis reaction to take place. Three different model esters of increasing complexity were successfully subjected to the 1,3-diene-ene ring-closing metathesis reaction. The best promoter for the simplest esters was the Grubbs first-generation precatalyst. A Hoveyda–Grubbs-type trifluoromethylamido-containing precatalyst developed by Mauduit's group gave satisfactory results for the most complex ester. In all experiments, the 12-Z-configured isomer was obtained as the major product. Subsequent microwave-promoted methanol elimination was achieved on the simplest model compound using camphorsulfonic acid (CSA) as a catalyst. Under these conditions, a E to Z isomerization of the double bond at C-4, as well as ca. 50 % isomerization of the 12-Z double bond into the corresponding 12-E isomer, were observed.

Rational design, synthesis, and evaluation of tetrahydroxamic acid chelators for stable complexation of zirconium(IV).

Metals of interest for biomedical applications often need to be complexed and associated in a stable manner with a targeting agent before use. Whereas the fundamentals of most transition-metal complexation processes have been thoroughly studied, the complexation of Zr(IV) has been somewhat neglected. This metal has received growing attention in recent years, especially in nuclear medicine, with the use of (89) Zr, which a β(+) -emitter with near ideal characteristics for cancer imaging. However, the best chelating agent known for this radionuclide is the trishydroxamate desferrioxamine B (DFB), the Zr(IV) complex of which exhibits suboptimal stability, resulting in the progressive release of (89) Zr in vivo. Based on a recent report demonstrating the higher thermodynamic stability of the tetrahydroxamate complexes of Zr(IV) compared with the trishydroxamate complexes analogues to DFB, we designed a series of tetrahydroxamic acids of varying geometries for improved complexation of this metal. Three macrocycles differing in their cavity size (28 to 36-membered rings) were synthesized by using a ring-closing metathesis strategy, as well as their acyclic analogues. A solution study with (89) Zr showed the complexation to be more effective with increasing cavity size. Evaluation of the kinetic inertness of these new complexes in ethylenediaminetetraacetic acid (EDTA) solution showed significantly improved stabilities of the larger chelates compared with (89) Zr-DFB, whereas the smaller complexes suffered from insufficient stabilities. These results were rationalized by a quantum chemical study. The lower stability of the smaller chelates was attributed to ring strain, whereas the better stability of the larger cyclic complexes was explained by the macrocyclic effect and by the structural rigidity. Overall, these new chelating agents open new perspectives for the safe and efficient use of (89) Zr in nuclear imaging, with the best chelators providing dramatically improved stabilities compared with the reference DFB.

Total Synthesis of the Z-Isomers of the Proposed and Revised Structures of Aspergillide B via an Iodocyclization and Ring-Closing Metathesis Strategy

The synthesis of Z -isomers of both the proposed and revised structures of aspergillide B is described. A divergent route is employed that involves kinetically controlled ring-closing metathesis for the construction of a 14-membered macrocyclic ring, ester formation under Yamaguchi conditions, a Wacker-type oxidative cyclization for creation of the C4 stereogenic center and a previously reported diastereoselective isomerization–iodocyclization strategy for the construction of the 2,6- trans -disubstituted tetrahydropyran subunit.