Ring-closing Metathesis Approach 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.

Stereoselective Synthesis of Highly Substituted O-Heterocycles via Matteson Homologation: A Ring-Closing Metathesis Approach

Matteson homologations of chiral boronic esters with the aid of unsaturated nucleophiles are powerful for gaining access to a range of different O-heterocycles via subsequent ring-closing metatheses. Using this protocol, six- to eight-membered rings become available and almost any position of the ring can be substituted and/or functionalized.

Construction of Unusual Amino Acid derivatives and Bis‐Fused Oxacycles via Ring‐Closing Metathesis

AbstractBenzofurans and benzoxepines are key structural elements present in several natural products and medicinally important targets. Here, we describe an easy‐to‐execute strategy for the synthesis of benzofurans and benzoxepines, by employing Claisen rearrangement and ring‐closing metathesis (RCM) as key steps. Amino acid L‐tyrosine and 1,8‐dihydroxy‐anthraquinone were converted into modified oxacycles in good yields. The RCM approach has been used to produce bis‐fused benzofuran, bis‐fused benzoxepine, and unusual α‐amino acid (AAA) derivatives containing heterocyclic units. Unusual AAA derivatives are among the most important source for new peptide drugs and these AAA derivatives containing oxacycles are useful in designing many natural and unnatural products. Bis‐fused benzofuran and bis‐fused benzoxepine derivatives were also synthesized by the application of double Claisen rearrangement and two‐directional RCM as key steps starting with 1,8‐dihydroxy anthraquinone.

Modular Approach to Benzofurans, 2H-Chromenes and Benzoxepines via Claisen Rearrangement and Ring-Closing Metathesis: Access to Phenylpropanoids.

Benzofurans, 2H-chromenes and benzoxepines are key structural elements present in several natural products and pharmaceuticals. Here, we describe an easy-to-execute strategy for the synthesis of benzofurans, 2H-chromenes and benzoxepines, by employing Claisen rearrangement and ring-closing metathesis as key steps. A variety of phenols were converted into useful oxacycles in good to excellent yields. The ring-closing metathesis approach has been used to produce phenylpropanoid natural products. Examples described here include, the naturally occurring benzofurans such as 7-methoxywutaifuranal, 7-methoxywutaifuranol, 7-methoxywutaifuranate and the O-prenylated natural products like boropinic acid, boropinols A and C.

Ring-Closing Metathesis Approaches towards the Total Synthesis of Rhizoxins.

Efforts are described towards the total synthesis of the bacterial macrolide rhizoxin F, which is a potent tubulin assembly and cancer cell growth inhibitor. A significant amount of work was expanded on the construction of the rhizoxin core macrocycle by ring-closing olefin metathesis (RCM) between C(9) and C(10), either directly or by using relay substrates, but in no case was ring-closure achieved. Macrocycle formation was possible by ring-closing alkyne metathesis (RCAM) at the C(9)/C(10) site. The requisite diyne was obtained from advanced intermediates that had been prepared as part of the synthesis of the RCM substrates. While the direct conversion of the triple bond formed in the ring-closing step into the C(9)-C(10) E double bond of the rhizoxin macrocycle proved to be elusive, the corresponding Z isomer was accessible with high selectivity by reductive decomplexation of the biscobalt hexacarbonyl complex of the triple bond with ethylpiperidinium hypophosphite. Radical-induced double bond isomerization, full elaboration of the C(15) side chain, and directed epoxidation of the C(11)-C(12) double bond completed the total synthesis of rhizoxin F.

A Ring Closing Metathesis Approach to the Formal Synthesis of (+)‐Callyspongiolide

AbstractAn enantioselective synthesis of macrocyclic core of (+)‐callyspongiolide is described, constituting a formal synthesis of this natural product. The synthetic strategy constructs the 14‐membered macrocyclic domain via Yamaguchi esterification followed by a challenging ring‐closing metathesis (RCM) to effect the final formation of the macrolactone.

Formal Synthesis of (±)-Aplykurodinone-1 Based on the Indium-Catalyzed Conia-Ene Reaction

A concise formal synthesis of (±)-aplykurodinone-1 starting from a commercially available material and based on the indium-catalyzed Conia-ene reaction has been accomplished. The synthesis features a Riley selenium dioxide oxidation, a Krapcho dealkoxycarbonylation, and a ring-closing metathesis approach. The synthetic strategy was also supported by a Saegusa oxidation and a classic Michael 1,4-conjugate addition.

Ring Closing Metathesis Approach for the Synthesis of o‐Terphenyl Derivatives

A linear synthesis of o‐terphenyl derivatives has been delineated using ring closing metathesis (RCM) as the key step. In this approach, benzil derivatives upon allyl Grignard addition provides diphenyl‐1,2‐diallyl dihydroxy derivatives which undergo ring closing metathesis to afford tetrahydro terphenyl derivatives. Aromatization‐driven dehydration then leads to a diverse set of electron rich and electron deficient o‐terphenyls. Furthermore, oxidative coupling of electron rich o‐terphenyls provides the corresponding triphenylene derivatives.

Studies Directed toward the Stereoselective Synthesis of Cytospolide E.

Our exhaustive effort toward the total synthesis of cytotoxic marine nonanolide cytospolide E has been detailed. To achieve this synthesis, we have explored both the ring-closing metathesis and lactonization-based macrocyclization strategies using a variety of precursors. Unfortunately, none of them provided the desired product. The ring-closing metathesis approach provided mainly the macrocycle with Z-olefin, whereas the macrolactonization strategy culminated in 8-epi-9-epi-cytospolide E following the regioselective formation of a 10-membered macrocycle over a 9-membered macrocycle.

Ring-Closing Metathesis Approach to Cage Propellanes Containing Oxepane and Tetrahydrofuran Hybrid System

The preparation of a variety of structurally interesting oxygenated cage compounds involving atom-economic processes such as Claisen rearrangement, Diels–Alder reaction, [2+2] photocycloaddition, and ring-closing metathesis (RCM) as key steps is reported. For the first time, oxepane ring system is introduced in cage framework using olefin metathesis as a key step. These cage systems assembled here are difficult to prepare by traditional methods. The synthetic sequence described here opens up new routes to higher order polycycles containing heteroatoms without the involvement of protecting groups. Transannular cyclization observed during Grignard addition and the RCM protocol used here may be applicable to generate unknown oxygenated cage systems.

Synthesis of deoxyelephantopin analogues.

Deoxyelephantopin is a sesquiterpene lactone that was reported to be as effective in the treatment of mammary tumours and lung metastasis as taxol based on a murine orthotopic cancer model. Its germacrene skeleton harbours three Michael acceptors that can potentially engage a target covalently. Its strained 10-membered ring is densely functionalised and represents an important synthetic challenge. We herein describe our studies towards deoxyelephantopins using a ring-closing metathesis approach and report some unexpected observations.

Piperlongumine derived cyclic sulfonamides (sultams): Synthesis and in vitro exploration for therapeutic potential against HeLa cancer cell lines

A novel modification of piperlongumine is designed, bearing a cyclic sulphonamide (sultam) and its synthesis is described. For the first time herein we report the synthesis and biological evaluation of the natural product derived cyclic sulfonamides using Grubbs second generation catalyst (Grubbs II) via ring closing metathesis approach. Synthesis of a series of piperlongumine derived sultams is done in a moderate to good yield using Wittig reaction, Ring-Closing Metathesis (RCM) and, amide synthesis by using mixed anhydride, approach. All synthesized compounds were evaluated for anticancer activity and some demonstrated dose dependent reduction in HeLa cell growth. Of these 7, 10 and 14 significantly reduced the cell growth. Consequently their calculated GI50 values were found to be 0.1 or <0.1 μM.

Medium-Ring Stereocontrol in the Temporary Silicon-Tethered Ring-Closing Metathesis Approach to the Synthesis of Polyketide Fragments

The temporary silicon-tethered ring-closing metathesis of chiral non-racemic allylic and homoallylic alcohols affords unsymmetrical Z -configured trisubstituted olefins that readily undergo stereoselective hydroboration and dihydroxylation to provide a novel approach to masked polypropionate and polyol fragments present in an array of biologically important natural products. For example, this strategy provides a convenient method for the construction of polyol fragments relevant to the secondary metabolites amphidinol 3 and lophodiol A, which have antifungal and cytotoxic activity, respectively.

Stereoselective Total Synthesis of Atractylodemayne A, a Conjugated 2(E),8(Z),10(E)-Triene-4,6-diyne.

The first total synthesis of the polyacetylene natural product atractylodemayne A is reported. Stereoselective construction of the conjugated 8(Z),10(E)-diene moiety was achieved through a tethered ring-closing metathesis approach, comprising a Ru-catalyzed RCM followed by a base-induced elimination. A Pd-catalyzed Cadiot-Chodkiewicz coupling was used for the synthesis of the diyne. Overall, atractylodemayne A was synthesized in nine steps for the longest linear sequence.

Synthesis of pyrimido[4,5-c]azepine- and pyrimido[4,5-c]oxepine-based γ-secretase modulators.

This Letter describes an efficient ring-closing metathesis approach to 2-chloro-4-amino-pyrimido[4,5-c]azepines and 2-chloro-4-amino-pyrimido[4,5-c]oxepines. These chlorides were applied to the synthesis of several potent γ-secretase modulators (GSMs).

The evolution of a stereoselective synthesis of the C1-C16 fragment of bryostatins.

The development of a synthesis of the C1-C16 fragment of bryostatins in which the key step is a stereoselective oxy-Michael reaction used to assemble the cis-2,6-disubstituted tetrahydropyran with the exocyclic alkene already installed, is described. Following early work using Stille reactions to prepare precursors for oxy-Michael reactions, a more efficient route was devised based on a ketophosphonate-aldehyde condensation to prepare the key dienone. Synthesis of the aldehyde required for the ketophosphonate condensation involved the highly stereoselective addition of a diorganocuprate derived from allylmagnesium bromide and copper(i) iodide to the methyl 5-hydroxyhex-2-ynoate prepared by ring-opening of a protected glycidol using lithiated methyl propiolate. Ester reduction, selective alcohol protection and oxidative cleavage of the terminal double bond gave the required aldehyde. The ketophosphonate was prepared in 13 steps from (R)-pantolactone using a synthesis based on a chelation controlled aldol condensation and an anti-selective reduction of a 3-hydroxyketone. Following the ketophosphonate-aldehyde reaction, selective deprotection followed by treatment with base effected the oxy-Michael reaction that gave the cis-2,6-disubstituted tetrahydropyran via thermodynamic control. Subsequent functional group manipulation led to the synthesis of a hydroxy ester that corresponded to the C1-C16 fragment of the bryostatins in 23 steps from (R)-pantolactone. The synthesis was repeated using slightly different protecting groups for a study of a ring-closing metathesis approach to the bryostatins.

Organometallic Routes to Novel Steroids Containing Heterocyclic C-17 Side-Chains

A range of novel steroid analogues bearing a C-17 side-chain containing a 20 R -hydroxyl group and a variety of heterocyclic substituents have been prepared by organometallic additions to 3-methoxypregnenolone. X-ray crystallography has been used to establish that the organometallic additions proceed with excellent Felkin–Anh control. This methodology has been extended, by use of the Achmatowicz rearrangement and ring-closing metathesis approaches, to prepare pyrandione and δ-lactone steroidal analogues reminiscent of the withanolide natural products.

Ring-closing metathesis approach for the synthesis of optically active l-proline-based macrocycles

Chiral macrocyclic compounds find many interesting applications in supramolecular chemistry, such as in chiral discrimination and chiral catalysis. In this study, the synthesis of novel optically active macrocyclic compounds based on l-proline is reported. The key step in the synthetic strategy is the utility of a ring-closing metathesis (RCM) technique. The synthesized macrocyclic compounds have been characterized by NMR, mass analysis, and single crystal X-ray crystallography. The application of these macrocyclic compounds as enantiodiscriminating agents have been demonstrated using (rac)-mandelic acid (2-hydroxy-2-phenylacetic acid) as a model example.

Simple Ring-Closing Metathesis Approach for Synthesis of PA11, 12, and 13 Precursors from Oleic Acid

Due to the concerns over limited fossil resources and increasing CO2 emissions, there is a strong interest in use of renewable resources in production of valuable synthetic materials. PA11, 12, and 13, more commonly known as Nylon, are high-strength polymers that find use in various industrial sectors. They are traditionally produced from petroleum-derived material or exotic fatty acids using reaction sequences that involve 4–6 steps. Herein we report a simple three-step synthesis of these polyamide precursors from oleic acid, an abundant natural fatty acid available from common plant sources as well as oleaginous micro-organisms such as microalgae. This approach represents the first example where ring-closing metathesis was used for the key C–C bond forming step, featuring introduction of amine functionality required in the final product. Our versatile approach allows preparation of nylon precursors of various chain-lengths from a single starting material for economical and sustainable production of these polymers.

ChemInform Abstract: Titanium Isopropoxide Promoted Tandem Self‐Cross and Ring‐Closing Metathesis Approach for the Synthesis of Macrotetralides.

A new approach is demonstrated for the synthesis of macrotetralides through an olefin metathesis reaction using Grubbs' second-generation catalyst with titanium isopropoxide as a cocatalyst. This study demonstrates a tandem self-cross and ring-closing metathesis approach to form macrocyclic ring systems with excellent (E) selectivity. The reaction was optimized with regard to functional group, catalyst, solvent, Lewis acid, concentration, and temperature.