Ring-rearrangement Metathesis Research Articles

Formal Synthesis of (+)-Sinefungin by Way of Sequential Asymmetric Metal Catalysis.

Here, we report a de novo approach toward (+)-sinefungin, a potent inhibitor of the physiological methyl transfer process. A key feature is represented by the sequential metal catalysis combining Pd-catalyzed hydroalkoxylation and ring-rearrangement metathesis. The unique advantage of the method is highlighted by the unprecedented complete control of the C6 stereocenter.

Synthetic Approach toward Diverse Oxa-Cages via Olefin Metathesis.

This article demonstrates a late-stage modification of the cage propellanes that are transformed into intricate oxa-cycles via ring-rearrangement metathesis (RRM) and regioselective ring-closing metathesis (RCM) as crucial steps. In addition, we also report the extended pentacycloundecane (PCUD)-based oxa-cages involving the domino cycloetherification followed by olefin metathesis. These oxa-cages involve a domino sequence in which the PCUD core unit remains intact. [Ru]-based Grubbs catalysts are used to execute the metathesis step to assemble these higher-order oxa-cage systems. Spectroscopic data assigned structures of various products and were further supported by single-crystal X-ray diffraction analysis. The synthetic approach to these cage polycycles involves high complexity generating processes such as Diels-Alder reaction, [2 + 2] photocycloaddition, and RRM as well as RCM.

Synthesis of Benzoxepine Derivatives via Two‐Fold Tandem Metathesis

AbstractHerein, we report the synthesis of the benzoxepine derivatives via a two‐fold ring rearrangement metathesis (RRM). Using the ring strain as deriving force, we provide a useful approach to assemble the seven‐membered benzo‐oxepine derivatives that are otherwise difficult to realize due to unfavorable entropic reasons. The synthetic route to the RRM products involves the Diels‐Alder reactions, diallylation, and RRM as key steps.

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 1,3‐cis‐Fused Tricyclic System through Regio‐ and Stereoselective Epoxidation and Ring‐Rearrangement Metathesis: Access To Basic Core of Presilphiperfolanols

AbstractHere, we report a new synthetic approach to a highly strained 1,3‐fused cis‐cis‐cis‐5/5/6 tricyclic system that appeared among the presilphiperfolanol analogues. The cis‐fusion of six‐membered ring to a diquinane moiety is accomplished by substrate‐controlled regio‐ and stereoselective epoxidation, and ring‐rearrangement metathesis (RRM) sequence. In this regard, both early‐, and late‐stage epoxidation and RRM reactions were studied. The target compound, cis‐cis‐cis‐5/5/6 tricyclic ketone with an α,β‐oxirane ring can serve as a key building block to produce various presilphiperfolanol analogues. The cis‐cis‐cis‐5/5/6‐tricyclic‐α,β‐epoxyketone can also act as masked enone and therefore useful for various chemical transformations in organic synthesis. The short synthetic sequence disclosed here may be useful to design various biologically important molecules. The newly reported molecules are well‐characterized by NMR, HRMS, and IR spectral data.

Stereocontrolled synthesis of some novel functionalized heterocyclic amino ester and amide derivatives with multiple stereocenters.

The synthesis of some novel functionalized fused-ring β-amino lactones and lactams with multiple chiral centers has been attempted from readily available strained bicyclic β-amino acids via a stereocontrolled synthetic route. The key step was ring-rearrangement metathesis of allyl/propargyl esters or N-allylated/N-propargylated amides of (oxa)norbornene β-amino acids. The RRM transformations [ring-opening metathesis (ROM)/ring-closing metathesis (RCM) or ring-opening metathesis (ROM)/ring-closing enyne metathesis (RCEYM)] have been investigated using some commercially available catalysts. Importantly, the procedure used in this synthetic process does not affect the configurations of the chiral centers. This means that the structure of the starting (oxa)norbornene β-amino acids predetermines the configuration of the formed products.

Synthesis of New β‐Amino Acid Scaffolds by Means of Ring‐Rearrangement Metathesis

AbstractThe synthesis of some novel functionalized azaheterocyclic β‐amino esters with multiple chiral centers via a stereocontrolled synthetic route has been carried out using some cyclooctene β‐amino acids as starting model compounds. The strategy of the method was planned to create some novel unsaturated N‐protected cyclic β‐amino esters that were subjected to ring‐opening metathesis (ROM) followed by selective ring‐closing metathesis (RCM). A number of experimental conditions were accomplished to investigate the activity of catalysts, yields, conversions, and substrate effect on ring‐rearrangement metathesis (RRM) transformation. Importantly, the procedure used in this synthetic process does not affect the configuration of the chiral centers; therefore, the stereochemistry of the starting cyclooctadiene β‐amino acids predetermined the structure of the new azaheterocyclic derivatives.

Ring-Rearrangement Metathetic Approach to Fused 6/5/6/5/6-Oxacyclic Ring System and Bipentalene Derivatives

AbstractWe have developed a useful synthetic route to a 6/5/6/5/6-oxacyclic ring system and bipentalene derivatives from dimeric 7-oxonorbornene derivatives by using ring-rearrangement metathesis as a key step. This method provides access to fused oxacycles containing eight stereogenic centers in just three steps and to bipentalene derivatives in two steps only.

Synthesis of Linear Tetraquinanes by [3+2] Cycloaddition, Chemoselective Allylation of 7-Ketonorbornene, and Ring-Rearrangement Metathesis as Key Steps

AbstractA nine-step synthetic sequence to linear tetraquinanes involving [3+2] cycloaddition, chemoselective allylation, and ring-rearrangement metathesis as key steps is reported. A chemoselective allylation of 7-ketonorbornene was realized for the first time by using indium powder and allyl bromide. By this method, norbornene ketones can be selectively allylated in the presence of a cyclopentanone moiety to give good yields of monoallylated Barbier-type products.

Synthetic Approaches to Crinipellin Based Tetraquinanes via Ring‐Rearrangement Metathesis and Ring‐Closing Metathesis

AbstractHerein, we have studied the synthetic approaches to tetraquinanes that are present in crinipellin type natural products by employing ring‐rearrangement metathesis (RRM) and ring‐closing metathesis (RCM) as key steps. These studies involve finding the suitable alkylation sequence to prepare desired RRM/RCM precursors. Methyl‐tetraquinane has been assembled by employing a regio‐, and stereoselective methylation followed by allylation sequence. Whereas, allyl‐tetraquinane was constructed by following the allylation‐RRM sequence. Interestingly, spiro‐quinanes rearranges to fused‐quinanes when olefinic moiety is present adjacent to the spiro‐ring under RRM conditions. Additionally, several di‐, and triquinane derivatives reported during this study were also found in diverse polyquinane natural products. The present strategy utilizes the vinyl derivative of exo‐dicyclopentadiene‐1‐one, and produce several intricate polycyclic systems. Our strategy may be useful to design various medicinally important “drug‐like” molecules.

A Modular Approach to Angularly Fused Polyquinanes via Ring-Rearrangement Metathesis: Synthetic Access to Cameroonanol Analogues and the Basic Core of Subergorgic Acid and Crinipellin.

We describe a modular approach to angularly fused polyquinanes that are core units of many natural products such as cameroonanol, subergorgic acid, and crinepellin, etc. in excellent yields by employing atom-economic ring-rearrangement metathesis as a key step. Our work highlights, the synthesis of cameroonanol analogues 1-6 and their ester derivatives by using the stereoselective reduction of the carbonyl group by using DIBAL-H- and DCC-mediated coupling as the key reactions. The subergorgic acid core 7 was produced by LDA-mediated kinetically controlled regio- and stereoselective ring-junction allylation as a critical step. Moreover, it is worth mentioning that the present strategy relies on a less explored exo-dicyclopentadiene-1-one (8) and produces highly congested polycyclic frameworks containing up to seven contiguous stereogenic centers including quaternary carbons up to two. All of the new molecules were characterized by NMR data. The structure and relative stereochemistry of some compounds were confirmed by chemical methods and further supported by single-crystal X-ray diffraction studies. The newly reported tri- and tetraquinane skeletons are present in many naturally occurring bioactive polyquinanes. Hence, this strategy is useful for designing various "druglike molecules" and expands the chemical space of cyclopentanoids that are useful in medicinal chemistry.

Synthesis of Angular Triquinane and [4.3.3]Propellane Derivatives via Ring‐Rearrangement Olefin Metathesis

AbstractHerein, a simple and useful approach to angular triquinane and [4.3.3]propellane moieties is described via olefin metathesis starting from exo and endo Diels‐Alder (DA) adducts containing norbornene unit. This approach involves DA reaction, and ring‐rearrangement metathesis (RRM) as key steps. Strained norbornene derivatives on allylation at bridgehead position(s) followed by ring‐opening metathesis and ring‐closing metathesis sequence delivered angular triquinane and also stereochemically different propellane derivatives.

Synthesis of Polycycles and Oxacycles via Tandem Metathesis of endo-Norbornene Derivatives

AbstractHere, we have demonstrated that the presence of a carbonyl group at C7 position prevents the olefin metathesis of endo-norbornene derivatives due to complexation of the metal alkylidene. Time-dependent NMR studies showed the presence of new proton signals in the metal alkylidene region, which indicate the formation of metal complex with the carbonyl group of the substrate. These observations were further proved by ESI-MS analysis. Whereas computational studies showed that the catalyst was interacting with the C7 carbonyl group and aligned perpendicular to that of norbornene olefin. These endo-keto norbornene derivatives were reduced to hydroxyl derivatives dia­stereoselectively. Ring-rearrangement metathesis (RRM) of these hydroxyl derivatives, produced the [6/5/6], and [5/6/5] carbo-tricyclic cores of natural products in one step. Whereas the RRM of O-allyl derivatives, delivered the oxa-tricyclic compounds in a single step with excellent yields.

Synthesis of spiro-annulated cyclobutane derivatives through ketene [2+2] cycloaddition and ring-rearrangement metathesis

Herein is reported a concise synthesis of spiro-annulated cyclobutane tetracyclic and pentacyclic derivatives by ketene addition, and ring-rearrangement metathesis (RRM) as key steps, starting with commercially available norbornadiene and dicyclopentadiene. The tetracyclic spiro-derivative contains a [5/5/4] core unit, which is the key building block to angular triquinanes synthesis. Whereas, the pentacyclic spiro-derivative contains a basic core skeleton of presilphiperfolanes, and other sesquiterpenoids.

Modular Approaches to Cyclopentanoids and their Heteroanalogs

AbstractCyclopentanoids and their derivatives are interesting targets in synthetic organic chemistry due to their extensive applications in various branches of chemical sciences like pharmaceuticals, natural and non-natural products. In view of these applications, several synthetic strategies have been developed in the past three to four decades. In this article, we describe our work towards the synthesis of cyclopentanoids and their heteroanalogs involving diverse synthetic strategies during the past two decades. Among these, photo-thermal olefin metathesis, ring-closing metathesis, ring-rearrangement metathesis, cyclopentane annulation, [2+2+2] cycloaddition and Diels–Alder reactions have been used to assemble cyclopentane rings of diverse architecture. 1 Introduction 2 Synthesis of Spiro[4.4]nonane (A1) Derivatives 3 Synthesis of Octahydropentalene (A2) Derivatives 4 Synthesis of Linear Triquinanes (A3) 5 Synthesis Spiro Triquinanes (A4) 6 Synthesis of Angular Triquinane (A5) Systems 7 Synthesis of Hexahydro-2′H-spiro[cyclopentane-1,1′-pentalene] (A6) Ring System 8 Synthesis of Dispiro[4.1.47.25]tridecane (A7) Ring System 9 Synthesis of Hexahydro-1H-3a,7a-propanoindene Ring System10 Synthesis of Linear Tetraquinanes (A11 and A12)11 Synthesis of Tetrahydro-1′H,3′H-dispiro[cyclopentane-1,2′-pentalene-5′,1′′-cyclopentane] (A13) Ring System12 Synthesis of Decahydro-1H,8H-dicyclopenta[a,h]pentalene (A14) Ring System13 Synthesis of Dodecahydro-1H-dicyclopenta[a,d]pentalene (A15) Ring System14 Synthesis of Octahydro-1′H-spiro[cyclopentane-1,2′-cyclopenta[c]pentalene] (A16) Ring System15 Synthesis of Decahydrospiro[cyclopentane-1,7′-cyclopenta-[a]pentalene] (A17) Ring System16 Synthesis of Compact Tetraquinane (A18)17 Synthesis of Higher Polyquinanes18 Conclusions19 Acronyms

Application of New Efficient Hoveyda-Grubbs Catalysts Comprising an N→Ru Coordinate Bond in a Six-Membered Ring for the Synthesis of Natural Product-Like Cyclopenta[b]furo[2,3-c]pyrroles.

The ring rearrangement metathesis (RRM) of a trans-cis diastereomer mixture of methyl 3-allyl-3a,6-epoxyisoindole-7-carboxylates derived from cheap, accessible and renewable furan-based precursors in the presence of a new class of Hoveyda–Grubbs-type catalysts, comprising an N→Ru coordinate bond in a six-membered ring, results in the difficult-to-obtain natural product-like cyclopenta[b]furo[2,3-c]pyrroles. In this process, only one diastereomer with a trans-arrangement of the 3-allyl fragment relative to the 3a,6-epoxy bridge enters into the rearrangement, while the cis-isomers polymerize almost completely under the same conditions. The tested catalysts are active in the temperature range from 60 to 120 °C at a concentration of 0.5 mol % and provide better yields of the target tricycles compared to the most popular commercially available second-generation Hoveyda–Grubbs catalyst. The diastereoselectivity of the intramolecular Diels–Alder reaction furan (IMDAF) reaction between starting 1-(furan-2-yl)but-3-en-1-amines and maleic anhydride, leading to 3a,6-epoxyisoindole-7-carboxylates, was studied as well.

Selective ring-rearrangement or ring-closing metathesis of bicyclo[3.2.1]octenes.

Explored was the competitive ring-closing metathesis vs. ring-rearrangement metathesis of bicyclo[3.2.1]octenes prepared by a simple and convergent synthesis from bicyclic alkylidenemalono-nitriles and allylic electrophiles. It was uncovered that ring-closing metathesis occurs exclusively on the tetraene-variant, yielding unique, stereochemically and functionally rich polycyclic bridged frameworks, whereas the reduced version (a triene) undergoes ring-rearrangement metathesis to 5-6-5 fused ring systems resembling the isoryanodane core.

Synthetic Strategies to Diverse Polyquinanes via Olefin Metathesis: Access to the Basic Core of Crinipellin, Presilphiperfolanol, and Cucumin.

A rapid and useful synthetic approach to various polyquinane-based natural products was accomplished efficiently by employing ring-rearrangement metathesis and ring-closing metathesis as key steps. Here, we report the synthesis of stereochemically well-defined cis-anti-cis triquinanes (1, 2), tetraquinanes (3, 4), a novel pentaquinane 5, and fused [5-5-5-6] tetracyclic systems (6, 7) that are present in crinipellin, presilphiperfolanol, cucumin, etc. Hence, the current strategy may be suitable for the synthesis of various complex natural and unnatural cyclopentanoid targets. Moreover, our approach to the newly synthesized pentaquinane 5 has paved the way for various complex polyquinanes/molecules having significant applications in theoretical and medicinal chemistry.

Synthesis of Aza-polyquinanes via Fischer Indolization and Ring-Rearrangement Metathesis as Key Steps

Herein, we describe a simple synthetic strategy to assemble aza-polyquinane systems containing an indole motif by employing the Fischer indolization and ring-rearrangement metathesis as key steps. The precursor used here is exo-dicyclopentadienone, which is derived from less explored exo-dicyclopentadiene. By using this approach, several aza-polyquinanes that contain indole units and fused medium rings (eight- and nine-membered rings) were synthesized in good yields.

Exploration of Ring Rearrangement Metathesis Reaction: A General and Flexible Approach for the Rapid Construction [5,n]-Fused Bicyclic Systems en Route to Linear Triquinanes.

Structurally diverse [5,n] bicyclic systems with cis ring junction stereochemistry were accessed readily through RRM (ring rearrangement metathesis) reaction of properly functionalized [2.2.1]norbornene skeletons. Several bicyclic enones, ketones, alcohols, and ethers acted as the substrates and yielded the respective linearly fused [5,n] bicyclic systems stereoselectively after the RRM reaction. Such [5,5]bicyclic enone scaffolds were then synthetically manipulated to core structural analogue of naturally occurring liner triquinane hirsutene having cis-syn-cis stereochemistry.