Ring-closing Olefin Metathesis Research Articles

Folding a Molecular Strand into a Trefoil Knot of Single Handedness with Co(II)/Co(III) Chaperones.

We report the synthesis of a right-handed (Δ-stereochemistry of strand crossings) trefoil knot from a single molecular strand containing three pyrazine-2,5-dicarboxamide units adjacent to point-chiral centers and six pyridine moieties. The oligomeric ligand strand folds into an overhand (open-trefoil) knot through the assistance of coordinatively dynamic Co(II) "chaperones" that drive the formation of a three-metal-ion circular helicate. The entangled structure is kinetically locked by oxidation to Co(III) and covalently captured by ring-closing olefin metathesis to generate a trefoil knot of single topological handedness. The stereochemistry of the strand crossings in the metal-coordinated overhand knot is governed by the stereochemistry of the point-chiral carbon centers in the ligand strand. The overhand and trefoil knots were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Removal of the metal ions from the knot, followed by hydrogenation of the alkene, yielded the wholly organic trefoil knot. The metal-free knot and parent ligand were investigated by circular dichroism (CD) spectroscopy. The CD spectra indicate that the topological stereochemistry of the knot has a greater effect on the asymmetry of the chromophore environment than do the point-chiral centers of the strand.

Total Synthesis of (+)-Euphorikanin A via an Atropospecific Cascade.

A total synthesis of the ingenane-derived diterpenoid (+)-euphorikanin A is described. Key to the strategy is a stereocontrolled one-pot sequence consisting of transannular aldol addition reaction, hemiketal formation, and subsequent semipinacol rearrangement that efficiently leads to the complete euphorikanin skeleton. Atroposelective ring-closing olefin metathesis proved critical for the stereospecific cascade, leading to formation of a (Z)-bicyclo[7.4.1]tetradecenone core. An additional salient feature of the route is pyrolysis of a bis-methylxanthate to cleanly furnish the natural product.

Expanding the Diversity of Azepino [3,2,1‐hi] Indoles via Transition Metal‐based Strategies such as Cross‐coupling Reaction and Ring Closing Metathesis

AbstractHere we report several useful synthetic strategies to azepino hi‐indoles, that are structural units in natural products by using commercially available indole derivatives such as 7‐bromo indole and 7‐formylindole via palladium catalyzed cross‐coupling reaction, ring closing olefin metathesis (RCM) as key steps. Later, we extended this strategy to seven‐membered fused indoles containing dioxano and dioxocano derivatives via allylation, double bond isomerization and RCM sequence as key step. Additionally, azepino indole having diene moiety was obtained by enyne metathesis and shown to be useful for Diels−Alder strategy. Along similar lines, we also report eight‐membered fused azocino indole using Grignard reaction and RCM as key steps and we demonstrated that the product obtained is useful for Fischer indolization to obtain bis‐indole derivative. Here several synthetically useful transformations were performed to generate diversity in the fused indole derivatives. All the compounds synthesized here are well characterized by NMR and HRMS data.Introduction

Synthetic Study of Natural Metabolites Containing a Benzo[c]oxepine Skeleton: Heterocornol C and D.

A versatile strategy for the enantioselective synthesis of a benzo[c]oxepine structural core containing natural secondary metabolites was developed. The key steps of the synthetic approach include ring-closing alkene metathesis for seven-member ring construction, the Suzuki-Miyaura cross-coupling reaction for the installation of the double bond and Katsuki-Sharpless asymmetric epoxidation for the introduction of chiral centers. The first total synthesis and absolute configuration assignment of heterocornol D (3a) were achieved. Four stereoisomers, 3a, ent-3a, 3b and ent-3b, of this natural polyketide were prepared, starting with 2,6-dihydroxy benzoic acid and divinyl carbinol. The absolute and relative configuration of heterocornol D was assigned via single-crystal X-ray analysis. The extension of the described synthetic approach is further presented with the synthesis of heterocornol C by applying the ether group reduction method to the lactone.

Highly Strained Oxygen-Doped Chiral Molecular Belts of the Zigzag-Type with Strong Circularly Polarized Luminescence.

Herein we report a two-directional cyclization strategy for the synthesis of highly strained depth-expanded oxygen-doped chiral molecular belts of the zigzag-type. From the easily accessible resorcin[4]arenes, an unprecedented cyclization cascade generating fused 2,3-dihydro-1H-phenalenes has been developed to access expanded molecular belts. Stitching up the fjords through intramolecular nucleophilic aromatic substitution and ring-closing olefin metathesis reactions furnished a highly strained O-doped C2 -symmetric belt. The enantiomers of the acquired compounds exhibited excellent chiroptical properties. The calculated parallelly aligned electric (μ) and magnetic (m) transition dipole moments are translated to the high dissymmetry factor (|glum | up to 0.022). This study provides not only an appealing and useful strategy for the synthesis of strained molecular belts but also a new paradigm for the fabrication of belt-derived chiroptical materials with high CPL activities.

A Star of David [2]catenane of single handedness

<h2>Summary</h2> Prime links with multiple interlocked components feature unconditional topological chirality. Although several prime links have been constructed to date, the control of their chirality with high stereoinduction is rare. Here, we report the stereoselective synthesis of an iconic molecular <mml:math><mml:mrow><mml:mn>6</mml:mn><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mrow></mml:math> link (Star of David [2]catenane) comprising two triply entwined 84-membered rings. The single-handed Star of David catenane is generated from the closure of a hexameric copper(I) circular helicate by ring-closing olefin metathesis, and the topological writhe (<i>w</i> = −6) was predetermined by the configuration of the chiral ligand. Conformation of the topology was proven through NMR spectroscopy and mass spectrometry, whereas the handedness of the link was further investigated through circular dichroism. Stereoselective synthesis of molecular nanotopologies with certain topological writhe by chiral information transfer opens the way to probe the consequences and potential of topological stereochemistry in chemistry, materials, and biology.

Syntheses of 6,7-Benzotropolones by a Sequence of Acylation (or α-Hydroxyalkylation/Oxidation), Ring-Closing Metathesis, and Hydrolysis/β-Elimination.

Starting from ortho-bromostyrenes, 12 6,7-benzotropolones were synthesized in 3 or 4 steps. Br/Li exchanges provided the respective ortho-lithiostyrenes. The latter were acylated by a Weinreb amide, if unhindered, or hydroxyalkylated by an aldehyde, if hindered (an oxidation ensuing thereafter). Acetonide-containing benzannulated nonatrienones resulted. Ring-closing olefin metatheses converted them into acetonide-containing benzocycloheptadienones. Treatment with aq. HCl caused hydrolyses followed by β-elimination reactions giving 6,7-benzotropolones.

Social Self-Sorting Synthesis of Molecular Knots.

We report the synthesis of molecular prime and composite knots by social self-sorting of 2,6-pyridinedicarboxamide (pdc) ligands of differing topicity and stereochemistry. Upon mixing achiral monotopic and ditopic pdc-ligand strands in a 1:1:1 ratio with Lu(III), a well-defined heteromeric complex featuring one of each ligand strand and the metal ion is selectively formed. Introducing point-chiral centers into the ligands leads to single-sense helical stereochemistry of the resulting coordination complex. Covalent capture of the entangled structure by ring-closing olefin metathesis then gives a socially self-sorted trefoil knot of single topological handedness. In a related manner, a heteromeric molecular granny knot (a six-crossing composite knot featuring two trefoil tangles of the same handedness) was assembled from social self-sorting of ditopic and tetratopic multi-pdc strands. A molecular square knot (a six-crossing composite knot of two trefoil tangles of opposite handedness) was assembled by social self-sorting of a ditopic pdc strand with four (S)-centers and a tetratopic strand with two (S)- and six (R)-centers. Each of the entangled structures was characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and circular dichroism spectroscopy. The precise control of composition and topological chirality through social self-sorting enables the rapid assembly of well-defined sequences of entanglements for molecular knots.

E/ZSwitchable Ring-Closing Metathesis in 1,1′-Bis(but-3-enyl)ferrocenes: Synthesis and Characterization of Axially Chiralansa[6]-Ferrocenes

A series of ansa[6]-ferrocenes were obtained through the ruthenium-catalyzed ring-closing olefin metathesis in 1,1′-bis(but-3-enyl)ferrocenes. The stereoselectivity of this reaction depended on the structure of substrates, that is, mixtures of (E) and (Z) isomers were obtained for alkyl-substituted 3a–3d while (Z)-isomers only for aryl-substituted 3e and 3f. The ansa[6]-ferrocenes are axially chiral in the solid state according to single-crystal X-ray analysis. In the case of methyl-substituted derivative 4a, spontaneous resolution of the racemate was observed during crystallization. The solution properties of (E) and (Z) isomers are remarkably different; the latter displays inequivalence of the two formally identical parts of the molecule at low temperature.

Hydrazone–Olefin and Oxime–Olefin Ring-Closing Metathesis

Key words hydrazones - olefination - oximes - ring-closing metathesis - ruthenium alkylidenes

Vernier template synthesis of molecular knots.

Molecular knots are often prepared using metal helicates to cross the strands. We found that coordinatively mismatching oligodentate ligands and metal ions provides a more effective way to synthesize larger knots using Vernier templating. Strands composed of different numbers of tridentate 2,6-pyridinedicarboxamide groups fold around nine-coordinate lanthanide (III) ions to generate strand-entangled complexes with the lowest common multiple of coordination sites for the ligand strands and metal ions. Ring-closing olefin metathesis then completes the knots. A 3:2 (ditopic strand:metal) Vernier assembly produces +31#+31 and -31#-31 granny knots. Vernier complexes of 3:4 (tetratopic strand:metal) stoichiometry selectively form a 378-atom-long trefoil-of-trefoils triskelion knot with 12 alternating strand crossings or, by using opposing stereochemistry at the terminus of the strand, an inverted-core triskelion knot with six alternating and six nonalternating strand crossings.

Vanadium Imido NHC Complexes for Ring-Closing Olefin Metathesis Reactions

Vanadium bis-phosphine imido and oxo chloride alkylidenes have been extensively applied in the ring-closing metathesis of various acyclic olefins. However, their reactions involving ethylene have shown limited productivity due to rapid decomposition. The primary degradation pathway involving V bis-phosphine imido complexes is β-H elimination at an unsubstituted metallacyclobutane. In contrast, β-H elimination is disfavored for V oxo species, but bimolecular decomposition precludes its high productivity. Herein, we present the synthesis of V imido NHC complexes that are the most productive V catalysts toward various terminal olefins in ring-closing metathesis reactions. Experimental and computational studies suggest that β-H elimination and bimolecular decomposition are disfavored for V imido NHC complexes.

Total Synthesis of Euonymine and Euonyminol Octaacetate.

Euonymine (1) and euonyminol octaacetate (2) share the core structure of euonyminol (3), the most hydroxylated member of the dihydro-β-agarofuran family. In 2, eight of the nine hydroxy groups of 3 are acetylated, and 1 has six acetyl groups and a 14-membered bislactone comprising a pyridine dicarboxylic acid with two methyl groups. The different acylation patterns provide distinct biological activities: 1 and 2 display anti-HIV and P-glycoprotein inhibitory effects, respectively. The 11 contiguous stereocenters and 9 oxygen functionalities of the ABC-ring system of 1 and 2 represent a formidable challenge, which is further heightened by the macrocyclic structure of 1. Here we disclose an efficient synthetic strategy for enantioselective total synthesis of 1 and 2. Starting from (R)-glycerol acetonide, we constructed the B-ring by an Et3N-accelerated Diels-Alder reaction, the C-ring by intramolecular iodoetherification, and the A-ring by ring-closing olefin metathesis. The 10 stereocenters were installed through a series of substrate-controlled stereoselective C-C and C-O bond formations by exploiting the three-dimensional structures of judiciously designed substrates. These newly developed reaction sequences led to protected euonyminol 5, which served as a common intermediate for assembling 1 and 2. Global deprotection of 5 and subsequent acetylation produced 2. Alternatively, the discriminative protective groups of 5 allowed for site-selective bis-esterification to generate bislactone. Combining [3 + 2]-cycloaddition and reductive desulfurization introduced the last remaining stereocenters of the two methyl groups on the macrocycle. Finally, deprotection and acetylation gave rise to fully synthetic 1 for the first time.

Confinement Effects for Efficient Macrocyclization Reactions with Supported Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido ...

Synthesis of Vanadium Oxo Alkylidene Complex and Its Reactivity in Ring-Closing Olefin Metathesis Reactions

V imido alkylidenes have been applied for the ring-opening metathesis polymerization involving cyclic olefins. However, those complexes found limited application in reactions with acyclic terminal olefins due to instability toward ethylene. Experimental and theoretical studies show that the β-hydride elimination from unsubstituted metallacyclobutene is the primary decomposition pathway in those systems. Herein, we report the synthesis of the first catalytically active V oxo alkylidene, VO(CHSiMe3)(PEt3)2Cl, which exhibits the highest reported productivity with various terminal olefins in ring-closing metathesis reactions among known V catalysts. Presented DFT studies indicate that β-hydride elimination is significantly disfavored for V oxo species.

A Unified Approach to Polycyclic Alkaloids of the Ingenamine Estate: Total Syntheses of Keramaphidin B, Ingenamine, and Nominal Njaoamine I.

Many polycyclic marine alkaloids are thought to derive from partly reduced macrocyclic alkylpyridine derivatives via a transannular Diels–Alder reaction that forms their common etheno-bridged diaza-decaline core (“Baldwin–Whitehead hypothesis”). Rather than trying to emulate this biosynthesis pathway, a route to these natural products following purely chemical logic was pursued. Specifically, a Michael/Michael addition cascade provided rapid access to this conspicuous tricyclic scaffold and allowed different handles to be introduced at the bridgehead quarternary center. This flexibility opened opportunities for the formation of the enveloping medium-sized and macrocyclic rings. Ring closing alkyne metathesis (RCAM) proved most reliable and became a recurrent theme en route to keramaphidin B, ingenamine, xestocyclamine A, and nominal njaoamine I (the structure of which had to be corrected in the aftermath of the synthesis). Best results were obtained with molybdenum alkylidyne catalysts endowed with (tripodal) silanolate ligands, which proved fully operative in the presence of tertiary amines, quinoline, and other Lewis basic sites. RCAM was successfully interlinked with macrolactamization, an intricate hydroboration/protonation/alkyl-Suzuki coupling sequence, or ring closing olefin metathesis (RCM) for the closure of the second lateral ring; the use of RCM for the formation of an 11-membered cycle is particularly noteworthy. Equally rare are RCM reactions that leave a pre-existing triple bond untouched, as the standard ruthenium catalysts are usually indiscriminative vis-à-vis the different π-bonds. Of arguably highest significance, however, is the use of two consecutive or even concurrent RCAM reactions en route to nominal njaoamine I as the arguably most complex of the chosen targets.

Ring-Closing Olefin Metathesis Catalyzed by Well-Defined Vanadium Alkylidene Complexes.

Vanadium-based catalysts have shown activity and selectivity in ring-opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring-closing olefin metathesis catalyzed by well-defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron-donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both β-hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.

Selectively Depolymerizable Polyurethanes from Unsaturated Polyols Cleavable by Olefin Metathesis

Inside Cover: In article number 2000571 by Brad H. Jones and coworkers, linear and crosslinked polyurethanes are prepared from traditional isocyanates and new polyols containing unsaturated moieties designed to favor ring-closing olefin metathesis and concomitant cleavage of the molecule. These polyurethanes are found to be efficiently depolymerized upon exposure to second generation Hoveyda–Grubbs catalyst under ambient conditions. The depolymerization products are multi-vinyl/cycloalkene species that can be re-polymerized.

Olefin metathesis of syn,syn-2,4,6-triallyl-hexahydro-1,3,5-tripropionyl-s-triazine and nitrolysis products

Ring closing olefin metathesis of syn,syn-2,4,6-triallyl-hexahydro-1,3,5-tripropionyl-s-triazine gave an intramolecular cyclized product and an intra/intermolecular dimerized product depending on the choice of organometallic ruthenium catalyst. The intramolecular cyclized product was further reduced and then reacted under nitrolysis conditions with trifluoroacetyl nitrate to give endo-8-(1-propyl)-7,9,10-trinitro-7,9,10-triazabicyclo[4.3.1]decane in 25% yield.

Design and synthesis of 14 and 15-membered macrocyclic scaffolds exhibiting inhibitory activities of hypoxia-inducible factor 1α

Inspired by the privileged molecular skeletons of 14- and 15-membered antibiotics, we adopted a relatively unexplored synthetic approach that exploits alkaloidal macrocyclic scaffolds to generate modulators of protein–protein interactions (PPIs). As mimetics of hot-spot residues in the α-helices responsible for the transcriptional regulation, three hydrophobic sidechains were displayed on each of the four distinct macrocyclic scaffolds generating diversity of their spatial arrangements. Modular assembly of the building blocks followed by ring-closing olefin metathesis reaction and subsequent hydrogenation allowed concise and divergent synthesis of scaffolds 1–4. The 14-membered alkaloidal macrocycles 2–4 demonstrated similar inhibition of hypoxia-inducible factor (HIF)-1α transcriptional activities (IC50 between 8.7 and 10 µM), and 4 demonstrated the most potent inhibition of cell proliferation in vitro (IC50 = 12 µM against HTC116 colon cancer cell line). A docking model suggested that 4 could mimic the LLxxL motif in HIF-1α, in which the three sidechains are capable of matching the spatial arrangements of the protein hot-spot residues. Unlike most of the stapled peptides, the 14-membered alkaloidal scaffold has a similar size to the α-helix backbone and does not require additional atoms to induce α-helix mimetic structure. These experimental results underscore the potential of alkaloidal macrocyclic scaffolds featuring flexibly customizable skeletal, stereochemical, substitutional, and conformational properties for the development of non-peptidyl PPI modulators targeting α-helix-forming consensus sequences responsible for the transcriptional regulation.