Highly efficient ruthenium catalysts for the formation of tetrasubstituted olefins via ring-closing metathesis

[reaction: see text] A series of ruthenium-based metathesis catalysts with N-heterocyclic carbene (NHC) ligands have been prepared in which the N-aryl groups have been changed from mesityl to mono-ortho-substituted phenyl (e.g., tolyl). These new catalysts offer an exceptional increase in activity for the formation of tetrasubstituted olefins via ring-closing metathesis (RCM), while maintaining high levels of activity in ring-closing metathesis (RCM) reactions that generate di- and trisubstituted olefins.     

An imine addition/ring-closing metathesis approach to the spirocyclic core of halichlorine and pinnaic acid

[reaction: see text] An approach to the spirocyclic core of halichlorine and pinnaic acid has been designed around an imine allylation/ring-closing metathesis sequence. This sequence has been used to generate several azabicylo[n.5] model systems. A newly reported metathesis catalyst was show to be highly effective for cyclization of these systems.     

Synthesis of substituted cyclohexenyl-based beta-amino acids by ring-closing metathesis

[structure: see text] A versatile ring-closing metathesis (RCM) approach has been developed for the preparation of cis and trans cyclohexenyl-based beta-amino acids that are either unsubstituted (3) or substituted (10 and 12) at the alpha-position.     

Polycyclic aromatic hydrocarbons by ring-closing metathesis

A strategy for the synthesis of polycyclic aromatic hydrocarbons (PAHs) by the ring-closing olefin metathesis (RCM) of pendant olefins on a phenylene backbone has been developed. RCM of 2,4',6',2' '-tetravinyl-[1,1';3',1' ']terphenyl and 2,2',5',2' '-tetravinyl-[1,1';4',1']terphenyl affords in high yield the isomeric [a,j] and [a,h] dibenzanthracenes, respectively. In contrast with other intramolecular annulation methods, such as Friedel-Crafts acylations, this reaction is completely regioselective. Since RCM is reversible and PAHs are often thermodynamic sinks, this strategy is an effective and general method for the preparation of PAHs. Density functional theory calculations support these results. Carbon disulfide is a suitable solvent for these reactions.     

Diastereotopic differentiation on phosphorus templates via the ring-closing metathesis reaction

[reaction: see text] A strategy is described in which the ring-closing metathesis reaction is utilized to desymmetrize a number of pseudo-C(2)-symmetric phosphorus templates 1-3. These reactions give excellent levels of selectivity (12-15:1) with vinyl phosphonamides containing a (E)-Ph group on the diastereotopic olefins. This approach is being developed as an effective method of obtaining P-chiral phosphonamides and phosphonates.     

A sequential Claisen/ring-closing metathesis approach to the synthesis of spirocyclic cyclopentanes and cyclohexanes

A new method for the formation of spirocycles is described using a sequential Ireland-Claisen/Metathesis strategy to construct spirocyclic systems. Optimization of the Ireland-Claisen conditions provides the key metathesis substrates. The metathesis reactions were highly regioselective by virtue of steric hindrance in the substrate olefins.     

Tandem cyclopropanation/ring-closing metathesis of dienynes

Certain dienynes give cyclorearrangement by tandem cyclopropanation/ring-closing alkene metathesis, triggered by either a ruthenium carbene or noncarbene ruthenium(II) precatalyst. The process represents a variation of enyne metathesis where presumed cyclopropyl carbene intermediates undergo a consecutive ring-closing metathesis. A mechanistic proposal is offered, and sequential use of catalysts provided a tandem ring-closing enyne/alkene metathesis product.     

Synthesis of unsaturated [1,2]oxazines by using sigmatropic rearrangements and the ring-closing metathesis reaction

Various substituted unsaturated [1,2]oxazines have been synthesized by using a [2,3]- or a [3,3]-sigmatropic rearrangement and a ring-closing metathesis reaction as key steps.     

Asymmetric total syntheses of (+)-mycoepoxydiene and related natural product (-)-1893A: application of one-pot ring-opening/cross/ring-closing metathesis to construct their 9-oxabicyclo[4.2.1]nona-2,4-diene skeleton

The total syntheses of (+)-mycoepoxydiene and (-)-1893A have been completed. The present synthetic strategy features the use of one-pot ring-opening/cross metathesis (ROM/CM) followed by a ring-closing metathesis (RCM) reaction, allowing for the concise construction of the 9-oxabicyclo[4.2.1]nona-2,4-diene framework from a 7-oxabicyclo[2.2.1]hept-2-ene derivative and 1,3-butadiene. The sequential metathesis product was converted into (+)-mycoepoxydiene through the oxidative rearrangement of a furfuryl alcohol to a pyranone, thereby establishing its absolute stereochemistry. From the common intermediate, a structurally related natural product (-)-1893A was also synthesized via the vinylogous aldol reaction.     

Total synthesis of (+)-phomopsolide C by ring-size selective ring-closing metathesis/cross-metathesis

[reaction: see text]A new strategy for the synthesis of chiral 6-substituted 5,6-dihydro-5-hydroxypyran-2-ones by ring-size selective ring-closing metathesis (RCM) and its application to a short total synthesis of (+)-phomopsolide C are described. The key bond-forming reactions in this approach are a chemoselective cross-metathesis (CM) and RCM.     

Synthesis of bridged azabicyclic structures via ring-closing olefin metathesis

A new strategy for the facile synthesis of azabicyclo[m.n.1]alkenes (m = 3-5; n = 3, 2) has been developed that involves the ring-closing metathesis (RCM) reaction of cis-2,6-dialkenyl-N-acyl piperidine derivatives. The requisite 2,6-dialkenylpiperidines may be readily prepared in six steps starting from glutarimide (11) or three steps from 4-methoxypyridine (25). In one example that establishes the practical utility of the procedure, the functionalized 8-azabicyclo[3.2.1]octane 32, which is a potential intermediate for the syntheses of various tropane alkaloids, was prepared. Additionally, a new route for the construction of the bridged tetrahydro-beta-carboline ring system 5 has been developed that features the ring-closing metathesis of the enyne 45 to construct the bridging ring in 46. This concise route to 46 also features a potentially general and useful procedure for the one-step preparation of a terminal alkyne from an ester function. Selective oxidation of the vinyl group in 46 afforded the unsaturated aldehyde 47, which may serve as a useful intermediate in syntheses of several Sarpagine alkaloids.     

A double ring-closing olefin metathesis approach to [3]catenanes

A [3]catenane with peripheral olefinic macrocycles was conveniently synthesized via a double ring-closing olefin metathesis. Highlights of this work include the synthesis of a 65-membered macrocycle featuring two phenanthroline ligands, a Cu(I)-templated synthesis of a [3]pseudorotaxane, and the key double ring-closing olefin metathesis to afford the desired [3]catenane in 71% yield.     

Templates for exploratory library preparation. Derivatization of a functionalized spirocyclic 3,6-dihydro-2H-pyran formed by ring-closing metathesis reaction

The preparation of a novel spirocyclic template from tert-butoxycarbonyl-4-piperidone is reported. The synthesis of N-(tert-butoxycarbonyl)-1-oxa-9-aza-spiro[5.5]undec-3-ene (4) for exploratory library generation involves ketone allylation, etherification, and ring-closing metathesis (RCM) reactions. Epoxidation of the alkene formed in the RCM followed by addition of volatile amines to the epoxides led rapidly to an exploratory library of structurally novel spirocyclic amino alcohols. The addition of amines to epoxides derived from 4 was determined to occur primarily at C3.     

Ruthenium-catalyzed ring-closing metathesis to form tetrasubstituted olefins

[structure: see text]. Increased efficiency for ring-closing metathesis to form tetrasubstituted olefins using N-heterocyclic carbene ligated ruthenium catalysts was achieved by reducing the size of the substituents at the ortho positions of the N-bound aryl rings.     

Cross and ring-closing metathesis of 1,3-diynes: metallotropic [1,3]-shift of ruthenium carbenes

The regio- and stereoselective enyne CM and RCM reactions involving 1,3-diynes have been developed. The ring-closing metathesis of enediynes induces a facile metallotropic [1,3]-shift of alkynyl ruthenium carbene species, thereby providing a unique entry into the synthesis of fully conjugated 1,5-diene-3-ynes.     

Stereoselectivity of macrocyclic ring-closing olefin metathesis

[reaction: see text] Macrocyclic ring-closing olefin metathesis using ruthenium catalyst 3 was performed to produce a 14-membered lactone. The E/Z ratio of lactone was high regardless of the R group (auxiliary) or the initial alkene stereochemistry. A kinetic study demonstrates that the high E/Z ratio is due to secondary metathesis reactions that isomerize the product to the thermodynamic E/Z ratio.     

Molybdenum-catalyzed ring-closing metathesis of allenynes

A ring-closing metathesis reaction of allenynes occurred at room temperature in the presence of a molybdenum alkylidene complex to give ring-closed vinylallenes. The vinylallene skeletons were constructed by a metathesis-type reaction between the alkyne moiety and the proximal carbon-carbon double bond of the allene moiety. [reaction: see text]     

Concise formal synthesis of (-)-peduncularine via ring-closing metathesis

[reaction: see text] A synthesis of the 6-aza[3.2.1]bicyclooctene (-)-2 has been completed by a short sequence of reactions that required only six operations from (S)-malic acid and featured a novel ring-closing metathesis to form the bridged bicyclic ring. Because 2 was previously converted into (-)-peduncularine (1), its preparation constitutes a formal enantioselective synthesis of 1.     

Synthesis of conformationally restricted dinucleotides by ring-closing metathesis

[reaction:see text] The ring-closing metathesis reaction has been used in the synthesis of conformationally restricted dinucleotides as well as a 3'-nucleotide analogue. From bis-allylic nucleoside phosphates obtained from simple nucleoside precursors, the synthesis of unsaturated cyclophosphates has been accomplished using either Grubbs' catalyst or an improved analogue. Hereby, the conformational freedom of the nucleic acid phosphordiester linkage has been efficiently constrained.     

Suzuki-Miyaura cross-coupling and ring-closing metathesis: a strategic combination to the synthesis of indeno[1,2-c]isoquinolin-5,11-diones

A variety of diversely substituted isoindeno[1,2-c]isoquinolin-5,11-diones has been readily assembled through a sequence involving a Suzuki-Miyaura cross-coupling reaction with enol phosphates combined with a ring-closing metathesis (RCM) reaction. Intramolecular carbocationic annulation reaction and ultimate oxidation of a latent hydroxyl functionality completed the synthesis of the target titled compounds.