Enyne Metathesis

A distinctive transformation based diversity oriented synthesis of small ring carbocycles and heterocycles from biocatalytically derived enantiopure α-substituted-β-hydroxyesters

A series of structurally novel small ring carbocyclic and heterocyclic molecules were accessed in an enantiopure fashion.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Ruthenium-catalyzed intramolecular [2+2+2] cycloaddition and tandem cross-metathesis of triynes and enediynes

[2+2+2] Cycloadditions can be applied to specifically build up derivatives of benzene and cyclohexadiene and, therefore, have attracted much attention. Herein, we present an intramolecular [2+2+2] cycloaddition of triynes catalyzed by the first-generation Grubbs ruthenium complex (Ru gen-1), which can efficiently afford benzene derivatives in good yields under mild conditions. Moreover, we also report on a novel tandem cross-metathesis transformation of intramolecular enediynes also catalyzed by Ru gen-1, which has not been observed previously in related reports. On the basis of deuterium labeling experiments, a possible reaction mechanism is presented.

Diastereoselective total synthesis of (±)-schindilactone A, Part 1: Construction of the ABC and FGH ring systems and initial attempts to construct the CDEF ring system

First-generation synthetic strategies for the diastereoselective total synthesis of schindilactone A (1) are presented and methods for the synthesis of the ABC, FGH, and CDEF moieties are explored. We have established a method for the synthesis of the ABC moiety, which included both a Diels-Alder reaction and a ring-closing metathesis as the key steps. We have also developed a method for the synthesis of the FGH moiety, which involved the use of a Pauson-Khand reaction and a carbonylative annulation reaction as the key steps. Furthermore, we have achieved the construction of the central 7-8 bicyclic ring system by using a [3,3]-rearrangement as the key step. However, unfortunately, when this rearrangement reaction was applied to the construction of the more advanced CDEF moiety, the anticipated annulation reaction did not occur and the development of an alternative synthetic strategy would be required for the construction of this central core.

Ene-yne cross-metathesis with ruthenium carbene catalysts

Conjugated 1,3-dienes are important building blocks in organic and polymer chemistry. Enyne metathesis is a powerful catalytic reaction to access such structural domains. Recent advances and developments in ene–yne cross-metathesis (EYCM) leading to various compounds of interest and their intermediates, that can directly be transformed in tandem procedures, are reviewed in this article. In addition, the use of bio-resourced olefinic substrates is presented.

Recent Progress on Enyne Metathesis: Its Application to Syntheses of Natural Products and Related Compounds

Olefin metathesis using ruthenium carbene complexes is a useful method in synthetic organic chemistry. Enyne metathesis is also catalyzed by these complexes and various carbo- and heterocycles could be synthesized from the corresponding enynes. Dienyne metathesis, cross enyne metathesis and ring-opening enyne metathesis have been further developed. Various complicated compounds, such as the natural products and the related biologically active substances, could be synthesized using these metatheses reactions. Skeletal reorganization using the transition metals and metallotropic rearrangement are also discussed.

Application of tandem ring-closing enyne metathesis: formal total synthesis of (-)-cochleamycin A

A tandem ring-closing metathesis of a silaketal-based dienyne substrate proceeded efficiently to provide a bicyclic siloxane, which upon removal of the silicon tether afforded an (E,Z)-1,3-dienediol. Further manipulation of this key functional motif rendered synthesis of the entire C1-C19 linear skeleton of (-)-cochleamycin A, a late-stage intermediate employed in the previous total synthesis of (+)-cochleamycin A by Roush and co-workers.

Advanced approach to polycyclics by a synergistic combination of enyne metathesis and Diels-Alder reaction

Enyne metathesis (EM) has widely been used to prepare various synthetic and natural products. Further, a sequential use of EM and Diels-Alder (DA) reactions generates highly functionalized and intricate polycarbocycles and heterocyclic frameworks. In this critical review we describe the application of a unique combination of EM and DA reactions to prepare various amino acid derivatives, natural products and heterocycles. Some of the heterocyclic targets include cyclic peroxides, siloxanes and various nitrogen (or oxygen) containing compounds. Use of ethylene and cyclooctadiene (COD) for improving the yield of EM products are also discussed (69 references).

Tandem enyne metathesis-metallotropic [1,3]-shift for a concise total syntheses of (+)-asperpentyn, (-)-harveynone, and (-)-tricholomenyn A

A tandem reaction sequence involving relay metathesis-induced enyne RCM and metallotropic [1,3]-shift is an effective tool to construct cyclic alkenes with embedded 1,5-dien-3-yne moieties from acyclic precursors containing a 1,3-diyne. Total syntheses of (+)-asperpentyn, (-)-harveynone, and (-)-tricholomenyn A have been accomplished by implementing this metathesis-based tandem reaction sequence as the key step.

Absence of the Thorpe-Ingold Effect by gem-Diphenyl Groups in Ring-Closing Enyne Metathesis

In tandem ring-closing metathesis of alkynyl silaketals containing two different tethered olefins, the gem-dimethyl group showed the expected Thorpe-Ingold effect, thereby giving good level of group selectivity. Unexpectedly, however, the corresponding gem-diphenyl group did not show any Thorpe-Ingold effect for the ring closure reaction.

Ring-Rearrangement Metathesis

Ring-rearrangement metathesis (RRM) refers to the combination of several metathesis transformations into a domino process, in which an endocyclic double bond of a cycloolefin reacts with an exocyclic alkene. RRM has proven to be a powerful method for the rapid construction of complex structures. The extension of the basic ring-opening-ring-closing metathesis process by further metathesis steps as well as an examination of the driving forces, limits, scope, recent advantages, and future perspectives of these domino sequences is presented with various examples, thus reflecting the high efficiency and utility of RRM in organic synthesis.

Dienyne Ring-Closing Metathesis Approach for the Construction of Taxosteroids

A cascade dienyne ring-closing metathesis approach has been applied to the synthesis of the tetracyclic carbon framework of a new class of hybrid compounds-the taxosteroids-possessing carbon frameworks incorporating moieties characteristic of both taxanes (such as AB rings) and steroids (i.e., CD system and side chain). This tandem cyclization is highly stereoselective, allowing the one-step formation of the bicyclo[5.3.1]undecene system characteristic of taxol. In this work we describe the scope and limitations of such cyclizations.

Formal Total Synthesis of (−)-Apicularen A by a Strategy Based on Ring-Closing Metathesis and Transannular Cyclization

A formal synthesis of (-)-apicularen A, a potent antitumor agent with unique biological properties, has been completed in a 15-step sequence starting from a known, enantiomerically pure hydroxyepoxide, which was generated by using the Jacobsen hydrolytic-kinetic-resolution methodology. The 12-membered macrocyclic lactone in the target was constructed by ring-closing metathesis, and the trans-tetrahydropyran ring system was created through the transannular etherification of a hydroxyalkene.

Ring closing enyne metathesis: A powerful tool for the synthesis of heterocycles

The ring closing metathesis (RCM) is a powerful method in organic synthesis for the preparation of cyclic compounds by formation of new carbon-carbon bonds. In the past years a particular subclass of the RCM, the ring closing enyne metathesis (RCEYM), has attracted attention due to its synthetic potential in the generation of ring structures with 1,3-diene moieties, which can subsequently be further functionalised. In this tutorial review mechanistic considerations will be described and the synthetic power of this useful and attractive carbon-carbon bond forming reaction will be illustrated by recent examples of RCEYM applications in the preparation of heterocyclic compounds.

Feasibility of associative mechanism in enyne metathesis catalyzed by grubbs complexes

The mechanism of enyne metathesis catalyzed by first and second generation Grubbs complexes has been computationally explored at the DFT level. The relative reactivity and the regioselectivity for the reaction of differently substituted alkenes and alkynes with model Ru complexes has been studied. The usually accepted dissociative mechanism for the alkene metathesis has been explored for alkynes, and compared with an associative pathway involving initial coordination of the alkyne to the 16-electron catalyst. Our results show that an associative mechanism would be the preferred pathway for the reaction of phosphine-based (first generation) Ru carbenes, at least for small phosphines such as PMe(3), whereas for the more reactive complexes containing a heterocyclic carbene as ligand (second generation catalysts), the dissociative process is far more favourable.

Diversity-Oriented Approach to Biologically Relevant Molecular Frameworks Starting with β-Naphthol and Using the Claisen Rearrangement and Olefin Metathesis as Key Steps

A diversity-oriented approach for the synthesis of various structurally different molecular frameworks from readily accessible and common precursors is described. A Claisen rearrangement followed by ring-closing metathesis or ethylene-promoted ring-closing enyne metathesis has been utilized as the key synthetic transformation to generate naphthoxepine derivatives. The ring-closing metathesis approach has also been used to generate spirocyclic compounds and the pleiadene framework.

Metathesis reactions in total synthesis

With the exception of palladium-catalyzed cross-couplings, no other group of reactions has had such a profound impact on the formation of carbon-carbon bonds and the art of total synthesis in the last quarter of a century than the metathesis reactions of olefins, enynes, and alkynes. Herein, we highlight a number of selected examples of total syntheses in which such processes played a crucial role and which imparted to these endeavors certain elements of novelty, elegance, and efficiency. Judging from their short but impressive history, the influence of these reactions in chemical synthesis is destined to increase.