Examples of Enyne Metathesis

Examples of enyne metathesis in natural products synthesis a) S. C. Schurer,... 

Fig. 4.13. Examples of enyne metathesis reactions, ring opening-cross metathesis cascades, and other metathesis sequences.

Scheme 30 Representative examples of enyne metathesis reactions occurring in tandem with other metathesis modes.

Scheme 23 Representative examples of enyne cross-metathesis.

In turn, the propensity of 1 to respond to steric hindrance can be used to control the site of initiation of an RCM reaction in a polyene substrate (Scheme 9) [20]. Thus, dienyne 25 reacts with the catalyst regioselectively at the least substituted site the evolving ruthenium carbene 26 undergoes a subsequent enyne metathesis leading to a new carbene 27, which is finally trapped by the disubsti-tuted olefin to afford the bicyclo[4.4.0]decadiene product 28. By simply reversing the substitution pattern of the double bonds, the complementary bicyclo [5.3.0] compound 32 is formed exclusively, because the cyclization cascade is then triggered at the other end of the substrate. Note that in both examples tri-substituted olefins are obtained by means of a ruthenium based metathesis catalyst [20] ... 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

The stereoselectivity of olefin formation is crucial to the utility of CM. To date, a general metathesis catalyst capable of effecting diastereomeric control over a broad range of substrates has yet to be realized. Of particular interest is the development of a Z-selective catalyst, as Z olefins are a prevalent structural motif within both natural products and pharmaceutical agents.Current examples of Z-selective olefin CM have proved to be substrate dependent. These include the CM of enynes with alkenes, acrylonitrile the CM reaction... 

It should be noted that while enyne metathesis is considered incompatible with molybdenum catalyst 1, ruthenium catalysts other than the Grubbs type also promote the reaction. Semeril et al. [103] reported efficient enyne RCM with a catalyst conveniently generated in situ from [RuCl2(p-cymene)]2,1,3-bis(mesityl)imidazolium chloride and caesium carbonate. Interestingly the authors found that the in situ derived system gave better results than the isolated catalyst. One of the most impressive examples of the use of enyne RCM is the total synthesis of (-)-longithorone by Layton et al. [104]. Inspired by a pro-... 

The term intramolecular enyne metathesis describes two types of processes. One involves a [2+2] cycloaddition of a multiple bond and a transition-metal carbene complex and the other is an oxidative cyclization catalyzed by low-valent transition-metal complexes, for example, Pt, Pd and Ru. The latter reaction is also called a skeletal reorganization. Both processes lead to similar products (Eq. 84). 

RCM reactions are most frequently employed in the synthesis of 2,5-dihydrofurans as well as dihydropyrrole derivatives . Likewise, RCM provides the most general approach to 3,6-dihydropyrans . In a specific example, dihydropyran 127 bearing a chiral oxacyclic diene can be constructed via enyne metathesis of the chiral ether 126 (Scheme 68) <2002T5627>. The analogous tetrahydropyridine derivatives are prepared by a similar RCM procedure . 

Our group has used a combined metathesis-PKR for the synthesis of tricyclic compounds in one step. The process starts from pure cobalt complexed dienynes 52. The cobalt cluster acts first as a protecting group to avoid undesired enyne metathesis processes. The methodology allows the formation of tricyclic [6.5.5] (53) and [7.5.5] (54) structures including, in some examples, oxygen or nitrogen. Tricycles 53 are obtained in a total stereoselective manner, while compounds 54 are formed as mixtures of two diastereomers (Scheme 17) [110]. 

The most likely scenario for enyne metathesis is an intramolecular combination of the alkene and alkyne moieties to form a ring, which is really a variation of RCM. Examples of intermolecular enyne metathesis (CM) have been successful if they are run in an atmosphere of ethene, using it as the alkene component. Even for some intramolecular enyne metatheses, preequilibration of the catalyst with ethene caused vastly improved yields.74 Equations 11.2475 and 11.2576 show... 

The catalytic enyne metathesis reaction was first discovered by Katz and Sivavec (421) in 1985 in which they reported the first examples of this process in... 

With this foundation and historical perspective of transition metal-promoted intramolecular enyne metathesis reactions of l,n-enynes in place, the remainder of this section will describe specific metal-catalyst systems with examples of their utilities for cycloisomerization reactions involving various substrates. 

Scheme 72. (a) Illustration and (h) example of enantioselective enyne-cioss metathesis using Rh(I) as a catalyst. 

The general reaction equation for alkene metathesis in a simple system, cross-metathesis of two different disubstituted alkenes, is depicted in Scheme 1. In this reaction, a transition metal catalyst establishes equilibrium between the starting alkenes, the ( )- and (Z)-stereoisomers of all possible substituent combinations, and ethylene. Related reaction processes have also been reported for alkynes (aikyne metathesis) and for combinations of alkenes and alkynes (enyne metathesis). Aikyne metathesis is less well developed compared to alkene metathesis and enyne metathesis. This review has been organized according to the basic modes of metathesis depicted in Scheme 2. Alkene metathesis is the more developed process and numerous examples of all the variants have been reported. Aikyne metathesis is less well developed and three variants exist aikyne cross-metathesis, aikyne metathesis polymerization, and ring-closing aikyne metathesis. 

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