Enynes, metathesis

Another interesting transformation is the intramolecular metathesis reaction of 1,6-enynes. Depending on the substrates and catalytic species, very different products are formed by the intramolecular enyne metathesis reaction of l,6-enynes[41]. The cyclic 1,3-diene 71 is formed from a linear 1,6-enyne. The bridged tricyclic compound 73 with a bridgehead alkene can be prepared by the enyne metathesis of the cyclic enyne 72. The first step of... 

N-Heterocycles, formation in transition metal-catalyzed enyne metathesis 98YGK433. 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

The cross metathesis of acrylic amides [71] and the self metathesis of two-electron-deficient alkenes [72] is possible using the precatalyst 56d. The performance of the three second-generation catalysts 56c,d (Table 3) and 71a (Scheme 16) in a domino RCM/CM of enynes and acrylates was recently compared by Grimaud et al. [73]. Enyne metathesis of 81 in the presence of methyl acrylate gives the desired product 82 only with phosphine-free 71a as a pre-... 

Enyne metathesis starting either from acetylenic boronates and homoallylic alcohols [104a,c] or from propargyl alcohols and allylboronates [104b] has recently been described. The resulting boronated dienes can be converted to allenes or cycloaddition products. The cross metathesis of vinylcyclopropyl-boronates directed toward the total synthesis of natural products has very recently been investigated by Pietruszka et al. [104d]. 

Grubbs et al. reported that the ruthenium-catalyzed RCM of a conjugated diene proceeds in such a way that the less hindered olefin moieties participate in the reaction. Consequently, RCM of 115 gives exo-methylene compound 116, and not exo-vinyl compound 117 (Scheme 24) [105]. This regioselectivity is complementary to that observed for enyne metathesis of 118, which gives exclusively 117 (Scheme 24) [106a]. 

Complementary results for ene-diene metathesis [105] and enyne metathesis... 

Hexacarbonyldicobalt complexes of alkynes have served as substrates in a variety of olefin metathesis reactions. There are several reasons for complex-ing an alkyne functionality prior to the metathesis step [ 125] (a) the alkyne may chelate the ruthenium center, leading to inhibition of the catalytically active species [125d] (b) the alkyne may participate in the metathesis reaction, giving undesired enyne metathesis products [125f] (c) the linear structure of the alkyne may prevent cyclization reactions due to steric reasons [125a-d] and (d) the hexacarbonylcobalt moiety can be used for further transformations [125c,f]. 

Enyne Cross Metathesis and Ring-Closing Enyne Metathesis.348... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Fig. 2a-c Typical enyne metathesis reactions ring-closing enyne metathesis (a) enyne cross metathesis (b and c)... 

Fig. 4 Commonly used (A D) and less commonly used (E, F) initiators for diene and enyne metathesis...

More research efforts have focused on the ring-closing enyne metathesis, which usually [176] provides conjugated vinyl cycloalkenes (cf Fig. 2a, exo mode) useful for further manipulation, but also allows tandem metathesis processes for the formation of polycyclic compounds. 

Clark s group also reported on ring-closing enyne metathesis for the preparation of six- and seven-membered cyclic enol ethers 428 n= 1,2) as potential building blocks for the synthesis of marine polyether natural compounds such as brevetoxins and ciguatoxins. Metathesis products 428 were obtained from ene-ynes 427 in 72-98% yield when the NHC-bearing catalyst C was used (Scheme 84) [179]. 

For leading references, and the application of this method to ring-closing enyne metathesis, see Storm Poulsen C, Madsen R (2002) J Org Chem 67 4441... 

Molander G, Dowdy EC (1999) Lanthanide- and Group 3 Metallocene Catalysis in Small Molecule Synthesis. 2 119-154 Monnier F, see Dixneuf (2004) 11 1-44 MoriM (1998) Enyne Metathesis. 1 133-154... 

In recent years, in addition to the ring-opening metathesis (ROM) and ring-closing metathesis (RCM), the enyne metathesis and the cross-metathesis (CM) have... 

As will be discussed later, the novel pentacyclic antitumor alkaloid roseophilin continues to attract much synthetic effort and several approaches relied on the venerable Paal-Knorr condensation for construction of the pyrrole moiety. For instance, Trost utilized this reaction upon diketone 1 to afford the tricyclic core 2 of roseophilin in a strategy featuring an enyne metathesis as a key step <00JA3801>, while another formal synthesis of this alkaloid utilized a radical macrocyclization to produce the ketopyrrole core <00JCS(P1)3389>. 

Diels-AIder reactions were utilized to prepare isoquinoline derivatives. Various tetrahydroisoquinoline-3-carboxylic acid derivatives were prepared by an enyne metathesis followed by a Diels-AIder reaction. For example the enyne 71 was treated with Grubb s catalyst to afford diene 72 in 65% yield. Subsequent Diels-AIder reaction and oxidation gave tetrahydroisoquinoline 73 in 93% yield <0OCC5O3>. Dihydrosoquinoline 75 was prepared... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

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] ... 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

Keywords Enyne metathesis, Enyne, Metathesis, Carbene complex, [2+2] Cycloaddition... 

Enyne Metathesis Using Fischer Carbene Complexes (W, Mo, Cr). 135... 

---