Esters Tebbe reagent

Recently, Nicolaou and coworkers have devised a novel, one-pot strategy for the direct transformation of acyclic olefinic esters to cyclic enol ethers [34]. Unlike the molybdenum alkylidene 1 (see Sect. 3.2), initial reaction between the Tebbe reagent 93 and an olefinic ester results in rapid carbonyl olefination to afford a diene intermediate. Subsequent heating initiates RCM to afford the desired cyclic product (Scheme 17). 

Scheme 18. The conversion of olefinic ester 125 to cyclic enol ether 127. (a) 4.0 equiv of Tebbe reagent (93), 25°C, 20 min then reflux, 5 h, 71% (b) 1.3 equiv of Tebbe reagent (93), 25°C, 20 min, 77% (c) 2.0 equiv of Tebbe reagent (93), 25°C, 20 min then reflux, 3 h, 65% (Nicolaou et al.) [34a]...

Preliminary investigations in this area involved treatment of olefinic ester 125 with a large excess (4 equiv) of the Tebbe reagent 93 (Scheme 18) [34a]. After 20 min at 25°C, the mixture was heated at reflux for 5 h. This resulted in the formation of tricyclic enol ether 127 in 71% overall yield. If only 1.3 equiv of Tebbe reagent 93 was employed and the reaction stopped after 20 min at 25°C,the olefinic enol ether 126 could be isolated in 77% yield. The proposed intermediacy of diene 126 in the initial tandem sequence was validated by its subsequent conversion into the cyclic enol ether 127 under the original reaction conditions [34a],... 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

CPv. / Ti Al Tebbe reagent methylenation of aldehydes, ketones, esters, thioesters, amides, carbonates can also induce olefin metathesis in situ preparation [711-713]... 

Experimental Procedure 3.2.4. Methylenation of an Ester with Tebbe Reagent 1 -Phenoxy-1 phenylethene [712,713]... 

Figure 5.11. Olefination of esters using the Tebbe reagent [109,110].

The Wittig reaction is carried out under strongly basic conditions, and is not possible with the five-membered keto ester 195, because an enolate of the ketone is formed. The Tebbe reagent, as described later, reacts with both esters and ketones to give a mixture of products. Selective reaction of the ketone in 195 without attacking... 

The Tebbe reagent reacts with some alkenes. The tricyclo[5.3.0.0] ring 207 was obtained nearly quantitatively by domino alkene metathesis and carbonyl alkenation of the norbomene-type ester 205 with the Tebbe reagent. This interesting reaction to give the intermediate 206 can be explained by the kinetic preference of the Tebbe reagent for the strained double bond over the ester. Alkenation of the ester in 206 produces 207. Capnellene (208) has been synthesized by applying this reaction as a key reaction [65],... 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

The aglycone segments 14 and 15 were next individually partnered with glycone 16 for the four step C-glycosidation sequence (Scheme 4). DCC mediated esterification (36) of 16 with 14 and 15 followed by treatment of the resulting esters with the Tebbe reagent (37) gave enol ethers 23 and 25 respectively, in 61 and 65% yields from 16. The key cyclization reactions on 23... 

The so-called Tebbe reagent 272 reacts with carbonyl compounds to form olefinic products 139a>, a reaction which is of particular synthetic value in case of esters (Equation 87) 139b). 

The Tebbe reagent (4.85) converts esters to enol (vinyl) ethers. 

In compounds containing both ketone and ester groups, the ketone selectively reacts in the presence of 1 equiv. of the Tebbe reagent (4.85), but with excess amount of the Tebbe reagent (4.85) both carbonyl groups are alkenated. 

Recently, Claisen rearrangement of allyl-vinyl ether prepared from glycal ester with Tebbe reagent was reported [91]. In contrast to the Ireland-Claisen rearrangement, in principle, a non-enolizable ester can be employed (O Scheme 63). This method was applied for the synthesis of C-disaccharide. 

A reaction that appears to be mechanistically similar to the Tebbe reaction was developed by Oshima in 1978. Diiodomethane or dibromomethane in the presence of zinc is treated with a Lewis acid to form, presumably, a divalent complex (72), which reacts with aldehydes and ketones to produce the corresponding methylene derivative (73 Scheme 18). This reagent complements the reactivity of the Tebbe reagent, in that the zinc methylenation is not reactive towards esters or lactones. Because it is an electrophilic reagent, it is suitable for the methylenation of enolizable ketones and aldehydes. 

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