Double Tebbe

Fig. 3.41. Ring expansion by tandem double Tebbe-Claisen methodology [699].

Using double Tebbe methylenation (77 to 78) followed by a Claisen rearrangement, Paquette and co-workers developed a concise, reliable, and efficient scheme for 4-cyclooctenones. Using this methodology, Paquette reported an alternate enantioselective route to a key bicyclic intermediate,... 

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]. 

Similarly, compound 73 was prepared from a persilylated D-galactono-1,4-lactone precursor in 60-80% yield (71). However, when unpurified Tebbe s reagent was employed, 1,4-lactones (for example 71a) gave a mixture of 72a and the product (74) of hydration of the double bond. The... 

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],... 

Various exocyclic enes, derived from both aldonolactone and ulose derivatives, have been made using unpurified Tebbe reagent. Reference to the generation of 2,6-anhydrohex-l-enitols from 1-halogenoketose derivatives is made in Chapter 6 and an annulated pyranoid derivative with a C-2 exo-double bond is noted in Chapter 14. 

Methylenation of carboxylic acid derivatives The Tebbe reagent 3 is extremely valuable as a reagent for the methylenation of carboxylic acid derivatives, which is generally unsuccessful using phosphorus ylides. Esters and lactones are readily transformed into enol ethers (Table 4.3), especially when a Lewis base such as THE is present in the reaction mixture. In the methylenation of a,j8-unsaturated esters, the internal olefin is not involved in the reaction, and the configuration of the double bond is maintained (entry 4). When carbonyl compounds bearing a terminal double bond are subjected to the methylenation, significantly lower yields are observed (entries 6 and 11), which may be attributable to competitive formation of a titanacycle from titanocene-methylidene 4 and the terminal olefin. 

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