Tebbe reagent Procedure

An alternative approach involves a two-step procedure, in which carbonyl olefination, using the Tebbe reagent 93, generates an acyclic enol ether-olefin (Scheme 16). In this case, subsequent RCM using molybdenum alkylidene 1 proceeds to give cyclic enol ethers. An efficient, one-pot carbonyl olefination-RCM approach has been developed by Nicolaou et al. for the formation of cyclic enol... 

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

Apart from the tandem metathesis/carbonyl o[efination reaction mediated by the Tebbe reagent (Section 3.2.4.2), few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp2TiCl2, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored. 

An improved procedure for the preparation of the Tebbe reagent 3 and the in situ preparation of 3 for large-scale reactions has been developed, requiring neither vacuum line nor Schlenk techniques [9] the crude reaction mixture formed by the combination of titanocene dichloride and two equivalents of trimethylaluminum can be employed for further reactions [10]. 

Barrett applied this procedure to the synthesis of sucrose (Scheme 5) [7]. 2,3,4,6-Tetra-O-benzyl-D-glucopyranose was converted into the a-glucopyranosyl ester 25 with excellent selectivity. In this case the methylenation using Tebbe reagent was unsuccessful. However, the Nozaki-Takai protocol gave the desired vinyl ether. After desilylation, the cyclization reaction gave only the P-fructofuranosyl derivative 26 when the reaction was performed in the presence of silica. 

Oxidative cleavage of C-allyl mannose 419 [157] following Wong s procedure [158] leads to acid 420 which is esterifled with 418 prior to methylenation with Tebbe s reagent (O Scheme 83). 

Chlorodimethylaluminum (generated from the methylenation reagent) is presumed to be the catalyst in the one-pot Tebbe methylenation -Claisen rearrangement procedure. This process proceeds with 98% E selectivity17s. 

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