Ester enolate 2,3 -Wittig rearrangement

As described above, the desired compound 17 with high degree of anti selectivity could be obtained from the starting materials, propargylic alcohols in only three steps. Thus,ester-enolate [2,3]-Wittig rearrangement can be considered as one of the most attractive synthetic methods for new types of trifluoromethyla-ted intermediates. However, switching of ( )- and (Z)-substrates did not lead to the diastereoselective construction of the different stereoisomers. The next... 

Table 8. Stereoselective Ester Enolate [2,3] Wittig Rearrangement of Substituted Methyl (Z)-2-[(4,4,4-Trifluorobut-2-enyl)oxy]acetates (Z)-25 to y.ti-U nsaturated anti-( )-a-Hydroxy-/3-(trifluoromethyl) Esters (E)-2617...

Disubstituted alkenes with a trans double bond can be generated routinely from Wittig rearrangements in the case of ester enolate Wittig rearrangements, cis selectivity can be achieved. For trisub-stituted alkenes, ( )- and (Z)-selective syntheses are known. 

Table 2 Counterion Effects on the cis/trans Selectivity of Ester Enolate Wittig Rearrangements ...

Ireland-Claisen rearrangement as an alternative approach to the syn isomer with a high degree of diastereoselectivity was also examined. This reaction might be similar to the system of ester-enolate [2,3]-Wittig shift in the case of OR (R=protective group). 

It has been demonstrated that the oxygen anion of initially formed product (36) effectively catalysed the [2,3]-Wittig rearrangement as a Lewis base. Other Lewis-base catalysts such as lithio or sodio 2-pyrrolidone promote the same [2,3]-Wittig rearrangement of silyl enolates generated from a-allyloxy ketones, whereas rearrangements of enolates from a-allyloxy esters were efficiently catalysed by ammonium 4-methoxybenzoate.24... 

Silyl enolates generated from a-allyloxy esters undergo the [2,3]-Wittig rearrangement on treatment with Lewis base such as tetrabutylammonium acetate or tetrabuty-lammonium 4-methoxybenzoate (Scheme 10).14... 

Generally, ester enolates of structure (202 R = M, R = Oalkyl) rearrange via a 3,3-shift, whereas the corresponding amide enolates (202 R = M, R = N(alkyl)2) and acid dianions (202 R = M, R = OM) prefer the 2,3-pathway (equation 20). Both pathways have been observed with ketone enolates (202 R = M, R = alkyl). With substrate (179), Koreeda and Luengo observed only traces of Wittig rearrangement product (205), except for the lithium enolate, where (205) accounted for up to 20% of the reaction mixture (equation 21). ° Thomas and Dubini, however, reported predominant formation of 2,3 Wittig products (207) and (209) under base treatment of ketones (206) and (208) (equation 22). ... 

Table 18 2,3-Wittig Rearrangement of o-Allyloxyacetic Ester Enolates LDA, THF,-100 C...

Synthesis of trifluoromethylated compounds 152 has been achieved via ester-enolate [2,3]-Wittig and [3,3]-lreland-Claisen rearrangements. Perfluorocyclo-butane phosphonium ylides, e.g. 153, have been used as a masked fluoride anion source in their reactions with alcohols and carboxylic acids which lead to alkyl-and acyl-fluorides. Ylides 153 are also reported to cleave Si-C and Si-O bonds, cause dimerisation of fluoro-olefins, and also react with acid chlorides or other activated aromatic compounds under halogen exchange. ... 

As noted previously, the [2.3] Wittig rearrangement of zirconium ester enolates demonstrates a strong preference for Z-oleftn formation. For secondary allylic ethers 16, the preferential formation of Z-i v -products suggests that rearrangement occurs through an Ra transition stale conformation in order to minimize the RA-G steric interaction62,6a. 

The auxiliary-mediated [2,3] Wittig rearrangement of nonracemic allyloxy acetates has to date received little attention. Enolates of the 8-phenylmenthyl esters 42 (R1 = CH3, Bu) undergo [2,3] rearrangement with good syn selectivity (>90%) and comparable levels of induced diastereose-lection85. 

Scheme 6.9. Rationale for the ul selectivity of dicyclopentadienylzirconium ester enolates in [2,3]-Wittig rearrangements.

Scheme 6.14. Possible transition structures for the [2,3]-Wittig rearrangement of the R-allylic ester enolates shown in Scheme 6.13. For amide enolates, see Scheme 6.22.

Scheme 6.15. [2,3]-Wittig rearrangements of chiral propargyloxy acetates (a) Zirconium ester enolates [81,82]. (b) Lithium endiolates, S = solvent [81-83].

The [2,3]-Wittig rearrangement of dienolates generated from a-allyloxy-substituted a, - and/or /S.y-unsaturated esters has been found to proceed with high diastereose-lectivity to give hexa-1,5-dienes in high yield. The greater reactivity of dienolate than enolate has been explained in quantum chemical terms. 

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