Silyl-Wittig rearrangement

Acylsilanes have been synthesized by several other approaches including (i) a reverse Brook rearrangement (silyl-Wittig rearrangement) of allyltriinethylsilyl ether followed by oxidation (Figure Si6.5)... 

Long before the silyl Wittig rearrangement was discovered, the base-catalyzed rearrangement of a-silylcarbinols to alkoxysilanes was known. This reaction, carefully studied by Brook and his students (4-6) takes place in exactly the opposite direction to the silyl Wittig reaction. In the... 

The silyl Wittig rearrangement of the lithiopropynyl silyl ethers (252) leads to the allenolates (253) in variable yield, the best yield being obtained where deprotonation is faster than desilylation (for example, when = H). The... 

Sigmatropic rearrangements of anions of (V-allyl amines have also been observed and are known as aza-Wittig rearrangements.291 The reaction requires anion stabilizing substituents and is favored by (V-benzyl and by silyl or sulfenyl substituents... 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

As a similar transformation of aza-[2,3]-Wittig rearrangement, a [2,3]-Stevens rearrangement to 126 of the A-silylated ylide derived from A-crotylglycine ester (125) was reported (equation 75) . 

Wittig rearranged products derived from (Z)-substrates. It is possible that this rearrangement proceeded via a six-membered transition state in which a lithium atom co-ordinates to an oxygen atom of a methoxy group to afford (Z)-ketene silyl acetal preferentially [38]. (Fig. 3)... 

Wittig rearrangement of allyl silyl ethers, followed by Simmons-Smith cyclopropana-tion and Collins oxidation, produces a-cyclopropyl acyl silanes, e.g. 22, in 10-85% yields (Scheme 55)85. 

Metal alkoxides, such as sodium benzylate, in catalytic amounts promote the [2,3]-Wittig rearrangement of silyl enolates (34), to afford the corresponding rearrangement product (35) in good yields at room temperature (Scheme 8).23... 

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... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

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... 

A Lewis-base-catalyzed [2,3]-Wittig rearrangement of the silyl enolate 1113 exclusively affords the chroman-4-one 1114 (Equation 436) <2005CL588>. 

Aza-Wittig rearrangement in the acyclic series are harder to control.141 The most reliable turn out to be those of allyl amides such as 192 in which the allyl group bears a (3-silyl substituent, whose function is to stabilise the anionic transition state of the reaction.142 143... 

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Asymmetric induction of the same kind controls the configuration at the allylic methine carbon in compound (134) (134) results from a chain-elongating Wittig rearrangement (mechanism Scheme 6) of sulfone (133 equation 35). Surprisingly, the sense of the asymmetric induction in the rearrangement of silyl ether (135 equation 36) is opposite to that of more than a dozen other ( )-ethers. ... 

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