Ketones 2,3 -Wittig rearrangement

Antithetic conversion of a TGT by molecular rearrangement into a symmetrical precursor with the possibility for disconnection into two identical molecules. This case can be illustrated by the application of the Wittig rearrangement transform which converts 139 to 140 or the pinacol rearrangement transform which changes spiro ketone 141 into diol 142. 

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... 

For allyl ethers 790 with R1 =Ph, treatment with LDA generates anions 791 which undergo [2,3]-Wittig rearrangement to more stable alkoxides 792 (Scheme 126). Spontaneous expulsion of benzotriazole anion from 792 generates (3,y-unsaturated ketones 793 that are isolated in high yields (86-92%) <1996JOC4035>. In the case of... 

This [1,4]-Wittig rearrangement system is applicable to the sequential [l,4]-rearrange-ment/aldol reaction which provides -hydroxy ketones with moderate diastereoselectivity (Table 4). 

It is notable that allyloxylation can also be performed in relatively good yields (Table 6) although allyl alcohols are easily oxidized anodically12. The allyloxylated sulfides thus obtained are easily converted into the corresponding ft, /-unsaturated ketones by a [2,3] Wittig rearrangement using bases as shown in equation 21. Anodic desilylation/carboxylation of a-thiomethylsilanes also takes place similarly as shown in an example in Table 6. 

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 ketones undergo the [2,3]-Wittig rearrangement in the presence of a catalytic Lewis base such as lithium 2-pyrrolidone, lithium acetamide, or lithium hexamethyldisilazide (Scheme ll).15... 

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. 

Naturally, l,3-diarylbenzo[c]furan-alkyne adducts cannot lose water on dehydration. As was found by Wittig and co-workers, in these cases a rearrangement takes place cycloalkyne adducts of type 200 yield ketones The rearrangement may occur even during chromat-... 

Ketone synthesis. Phosphonates of this type serve as acyl anion equivalents. Thus lithiation followed by alkylation gives 2, which can be hydrolyzed to ketones 3 by base. However, the alkylation must be conducted at -78- 20° to prevent a Brook-Wittig rearrangement to 4. 

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). ... 

Wacker process 339 Walden inversion 36 Wieland-Miescher ketone 257 [1,2]-Wittig rearrangement 310 Wurtz-type coupling 290... 

The tert-butoxide-catalyzed [2.3J Wittig rearrangement of ketone 10 has also been reported ... 

Methyl ketone 398, which is available from 379 according to procedures previously discussed, has been used as a chiral precursor for rearrangement studies. The requisite substrate 399 is prepared by addition of vinylmagnesium bromide to 398, which produces a tertiary alcohol with > 95% optical purity. Deprotonation of oxazoline ether 399 with Ai-butyllithium results in a [2,3] Wittig rearrangement and furnishes 400 as the sole product [125]. This remote transfer of chirality is of potential use in constructing fragments associated with a variety of macrolides. 

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