Sulfides 1,2 -Wittig rearrangements

The sulfur analogue of the Wittig rearrangement is an isomerization of a-metalated sulfides to skeletally rearranged metal thiolates, which yield thiols (equation 80). 

The pioneering work on thia-[2,3]-Wittig rearrangement was reported by Rautenstrauch in 1971 °. The reaction of allyl sulfides 137 with n-BuLi at —30°C for 1.5-4 h gave homoallyl sulfide or thiol 138 ([2,3]-product) exclusively (equation 81). In contrast, a similar reaction of vinyl sulfide 139 gave no [1,2]-rearrangement product 141 albeit a-thio-carbanion was generated as shown by its methylation to 140 (equation 82) °. 

Snider and colleagues have developed the sequential ene reaction/thia-[2,3]-Wittig reaction which provide appropriately functionalized product 152 at allylic position on simple alkene 150 in two steps involving intermediate 151 (equation 87) . Thia-[2,3]-Wittig rearrangement was often utilized as a key step of natural product synthesis. Masaki and colleagues have demonstrated that the potassium enolate thia-[2,3]-rearrangement of aUyl sulfide 153 to 154 is useful for the synthesis of terpenoid diol component 155 of the pheromonal secretion of the queen butterfly (equation 88) . 

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. 

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. 

Alkylation of benzyl alkyl ethers. The anion of a benzyl alkyl ether is unstable and rearranges to the corresponding alkoxide (Wittig rearrangement). However, coordination of the ether to CrfCO), stabilizes the oi-anion and permits alkylation to give an a-substituted benzyl ether. Complexation also stabilizes the anion of a benzyl alkyl sulfide. The products undergo decomplexation on exposure of ether solutions to air and sunlight (overall yields 65-80%). 

Wittig rearrangement of the cyclic sulfides and methylation of the products... 

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

Bruckner s approach started from L-arabinose converted in five steps to allylic alcohol 175, and to alkoxy sulfone 176 thereafter (Scheme 23). Key [2, 3] Wittig rearrangement of lithiated 176 in the presence of allyl lithium led to the desired compound 178 and its C17 isomer (ratio 1.4 1) via the intermediate 177. Regioselective epoxidation and thiophenate epoxide ring opening furnished sulfide 179 transformed in four steps to the C14-C20 building block 180. 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Hori and co-workers accomplished the first synthesis of azathianaphthalene and azathiaphenanthrene in 1979 <79TL3969>. Their approach began with the formation of an olefin from o rt/20 -ni t ro b e n za 1 dehyde 43, via a Wittig reaction with an ylide and a subsequent reduction with zinc to afford cis and trans ortho-aminostyryl methyl sulfide 45. The cis-olefin was then treated with NCS, AgCKAi and KOH to yield 2-methyl- l-aza-2-thianaphthalene 47 in 41% yield. 9-Methyl- 10-aza-9-thiaphenanthrene 48a and 9-ethyl-10-aza-9-thiaphenanthrene 48b were obtained in a similar fashion in almost quantitative yields, whereas 6-benzyl-67/-d i b e n zo [ c, e] [ 1,2 J t h i azi n cs 50 were isolated in moderate yields via a 1,2-rearrangement (Scheme 13) <90TL7021>. 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

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