Sulfonium ylides, cyclic 2.3- sigmatropic rearrangements

Sigmatropic rearrangements of cyclic sulfonium and ammonium ylides have been extensively used in ring contraction and ring enlargement methods. An example is shown in Scheme 55. 

The epoxy sulfones were prepared by exhaustive peracid oxidation of the corresponding alkene sulfides. These were generated by ring expansion of cyclic 1-methyl-2-vinyl sulfonium salts 1 via 2,3-sigmatropic rearrangement of the methanides. From the five-membered sulfonium ylide la, (Z)- and ( )-thiacyclooct-4-enes 2 were obtained as an 85 15 mixture. Their chromatographic separation turned out not to be feasible due to concomitant EjZ isomerization on the silica gel column. Since the separation of these epoxy sulfone mixtures obtained by exhaustive oxidation also proved unsuccessful, it was found expedient to first oxidize the mixture of sulfides to an alkene sulfone mixture ( )-3, which could be separated and eventually epoxidized into epoxides cis- and trans-4 and cis- and trans-5. 

Sulfur and ammonium ylides are susceptible to [2,3]-sigmatropic rearrangements under mild conditions with efficient transfer of chirality, as Trust and Ham-men showed in 1973 [1626], Most of the applications of these rearrangements have been conducted on chiral synthons [1622]. Kurth and coworkers [1627, 1628] have applied this reaction to the synthesis of P-chiral, y,8-unsaturated acids via the rearrangements of cyclic sulfonium ylides 10.21, starting from thiol 10.22 (Figure 10.9). 

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