Epoxidation sulfonium ylide-mediated

Until this work, the reactions between the benzyl sulfonium ylide and ketones to give trisubstituted epoxides had not previously been used in asymmetric sulfur ylide-mediated epoxidation. It was found that good selectivities were obtained with cyclic ketones (Entry 6), but lower diastereo- and enantioselectivities resulted with acyclic ketones (Entries 7 and 8), which still remain challenging substrates for sulfur ylide-mediated epoxidation. In addition they showed that aryl-vinyl epoxides could also be synthesized with the aid of a,P-unsaturated sulfonium salts lOa-b (Scheme 1.4). 

It should be mentioned at this stage that in addition to the catalytic version, numerous sulfur-ylide mediated asymmetric diastereoselec-tive epoxidations which require the use of stoichiometric amounts of the sulfur-ylide were reported. In these cases, the sulfonium salts had to be prepared in a first step, and were used in a subsequent diastereoselective epoxidation step. For some selected references for this type of stoichiometric asymmetric two-step epoxidation, see (a) T. Durst, L. Breau, R. N. 

The ylide, of general structure (4.132), is formed by reaction of the corresponding sulphide (4.133) with a metallocarbenoid resulting from transition metal salt-mediated decomposition of a diazo compound (4.134) or N-tosyl hydrazone salt (4.135). Reaction of the sulfonium ylide (4.132) with aldehyde (4.136) yields the trans-epoxide (4.137) as the major product and sulfide (4.133) which is then returned to the catalytic cycle (Figure 4.10). 

In catalytic epoxidation reactions an alternative to the ylide generation method via alkylation/deprotonation is the transition metal-mediated carbene transfer from diazo compounds to sulfide catalysts. In 1994, Aggarwal and coworkers employed this method in the enantioselective catalytic epoxidation of aldehydes [25]. Using 20mol% of non-racemic sulfide 17 and lmol% of Rh2(OAc)4 together with the slow addition of PhCHN2, a 58% yield and 11% ee were obtained in the epoxidation of benzaldehyde (Scheme 20.10). The enantioselectivity was similar to the results obtained by Breau and Durst using preformed sulfonium salts [26]. 

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