Sulfur yhdes

Sulfur ylides are among the most interesting carbon nucleophiles and their synthetic importance has been recently reviewed.One especially interesting use of these ylides is their application to the synthesis of cyclopropane derivatives using unsaturated oxazolones. For example, stabilized sulfur yhdes react with unsaturated oxazolones 629 via a Michael reaction to give oxazolone spirocyclopropanes 630 as shown in Scheme 7.202 and Table 7.46 (Fig. 7.57), whereas the less stabilized sulfur ylides give ring-opened products 631 as the major compounds (Scheme 7.202). ... 

Trost, B M, Melvin, L S, Jr 1975 A, Sulfur Yhdes, Academic Press New York... 

Catalytic asyrrunetric epoxidation and aziridination mediated by sulfur yhdes 98SL329. 

Suggest a mechanism for the following reaction of a sulfur yhde with a ketone. 

Sulfide 2a was later employed by Tang and coworkers in the reaction of 4-chlorobenzaldehyde with 3-trimethylsilylallyl bromide for the synthesis of viny-loxiranes. In the presence of 20mol% of 2a, the trans vinyl epoxide was obtained as the major diastereoisomer in 40% yield and 37% ee [21]. Higher concentrations of both aldehyde and aUyUc bromide were found to be beneficial to the yields, because the high concentration of aUyHc bromide favors the formation of the sulfonium salt and the high concentration of aldehyde could probably allow capture of the sulfur yhde before the [2,3]-sigmatropic rearrangement (Scheme 20.6). This reaction with thiolane (THT) as a catalyst could also be carried out without solvent, but the yield was lower than that in t-BuOH. 

In the same area, Xiao et al. have developed asymmetric cyclopropanations of P,y-unsaturated a-ketoesters with stabilized sulfur yhdes catalyzed by C2-symmetric ureas [132]. Although the stereoselectivities had to be further improved (de 2% and ee <80%), this novel methodology opened up access to other highly functionalized 1,2,3-trisubstituted cyclopropanes. In 2007, Cao et al. reported a novel... 

---