Benzylsulfonium ylides

In order to account for the origin of the enantioselectivity and diastereoselectivity of benzylidene transfer, it is necessary know whether the sulfur ylide reactions are under kinetic or thermodynamic control. From cross-over experiments it was found that the addition of benzylsulfonium ylide to aldehydes was remarkably finely balanced (Scheme 9) [28]. The trans-epoxide was derived directly from irreversible formation of the anti-betaine 4 and the cis-epoxide was derived from partial reversible formation of the syn-betaine 5. The higher transselectivity observed in reactions with aromatic aldehydes compared to aliphatic aldehydes was due to greater reversibility in the formation of the syn-betaine. 

Epoxides can also be formed by a fundamentally different "C -1- C=0" process, whereby an ylide serves as a carbon donor. In additions of benzylsulfonium ylides to aromatic aldehydes e.g., 47) a strong lraii.s selectivity is observed, which is rationalized as the result of irreversible... 

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