Reaction with Sulfonium Ylides. Corey Synthesis

Reaction with Sulfonium Ylides. Corey Synthesis 

Oxiranes can be prepared in excellent yield from carbonyl compounds by alkylidene transfer with sulfonium ylides. The reaction is generally carried out with dimethylsulfonium methylide 77, dimethylsulfoxonium methylide 78, or related compounds such as anionoid species originating from sulfylimines 79 and sulfox-imines 80 that can undergo addition to the electrophilic carbonyl carbon. 

A betaine intermediate is formed, the fragmentation of which leads to formation of an oxirane ring by intramolecular substitution by the anionic oxygen (Eq. 89). The sulfonium ylides also react with other types of compounds containing an electrophilic unsaturated bond (C=C, C=N), giving cyclopropane derivatives and aziridines. 

This ylide chemistry is treated in a number of monographs and reviews. The formation and properties of 77 and 78 were first dealt with by Corey et al. Reactive ylides are prepared from dialkyl sulfide and dialkyl sulfoxide in a non-aqueous medium. The solvent for the epoxidation reaction may be an aqueous basic solution. No side-reactions occur in a two-phase system or under the more recent phase-transfer conditions.  

While the mechanism of the reaction has been clarified in accordance with the scheme outlined in Eq. 89, its stereochemistry is not known in every respect. The sulfoxonium ylides are more stable and behave as better leaving groups than the sulfonium ylides. Choice of the reagent is governed by stereochemical considerations, because their stereoselectivities differ. In general, 77 attacks from 

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C. Reaction with Sulfonium Ylides. Corey Synthesis... 

Chiral epoxides frequently play a key role as intermediates in organic synthesis and the development of methods for the catalytic asymmetric synthesis of such compounds therefore remains an area of intensive research. Methods have focused principally on the asymmetric electrophilic oxidation of alkenes and good enantioselectivity has been achieved [1]. An alternative to oxidative processes for the synthesis of epoxides is the reaction of sulfur ylides with aldehydes and ketones [2,3,4,5,6]. Sulfur ylide epoxidation is a carbon-carbon bond forming reaction and is complementary to oxidative methods. The standard conditions for this reaction utilize the original Corey method treatment of a sulfonium salt with a strong base in the presence of or followed by the addition of an aldehyde... 

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