Dimethyloxosulfonium methylide reaction with ketones

Methylenecyclohexane oxide has been prepared by the oxidation of methylenecyclohexane with benzonitrile-hydrogen peroxide or with peracetic acid by treatment of 1-chlorocyclo-hexylmethanol with aqueous potassium hydroxide and by the reaction of dimethylsulfonium methylide with cyclohexanone. This reaction illustrates a general method for the conversion of ketones and aldehydes into oxiranes using the methylene-transfer reagent dimethyloxosulfonium methylide. The yields of oxiranes are usually high, and the crude products, in most cases, are of sufficient purity to be used in subsequent reactions (e.g., rearrangement to aldehydes) without further purification. 

An interesting aspect of this reaction is the contrasting stereoselective behaviour of the dimethylsulfonium and dimethyloxosulfonium methylides in reactions with cyclic ketones (E.J. Corey, 1963 B, 1965 A C.E. Cook, 1968). The small, reactive dimethylsulfonium ylide prefers axial attack, but with the larger, less reactive oxosulfonium ylide only the thermodynamically favored equatorial addition is observed. 

Tropolones. A new tropolone synthesis involves in the first step reaction of dimethyloxosulfonium methylide with the quinone monoketal 1 to form the cyclo-propyl ketone 2. Reaction of 2 with isopropylmagnesium bromide followed by dehydration and deketalization gives 3. On treatment with base, the enolate undergoes ring opening to form, after silylation, the dihydrotropolone derivative 4. This product is converted into 3-dolabrin (5) by oxidation followed by demethylation. 

As in the case of the phosphorus ylides, it is believed that d-orbitals on the sulfur participate in bonding, resulting in stabilization of the carbanion site. On reaction with unconjugated ketones and aldehydes, dimethyloxosulfonium methylide gives epoxides. The reaction course involves carbonyl addition, followed by an intramolecular nucleophilic displacement. Several specific examples are given in... 

There is an important difference between the reactions of these sulfur ylides and the analogous phosphorus ylides with carbonyl compounds. Whereas phosphorus ylides react with aldehydes and ketones to yield alkenes, dimethylsulfonium methylide and dimethyloxosulfonium methylide yield epoxides. Instead of a four-center elimination in the zwitterionic intermediate, intramolecular nucleophilic displacement by oxygen occurs. 

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