Sulfoxonium ylids

So what has this got to do with cyclopropanes If sulfur ylids react with enones either the epoxide 74 or the cyclopropane 76 may be formed.19 The general rule is that sulfonium ylids from 69 give epoxides but sulfoxonium ylids give cyclopropanes 76. 

The sulfoxonium ylid 78 is more stable and is therefore liable to do conjugate rather than direct addition (chapter 21). The intermediate eliminates dimethyl sulfoxide 79 to give the cyclopropane 76. The intermediate is long lived and the single bond that was the alkene can rotate so the geometry of the alkene is lost. In this case we expect the more stable trans cyclopropane to be formed by choice. 

Sulfoxonium ylids react with unsaturated carbonyl compounds in the same way as the stabilized ylids that you have met already do—they form cyclopropanes rather than epoxides. The example below shows one consequence of this reactivity pattern-—by changing from a sulfonium to a sulfoxonium ylid, high yields of either eposjde or cyclopropane can be formed from an unsaturated carbonyl compound (this one is the terpene carvone). 

The formation and reactions of sulfoxonium ylids demonstrate how sulfoxides occupy a useful and interesting part of the middle ground between sulfides and sulfones—they are weakly nucleophilic, like sulfides (and can be alkylated with methyl iodide to give sulfoxonium salts as we have just seen), but at the same time they stabilize anions almost as well as sulfones. However, sulfoxides are perhaps the most versatile of the three derivatives because of a good deal of chemistry that is unique to them. There are two reasons why this should be so. 

The photolytic rearrangement of sulfoxonium ylids is envisaged as taking place via a ketene in a process reminiscent of the Arndt-Eistert reaction viz. 

A good example is the natural product eucarvone 16 mentioned in the original Corey papers.5 The simple sulfonium ylid 5 gives the epoxide 16 in 93% yield by direct addition while the sulfoxonium ylid 10 gives the cyclopropane 14 in 88% yield by conjugate addition to the nearer alkene. In an extended conjugated system the alkene nearest to the carbonyl group is normally the most electrophilic. 

The first reaction is a straightforward cyclopropane formation with a sulfoxonium ylid and a conjugated ketone. The only unusual feature, the MeO group, makes no difference. 

Interactive mechanism for three-membered ring formation with sulfonium and sulfoxonium ylids... 

Sulfoxonium ylids, carbonyl-stabilized, reactions via — 21, 882 Sulfoxylic acid esters... 

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