Asymmetric synthesis with sulfur ylides

A number of attempts have been made to use optically active sulfur ylides to transfer the chirality of sulfur to carbon in the formation of epoxides and cyclopropanes. The results were somewhat disappointing. Thus, virtually no asymmetric induction was observed with the ylide (1) [475], With the stabilized ylides (2), e.e. values in the range 7-43% were reported [476]. Better results were obtained with sulfonium ylides derived from Eliel oxathiane [477]. Optically active diaryl epoxides could be prepared under PTC in high yields and good e.e. values. 

Durst [478, 479] has shown that the sulfonium ylide (3) transfers its benzylidene group to some carbonyl compounds with e.e. values approaching enantiomeric purity, although the reaction was not yet amenable to synthetic utility (low overall yields, side reaction). However, an interpretation of the difference of behaviour of (3) and (4) towards PhCHO (e.e. values, respectively, 96% and less than 3%) led the authors to propose a [2 + 2] cycloaddition mechanism rather than the commonly accepted nucleophilic antiperiplanar addition for the reaction of a sulfur ylide with a carbonyl compound [479]. Clearly, more work is needed in this area. 

Syntheses of cyclopropanes and oxiranes of high optical activity could be achieved [480] through reactions of anions derived from optically active sulfoximines or sulfoxides, respectively, with Michael acceptors or aldehydes, separation of the diastercoisomeric adducts and conversion of these to the cyclized products. 

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In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

A very convenient asymmetric synthesis of cyclopropane or epoxide systems developed by Johnson (184) is based on the use of chiral sulfur ylides as the agents that induce optical activity. Generally, this method consists of the asymmetric addition of a chiral sulfur ylide to the C=C or C=0 bond and subsequent cyclization of the addition product to form a chiral cyclopropane or epoxide system together with chiral sulfinamide. A wide range of chiral... 

The enantiomeric synthesis of rranj-3,4-disubstituted tetrahydrothiophenes using a sulfur ylide cycloaddition has been reported <990L1667>. The sulfur ylide derived from the action of cesium fluoride on sulfide 111 underwent an asymmetric cycloaddition with chiral a,p-unsaturated camphorsultam amide 112 giving tetrahydrothiophene 113 (80% de). The configuration was confirmed by cleavage of the chiral auxiliary followed by reductive desulfurization with Raney-Ni which gave known carboxylic acid 114. 

Chiral induction can also be quite effective when the locus of asymmetry is attached to the sulfur ylide itself The sulfonium salt 637, derived from Eliel s oxathiane, can be used to deliver a benzylic center to tosylimines (e.g., 638) and efficiently produces phenylaziridines with a very high degree of asymmetric induction. The method is amenable to gram-quantity synthesis, and the chiral auxiliary can be easily recovered. In general, cisitrans-mxxtmcs are obtained, depending upon the steric bulk of the imine substituent (Scheme 156) <2004JOC1409>. 

Dialkylamino-aryloxosulfonium alkylides may be employed for enantioselective epoxidation if the ylide with its chiral sulfur center is resolved into its enantiomeric form, " An enantioselective oxirane is obtained by means of a chiral phase-transfer catalyzed procedure with dimethylsulfonium methylide. The utilization of arsonium ylides was reported some time ago. ° A highly stereoselective synthesis of trans-epoxides with triphenylarsonium ethylide has recently been described.Optically active arsonium ylide has been used in the asymmetric synthesis of diaryloxiranes. ... 

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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