Epoxidations sulfur ylide epoxidation

The Corey-Chaykovsky reaction entails the reaction of a sulfur ylide, either dimethylsulfoxonium methylide (1, Corey s ylide, sometimes known as DMSY) or dimethylsulfonium methylide (2), with electrophile 3 such as carbonyl, olefin, imine, or thiocarbonyl, to offer 4 as the corresponding epoxide, cyclopropane, aziridine, or thiirane. ... 

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. 

Isolated carbonyls always give epoxides from the Corey-Chaykovsky reaction. Take the aldehyde substrate as an example. Spiro epoxide 30 was produced from the reaction of trisnorsqualene aldehyde 28 (R20 represents the polyene side-chain with 20 carbons) with substituted sulfur ylide 29, prepared in situ from cyclopropyldiphenylsulfonium tetrafluoroborate and KOH. " For the epoxidation of ketones, the Corey-Chaykovsky reaction works well for diaryl- (31), arylalkyl- (32), ... 

Stereoselective epoxidation can be realized through either substrate-controlled (e.g. 35 —> 36) or reagent-controlled approaches. A classic example is the epoxidation of 4-t-butylcyclohexanone. When sulfonium ylide 2 was utilized, the more reactive ylide irreversibly attacked the carbonyl from the axial direction to offer predominantly epoxide 37. When the less reactive sulfoxonium ylide 1 was used, the nucleophilic addition to the carbonyl was reversible, giving rise to the thermodynamically more stable, equatorially coupled betaine, which subsequently eliminated to deliver epoxide 38. Thus, stereoselective epoxidation was achieved from different mechanistic pathways taken by different sulfur ylides. In another case, reaction of aldehyde 38 with sulfonium ylide 2 only gave moderate stereoselectivity (41 40 = 1.5/1), whereas employment of sulfoxonium ylide 1 led to a ratio of 41 40 = 13/1. The best stereoselectivity was accomplished using aminosulfoxonium ylide 25, leading to a ratio of 41 40 = 30/1. For ketone 42, a complete reversal of stereochemistry was observed when it was treated with sulfoxonium ylide 1 and sulfonium ylide 2, respectively. ... 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

Since cbiral sulfur ylides racemize rapidly, they are generally prepared in situ from chiral sulfides and halides. The first example of asymmetric epoxidation was reported in 1989, using camphor-derived chiral sulfonium ylides with moderate yields and ee (< 41%) Since then, much effort has been made in tbe asymmetric epoxidation using sucb a strategy without a significant breakthrough. In one example, the reaction between benzaldehyde and benzyl bromide in the presence of one equivalent of camphor-derived sulfide 47 furnished epoxide 48 in high diastereoselectivity (trans cis = 96 4) with moderate enantioselectivity in the case of the trans isomer (56% ee). ... 

Catalytic asymmetric epoxidation and aziridination mediated by sulfur ylides 98SL329. 

Until this work, the reactions between the benzyl sulfonium ylide and ketones to give trisubstituted epoxides had not previously been used in asymmetric sulfur ylide-mediated epoxidation. It was found that good selectivities were obtained with cyclic ketones (Entry 6), but lower diastereo- and enantioselectivities resulted with acyclic ketones (Entries 7 and 8), which still remain challenging substrates for sulfur ylide-mediated epoxidation. In addition they showed that aryl-vinyl epoxides could also be synthesized with the aid of a,P-unsaturated sulfonium salts lOa-b (Scheme 1.4). 

The first attempt at a catalytic asymmetric sulfur ylide epoxidation was by Fur-ukawa s group [5]. The catalytic cycle was formed by initial alkylation of a sulfide (14), followed by deprotonation of the sulfonium salt 15 to form an ylide 16 and... 

Epoxy Esters, Amides, Acids, Ketones, and Sulfbnes 1.2.3.1 Sulfur Ylide-mediated Epoxidation... 

In general sulfur ylide-mediated epoxidation cannot be used to form an epoxide with an adjacent anion-stabilizing group such as an ester, as the requisite ylide is too stable and does not react with aldehydes [23], With the less strongly electron-withdrawing amide group, however, the sulfur ylide possesses sufficient reactivity for epoxidation. The first example of an asymmetric version of this reaction was by... 

Aziridination remains less well developed than epoxidation. Nevertheless, high selectivity in inline aziridination has been achieved through the use of chiral sulfi-nimines as auxiliaries. Highly successful catalytic asymmetric aziridination reactions employing either sulfur ylides or diazo esters and chiral Lewis acids have been developed, although their scope and potential applications in synthesis have yet to be established. 

The reaction between an aldehyde and a carbon nucleophile, such as a sulfur ylide, constitutes an alternative approach to the synthesis of epoxides. Since alkenes, which are the normal epoxidation substrates, are often formed from aldehydes, this approach can be highly efficient. On the other hand, the synthesis of appropriate carbon nucleophiles usually requires additional steps. 

The sulfur ylide-mediated epoxidation of aldehydes has been thoroughly investigated [70, 71]. The chiral sulfur ylides reported by Aggarwal have been most broadly applicable, and a catalytic, asymmetric process yielding aromatic transepoxides has been developed [72]. In this process, the sulfur ylides are produced in situ from diazo compounds, generated in turn from tosylhydrazone salts (Scheme 9.15) [73],... 

Metzner and co-workers reported a one-pot epoxidation reaction in which a chiral sulfide, an allyl halide, and an aromatic aldehyde were allowed to react to give a trons-vinylepoxide (Scheme 9.16c) [77]. This is an efficient approach, as the sulfonium salt is formed in situ and deprotonated to afford the corresponding ylide, and then reacts with the aldehyde. The sulfide was still required in stoichiometric amounts, however, as the catalytic process was too slow for synthetic purposes. The yields were good and the transxis ratios were high when Ri H, but the enantioselectivities were lower than with the sulfur ylides discussed above. 

The major limitation of asymmetric sulfur ylide epoxidations is that only aromatic vinylepoxides can be formed efficiently and with high selectivity. When an aliphatic aldehyde is allowed to react with a semistabilized or nonstabilized sulfur ylide, poor diastereoselectivities and yields are observed, due to problems in controlling the ylide conformation and competing ylide rearrangement reactions [71]. However, some racemic, aliphatic vinylepoxides have been successfully formed by sulfur ylide epoxidations, although varying diastereoselectivities were observed [78-80],... 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

Sulfur ylides are a classic reagent for the conversion of carbonyl compounds to epoxides. Chiral camphor-derived sulfur ylides have been used in the enantioselective synthesis of epoxy-amides <06JA2105>. Reaction of sulfonium salt 12 with an aldehyde and base provides the epoxide 13 in generally excellent yields. While the yield of the reaction was quite good across a variety of R groups, the enantioselectivity was variable. For example benzaldehyde provides 13 (R = Ph) in 97% ee while isobutyraldehyde provides 13 (R = i-Pr) with only 10% ee. These epoxy amides could be converted to a number of epoxide-opened... 

The intramolecular addition of sulfur ylides to imines (e.g. 72) has proven to be an excellent route to fused-ring aziridines (e.g. 73) <06AG(I)7066>. The addition of a sulfonamide to a vinylsulfonium salt leads to the formation of the sulfur ylide 72. The ylide then undergoes an intramolecular addition to form the product fused-ring aziridine 73. This method has also been used for the synthesis of fused-ring epoxides. 

An alternative to the synthesis of epoxides is the reaction of sulfur ylide with aldehydes and ketones.107 This is a carbon-carbon bond formation reaction and may offer a method complementary to the oxidative processes described thus far. The formation of sulfur ylide involves a chiral sulfide and a carbene or carbenoid, and the general reaction procedure for epoxidation of aldehydes may involve the application of a sulfide, an aldehyde, or a carbene precursor as well as a copper salt. This reaction may also be considered as a thiol acetal-mediated carbene addition to carbonyl groups in the aldehyde. 

In the design of chiral sulfides for sulfur ylide-mediated asymmetric epoxidation of aldehydes, two factors are important. First, a single sulfur ylide should be produced. Otherwise, the diastereomeric sulfur ylides may react with aldehydes in different ways and thus cause a drop in stereoselectivity. This may be achieved by choosing a rigid cyclic structure to make one of the lone pairs more accessible than the other. Second, the structure should be amenable to structural modification in order to study the electronic and steric effects of the sulfur on the enantioselectivity of the epoxidation reaction. 

The most remarkable feature of the chemistry of these ylides is their efficient participation in typical sulfur ylide chemistry, i.e. their ability to form epoxides with carbonyl partners (Eq. 27 a)29 and spiropentanes with enones (Eq. 27 b)30. ... 

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... 

Sulfur ylides behave similarly to phosphorus ylides, but the final products are different. Figure 10-31 shows the mechanism for the prepciration of a sulfur ylide and the reaction of the sulfur ylide with a carbonyl group. Notice that the mechanism for the formation of the sulfur ylide is similar to the formation of a phosphorus ylide. However, the last step in the sulfur ylide mechanism is an internal S, 2 reaction, which eliminates the original thioether (dimethyl sulfide). The reaction of a sulfur ylide with a ketone yields epoxides, whereas the product of a phosphorus ylide with a ketone is an alkene. 

The carbon fragment used in this approach can also be provided by sulfur yUdes. In this arena, Metzner and co-workers <99JCS(P1)731> developed a novel asymmetric variant employing (+)-(2/J,5/J)-2,5-dimethylthiolane (53) as the chiral auxiliary to prepare rrons-(S,S)-stilbene oxide (56). Chiral epoxides have also been prepared from aldehydes using sulfur ylides derived from the products of Baker s yeast reductions <99SL1328>. 

Sulfur ylides react with aldehydes and ketones to form epoxides (oxiranes) ... 

Dimethyloxosulfonium methylide reacts with the triketone (212) to give the fused 2-hydroxymethylenedihydropyran (213) (76H(4)1755). The reaction is thought to proceed through a zwitterion and the epoxide as indicated in Scheme 41. Intramolecular nucleophilic attack leads to the dihydropyran and the overall process may be regarded as a transfer of methylene from the sulfur ylide. 

These reactions rapidly found wide use and success, and many other sulfur ylides have been prepared and exploited [194, 195, 203, 204]. Various experimental procedures are to be found in the detailed monograph by Trost and Melvin [204] for sulfonium salts, ylides, epoxidations and cyclopropanations. 

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. 

Both reagents transfer a methylene group in efficient and selective pathways. So it is not surprising that sulfur ylides have been widely used as synthetic tools for the preparation of epoxides. The reactions can make use of sulfonium salts under phase transfer catalytic conditions, and the cheap and easily accessible trimethyl sulfonium methyl sulfate and triethylsulfonium ethyl sulfate were found to show a high reactivity under such conditions [450]. 

Kinetic control and thermodynamic control account, respectively, for epoxide formation and cyclopropanation [204]. Various Michael acceptors have been converted to cyclopropane derivatives with sulfur ylides. Some of the reagents used to transfer CHCOR [456], CHCOOR [457], CHMe... 

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