Epoxidation With sulfur ylids

A modern example of an agent used against human fungal infections is Pfizer s fluconazole, which actually contains two triazoles. The first is added as the anion to an a-chloroketone and the second is added to an epoxide made with sulfur ylid chemistry (you will meet this in Chapter 46). Note that weak bases were used to catalyse both of these reactions. Triazole is acidic enough for even... 

Epoxides can be converted directly to episulfides by treatment with NH4SCN and ceric ammonium nitrate.Diazoalkanes, treated with sulfur, give episulfides.It is likely that R2C=S is an intermediate, which is attacked by another molecule of diazoalkane, in a process similar to that shown in 16-46. Thioketones do react with diazoalkanes to give episulfides.Thioketones have also been converted to episulfides with sulfur ylids.Carbenes, such as the dichlorocarbene from CHCI3 and base, react with thioketones to give an... 

An interesting approach to azasugars (e. g. 116 in O Scheme 31) from the epoxsides built at the anomeric centers of unprotected carbohydrates was presented recently [78]. Such epoxides (e.g. 115) were obtained with very high selectivity hy reaction of the corresponding hemi-acetals (e. g. 114) with sulfur ylid (O Scheme 31). This reaction is applicable for various sugar aldehydes [79]. 

Some 2,3-epoxyamides of aldonic acids have been prepared by reaction of acyclic aldose derivatives with sulfur ylids (Scheme 1). 5y -epoxidation of some racemic 2-benzyloxy-4-alkenamides by way of iodohydrin formation and subsequent base treatment has afforded epoxides such as 2. Epoxidation of hex-2-enopyranosyl phosphonates has been effected with H202/sodium tungstate. ... 

Aldehydes and ketones can be converted to epoxides in good yields with the sulfur ylids dimethyloxosulfonium methylid (60) and dimethylsulfonium methylid (61). For most purposes, 60 is the reagent of choice, because 61 is much less... 

The relative rates and stereochemistry of epoxidation reactions of 5-substituted-adamantan-2-ones with two sulfur ylids (methylenedimethylsulfurane and its oxy-sulfurane analogue) have been studied in DMSO and in benzene.318... 

The simplest sulfur ylids are formed from sulfonium salts 69 by deprotonation in base. These ylids react with carbonyl compounds to give epoxides.18 Nucleophilic attack on the carbonyl group 70 is followed by elimination 71 of dimethylsulfide 72 and formation of the epoxide 73. You should compare diagram 71 with diagram 23 in chapter 15. The phosphonium ylid reacted by formation of a P-0 bond and an alkene in the Wittig reaction. The sulfonium compound reacts by formation of a C-O bond 71 as the S-O bond is much weaker than the P-0 bond. The sulfonium salt 69 can be reformed by reaction of 72 with Mel. 

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. 

Just as sulfur ylids react with the carbonyl of an aldehyde or ketone to give an epoxide, tellurium ylids react with imines to give an aziridine. The reaction of an aUyhc tellurium salt, RCH=CHCH2Te Bu2 Br, with lithium hexamethyldisila-zide in HMPA/toluene leads to the tellurium yhd via deprotonation. In the presence of an imine, the ylid add to the imine and subsequent displacement of Bu2Te generates an aziridine with a pendant vinyl group. ... 

Reactions of chiral allylic boranes with carbonyl compounds Reactions of chiral allyl boranes with imines Asymmetric Addition of Carbon Nucleophiles to Ketones Addition of alkyl lithiums to ketones Asymmetric epoxidation with chiral sulfur ylids Asymmetric Nucleophilic Attack by Chiral Alcohols Deracemisation of arylpropionic acids Deracemisation of a-halo acids Asymmetric Conjugate Addition of Nitrogen Nucleophiles An asymmetric synthesis of thienamycin Asymmetric Protonation... 

Asymmetric epoxidation with chiral sulfur ylids... 

A very different type of chemistry occurs when sulfur ylids add to carbonyl compounds. Epoxides are formed and recent progress with chiral sulfur ylids allows good asymmetric induction in this reaction. The easily prepared C2 symmetric sulfide 96 reacts with alkyl halides and then with aryl and alkyl aldehydes to give good yields of trans epoxides 97 with reasonable ees.19... 

The reaction of an aldehyde with a phosphorus ylid was noted several times in previous sections. Sulfur ylids such as dimethylsulfonium methylid used here also react with aldehydes, in this case with 5.81, to give an epoxide. Subsequent opening... 

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