Ylids, sulfur reaction with aldehydes

The generally accepted mechanism for the reaction between sulfur ylids and aldehydes or ketone is which is similar to that of the reaction of sulfur ylids with... 

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]. 

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. ... 

Reaction with an aldehyde starts by the removal of a proton with quite a weak base (usually a tertiary amine) to give the ylid 189 in which the negative charge is stabilised partly by electrostatic interaction (the ylid) and partly by the sulfur atom. Nucleophilic attack on the aldehyde gives 190 which can transfer a proton from C to O to give uncharged 191. 

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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