heavily influence the selectivity of the reaction as per the criteria above. If the ylide carbon is substituted with an alkyl group the reagent is referred to as an unstabilized ylide. The size of the alkyl groups are the major factors in selectivity with these reagents. The R-groups on the sulfur, though typically methyls, have been used to synthesize reagents that can perform enantioselective variants of the reaction (See Variations below). The size of the groups can also influence diastereoselectivity in alicyclic substrates. Synthesis of epoxides Reactions of sulfur ylides with ketones and aldehydes to form epoxides are by far the most common application of the Johnson–Corey–Chaykovsky reaction. Examples involving complex substrates and 'exotic' ylides have been reported, as shown below. The reaction has been used in a number of notable total syntheses including the Danishefsky Taxol total synthesis, which produces the chemotherapeutic drug taxol, and the Kuehne Strychnine total synthesis which produces the pesticide strychnine. Synthesis of aziridines The synthesis of aziridines from imines is another important application of the Johnson–Corey–Chaykovsky reaction and provides an alternative to amine transfer from oxaziridines. Though less widely applied, the reaction has a similar substrate scope and functional group tolerance to the carbonyl equivalent. The examples shown below are representative; in the latter, an aziridine forms in situ and is opened via nucleophilic attack to form the corresponding amine. Synthesis of cyclopropanes For addition of sulfur ylides to enones, higher 1,4-selectivity is typically obtained with sulfoxonium reagents than with sulfonium reagents. Many electron-withdrawing groups have been shown compatible with the reaction including ketones, esters, and amides (the example below involves a Weinreb amide). With further conjugated systems 1,6-addition tends to predominate over 1,4-addition. Other reactions In addition to the reactions originally reported by Johnson, Corey, and Chaykovsky, sulfur ylides have been used for a number of related homologation reactions that tend to be grouped under the same name. With epoxides and aziridines the reaction serves as a ring-expansion to produce the corresponding oxetane or azetidine. The long reaction times required for these reactions prevent them from occurring as significant side reactions when synthesizing epoxides and aziridines. Several cycloadditions wherein the ylide serves as a "nucleophilic carbenoid equivalent" have been reported. Living polymerizations using trialkylboranes as the catalyst and (dimethyloxosulfaniumyl)methanide as the monomer have been reported for the synthesis of various complex polymers. Enantioselective variations The development of an enantioselective (i.e. yielding an enantiomeric