Solladié-Cavallo’s group used Eliel’s oxathiane 1 (derived from pulegone) in asymmetric epoxidation (Scheme 1.3). This sulfide was initially benzylated to form a single diastereomer of the sulfonium salt 2. Epoxidation was then carried out at low temperature with the aid of sodium hydride to furnish diaryl epoxides 3 with
high enantioselectivities, and with recovery of the chiral sulfide 1.
Using a phosphazene (EtP2) base, they also synthesized aryl-vinyl epoxides 6a-c Table 1.1). The use of this base resulted in rapid ylide formation and efficient poxidation reactions, although it is an expensive reagent. There is potential for yclopropanation of the alkene when sulfur ylides are treated with a,b-unsaturated
aldehydes, but the major products were the epoxides, and high selectivities could e achieved (Entries 1–4). Additionally, heteroaromatic aryl-epoxides could be prepared ith high selectivities by this procedure (Entries 5 and 6). Although high electivities have been achieved, it should be noted that only one of the two enantiomers f 1 is readily available.
The Aggarwal group has used chiral sulfide 7, derived from camphorsulfonyl hloride, in asymmetric epoxidation. Firstly, they preformed the salt 8 from ither the bromide or the alcohol, and then formed the ylide in the presence of a ange of carbonyl compounds. This process proved effective for the synthesis of aryl-aryl, aryl-heteroaryl, aryl-alkyl, and aryl-vinyl epoxides (Table 1.2, Entries 1 –5).
Until this work, the reactions between the benzyl sulfonium ylide and ketones to give trisubstituted epoxides had not previously been used in asymmetric sulfur lide-mediated epoxidation. It was found that good selectivities were obtained with yclic ketones (Entry 6), but lower diastereo- and enantioselectivities resulted with cyclic ketones (Entries 7 and 8), which still remain challenging substrates for sulfur ylide-mediated epoxidation. In addition they showed that aryl-vinyl epoxides sould also be synthesized with the aid of a,b-unsaturated sulfonium salts 10a-b (Scheme 1.4).
