Stereoselectivity Enantioselective oxirane

Dialkylamino-aryloxosulfonium alkylides may be employed for enantioselective epoxidation if the ylide with its chiral sulfur center is resolved into its enantiomeric form, " An enantioselective oxirane is obtained by means of a chiral phase-transfer catalyzed procedure with dimethylsulfonium methylide. The utilization of arsonium ylides was reported some time ago. ° A highly stereoselective synthesis of trans-epoxides with triphenylarsonium ethylide has recently been described.Optically active arsonium ylide has been used in the asymmetric synthesis of diaryloxiranes. ... 

Pyridyl oxiranes (Figure 6.64) were stereoselectively hydrolyzed by EH from A. niger to give the (S)-epoxide and the (R)-diol [183]. The reactions proceed with high regio- and enantioselectivity on the least hindered carbon atom of the... 

The patterns of regio- and stereoselectivities become more complex in disubstituted oxiranes. Beginning with 2,2-disubstituted oxiranes, attack is always at the accessible C-atom. In terms of substrate enantioselectivity, it was found that 2-butyl-2-methyloxirane (2-methyl-1,2-epoxyhexane, 10.43, R = Bu) was hydrolyzed with a preference for the (5)-enantiomer. This substrate enantioselectivity was lost for branched analogues, namely 2-(/er/-bu-tyl)-2-methyloxirane (10.43, R = /-Bu) and 2-(2,2-dimethylpropyl)-2-meth-yloxirane (10.43, R = (CH3)3CCH2) [124], Thus, it appears that the introduction of a geminal Me group suppresses the enantioselectivity seen with branched monoalkyloxiranes, and reverses it for straight-chain alkyloxiranes. 

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

---