Chemistry of oxaziridines. 14. Asymmetric oxidation of ketone enolates using enantiomerically pure (camphorylsulfonyl)oxaziridine

Article abstract of DOI:10.1021/ja00174a035

The reagent-controlled asymmetric oxidation of tri- and tetrasubstituted ketone enolate anions 4 and 8 by enantiomerically pure (camphorylsulfonyl)oxaziridine 2 has been investigated. The stereoselectivities for oxidation of trisubstituted enolates 4a-d are good to excellent, 60-95% ee, while those for tetrasubstituted enolates 4e and 8 are lower; i.e., 21-30% ee. Isolated chemical yields for both types of enolate anions are good to excellent. The sodium enolate anions of 4a-d, which could be oxidized at -78°C, gave both higher yields and stereoselectivities than the corresponding lithium or zinc enolates, which required warming to higher temperatures for complete oxidation. The presence of HMPA generally had a deleterious effect on the stereoinduction. However, for oxidation of (E)- and (Z)-4d the highest ee's were observed in the presence of this additive. Investigation of the stereoselective trends reveals that the enolate substitution pattern and the enolate solution structure are the most important stereocontrol elements. The role that the enolate geometry has in the stereoinduction is less clear although Z enolates seem to exhibit higher stereoselectivities than the E enolates. The results obtained in this study have been formulated into a mechanistic rational involving an S_N2-type substitution of the enolate anion on oxaziridine 2 via an "open" transition state.