[Stereoselective epoxidation with bulky dioxiranes generated from substituted cyclohexanones].

Abstract

Dioxiranes have recently been shown to be important and versatile oxidants, which are generated from potassium monoperoxysulfate (KHSO5) and ketones. Dimethyldioxirane, a dioxirane generated from acetone as a ketone, is particularly useful as an oxidation reagent with a broad scope of synthetic applications. Several papers have been reported about stereoselective epoxidation using dimethyldioxirane. However, there have been only a few examples using dioxiranes generated from other ketones. Dioxiranes derived from sterically hindered ketones are expected to be "bulky oxidant", and to be useful for stereoselective epoxidation. In a CH2Cl2-buffered water (two-phase system) in the presence of phase-transfer agent under neutral condition (pH 7.8) according to the reported procedure, the epoxidations with hindered alpha-substituted cyclohexanones proceeded in very low yields. After a survey of possible conditions we found that basic (pH 11) CH2Cl2-MeOH-buffer system was effective for the epoxidations of olefins with Oxone (active constituent KHSO5) and substituted cyclohexanones. We carried out epoxidation of cyclohexen derivatives with dioxiranes derived from alpha-substituted cyclohexanones in a CH2Cl2-MeOH-buffer solvent system at pH 11. High trans selectivities were obtained. Furthermore, according to this method acyclic silyl ethers were stereoselectively oxidized to afford erythro epoxides.