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.
We have developed an efficient method for the preparation of enol silyl ethers using novel agents, silazanes together with NaH or DBU catalyst, wherein TMS and TBDMS groups were smoothly and chemoselectively introduced into ketones and aldehydes under mild conditions.
A highly efficient method of silylation using a novel agent, Si-BEZA (silylbenzamide), together with a pyridinium triflate catalyst was developed, wherein a variety of silyl groups can be introduced into sterically crowded alcohols under mild conditions.
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