Science of Synthesis

Matsumoto, K.; Katsuki, T.; Arends, I. W. C. E., Science of Synthesis: Stereoselective Synthesis, (2011) 1, 69.

General Introduction

Epoxides are an important class of molecules that are useful and versatile chiral building blocks in organic transformations, especially in stereoselective synthesis, because of their high reactivity, stereospecificity, and regioselectivity in ring-opening processes.[‌1‌] The epoxide group is also an important structural motif that is found in many biologically active compounds, and several epoxide-containing compounds have pharmaceutical applications. Although many methods have been reported for preparing epoxides in a stereoselective manner, the epoxidation of C=C bonds is the most direct and well-studied method (‌Scheme 1‌).[‌2‌–‌4‌] The epoxidation of C=C bonds is generally categorized into two classes: electrophilic epoxidation and nucleophilic epoxidation. In electrophilic epoxidation, an alkene is attacked by an electrophilic oxidant, such as a high-valent transition metal oxo complex or a dioxirane, to give the corresponding epoxide. Alkenes that are rich in electrons generally undergo rapid electrophilic epoxidation reactions, whereas electron-deficient alkenes, such as α,β-unsaturated carbonyl compounds, react more readily with nucleophilic oxidants through the Weitz–Scheffer process. This involves 1,4-addition of the oxidizing agent to the β-carbon, and subsequent nucleophilic attack by the enolate intermediate on the now-electrophilic oxygen atom. This section concentrates on catalytic enantioselective electrophilic epoxidation reactions, including enzymatic reactions.[‌5‌–‌7‌] The epoxidation reaction is further categorized into two subclasses: epoxidation of allylic alcohols and related compounds, and the epoxidation of nonfunctionalized alkenes.

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References