α-Hydroxylation of Enolates and Silyl Enol Ethers

Abstract The autoxidation of enolizable carbonyl compounds was reported as early as 1871, and the accelerating action of base is well documented. The products of these reactions, however, were usually mixtures of compounds resulting from decomposition of the unstable alpha‐hydroperoxy intermediates. It was not recognized until the 1960s that these intermediates can be reduced to alpha‐hydroxy compounds with zinc dust. This observation laid the foundation for what has become on of the simplest and most widely used strategies for introducing a hydroxyl group adjacent to a carbonyl group, namely the oxidation of metal enolates. The alpha‐hydroxy carbonyl array not only occurs in many biologically active molecules, but also serves as an important building block for synthesis. While the oxygenation of enolates still play an important role in the synthesis of alpha‐hydroxy carbonyl compounds, a variety of new, more convenient oxididizing agents, including chiral nonracemic ones, have been introduced. In addition, sily enol ethers and silyl ketene acetals can serve as educts for alpha‐hydroxycarbonyl compounds. This chapter covers the literature of alpha‐hydroxylation of metal enolates and silyl enol ethers up to the end of 2000. Nitriles and aza‐enolates are included. The related alpha‐alkoxylation, alpha‐acyloxylation, and alpha‐sulfonyloxylation of metal enolates are briefly discussed.