Diastereoselection preformed lithium enolates

Heathcock et al. have reported a study of the diastereoselectivity of the conjugate addition of preformed lithium enolates with a,8- unsaturated ketones. They found a strong correlation between enolate geometry and product stereochemistry... 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium, titanium, zirconium, and tin, ° silyl enol ethers, enol borinates,and enol borates, R CH=CR"—OB(OR)2. The nucleophilicity of silyl enol ethers has been examined. In general, metallic Z enolates give the syn (or erythro) pair, and this reaction is highly useful for the diastereoselective synthesis of these products. The ( ) isomers generally react nonstereoselectively. However, anti (or threo) stereoselectivity has been achieved in a number of cases, with titanium enolates, with magnesium enolates, with certain enol bor-inates, and with lithium enolates at — 78°C. ... 

Another type of preformed reagent (20) has been used to carry out diastereoselective Mannich reactions. The lithium salts 19 are treated with TiCI4 to give 20, which is then treated with the enolate of a ketone.190... 

---