Silyl enol ethers alkali metal enolates

Titanium enolates. These enolates have generally been prepared by transmet-talation of alkali-metal enolates or silyl enolate ethers. Surprisingly, Evans et al. find that a titanium enolate can be prepared directly from the oxazolidinone 1 by reaction with TiCl4 (1 equiv.) in CH2C12 and shortly thereafter with ethyldiisopropyl-amine (or triethylamine) at 0°. The enolate may actually be an ate complex (2a)... 

Alkali Metal Enolates from Enol Acetates and Silyl Eru>l Ethers 1.423 Alkali Metal Enolates by Miscellaneous Methods... 

A series of alkyl-aryl ketones 31 was converted into cA-enolates 32 in a highly selective manner by deprotonation with LDA in THF, the cisttmns ratio uniformly surpassing 99 1. Only in the substrates 34 with a-t-butyl or a-phenyl substituents the selectivity was moderate, so that, for these ketones, an alternative protocol was elaborated that used the additive DMPU for obtaining sodium enolates cis-35 exclusively. These conditions were also applied for preparing cA-configured sodium enolates 38 from N-acyl pyrroles 37. In all cases, the alkali metal enolates were quenched by silylation to give silyl enol ethers 33, 36, and 39, respectively (Scheme 2.9) [43]. 

Alkali Metal Enolates by Cleavage of Enol Acetates or Silyl Enol Ethers... 

The two main problems in the preparation of silyl enol ethers are control of regios-electivity, kinetic and thermodynamic, and stereoselectivity, (E) and (Z). Although many useful procedures are now available for the kinetic deprotonation of ketones by use of alkali metal dialkylamides, there are few practical procedures for thermodynamic deprotonation. Recently, the author and Yamamoto et al. found that the regio- and stereoselective isomerization of a kinetic silyl enol ether to a thermodynamic ether was catalyzed by LBA [138]. 

---