Diastereoselective alkylation of glycinates. 4. The role of the metal cation and additives

Abstract

The alkylation of the sodium, potassium, zinc, and zirconium enolates of the R-camphor imine of tert-butyl glycinate have been examined and the stereochemical results compared with those obtained with the lithium enolate. No difference was noted for the sodium enolate but inferior chemical yields and stereoselectivities were obtained with the potassium enolate, perhaps due to its increased ionic character. Substituting TMEDA for HMPA in these reactions led in all but one case to reduced stereoselectivities. The results support and are interpreted in terms of the π–metal association model previously proposed.