Diastereoselective Aldol Reactions via Zirconium Enolates

Zirconium enolates are prepared by the reaction of lithium enolates with Cp2ZrCl2. Aldol reactions mediated by zirconium enolates are characterized by high syn selectivity and good yields as a result of stereo control of the ligands on the metal (Eq. 1) [2]. Even at -78 °C zirconium enolates are reactive in addition to aldehydes because of the high Lewis acidity of the metal. The reaction of (Z)-enolates with aldehydes proceeds via chair-like conformation the conformation is boat-like for (E)-eno-lates [2a]. Thus both ( )- and (Z)-enolates (2) prepared from ketone 1 give predominantly syn aldols syn- i. 

Zirconium enolates can also be derived from amides. It is of particular interest that diastereoselective functionalization of /3-lactams has been performed in high yields (Eq. 2) [2d], 

Zirconium enolates and the reaction products have been structurally characterized [2i-2k]. 

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